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The Sonoran Desert is one of four traditionally recognized deserts in North America, the others in this traditional view being the Chihuhuan, Great Basin, and Mojave Deserts. Some recent (and perhaps more popularized than scientifically based) interpretations added the Colorado Desert as a fifth desert whereas the older perspective regarded the Colorado as one of seven parts (designated as subdivisions by Shreves; explained in detail below) of the greater Sonoran Desert. Broadly speaking the Sonoran and Chihuhuan Deserts are "hot" deserts with their southern portions extending below 30 degrees North latitude into the subtropical belts of high pressure (ie. the horse latitudes). The Great Basin Desert coincides closely (not exactly of course) with the Great Basin section of the Basin and Range province, from which it's designation was derived. The Great Basin Desert is a "cold" desert. The Mojave Desert is the smallest of the traditional four North American deserts and is situated between the "hot" Sonoran Desert and the "cold" Great Basin and has often been characterized as an "intermediate" desert. Brown (1994, ps.143, 144, 156, 180) divided North American deserts ("desertlands" was the overall or general heading) as follows: 1) cold-temperate desertlands- Great Basin desertscrub, 2) warm-temperate desertlands- Mohave desertscrub and Chihuhuan desertlscrub, 3) tropical-subtropical desertlands- Sonoran desertscrub.
All of the North American deserts occur within the vast Basin and Range physiographic province, and all of them have the telltale physiography of valleys or geographic depressions called "basins" situated between mountain ranges of varying elevations but which typically run parallel to the valleys. Ergo: "basin and range" topography. Most of the vast region of the North American deserts was designated as Desert climate (BW) by the Koppen-Geiger system of climate classification although parts of the perimeters of these deserts was classified as Steppe climate (BS). Obviously there is no exact line between these. Big sagebrush-dominated Great Basin Desert blends into the bunchgrass-sagebrush shrub steppe and the creosotebush-dominated Chihuhuan Desert merges with the semidesert grassland. Boundaries of the Sonoran Desert are for the most part more distinct than those of the former two large deserts.
Good summary discussions of the Koppen-Geiger climate classification system are available in most of the standard texts on Climatology or Meteorology including the comprehensive Strahler and Strahler (1978, ps. 144-146). For details of the Basin and Range physiographic province the definitive authority remains the description and maps of Fenenman (1931, Chapter VIII, esp. ps. 367-379 for Sonoran Desert and Salton Basin sections). The more recent text of Hunt (1974, Chapter 16, esp. ps. 499-502 for Sonoran Desert and Salton Trough) is also worth reading as is the work of Laity in Orme (2002, Chapter 19, ps. 390-393 for Sonoran Desert). Readers should avoid confusion over the difference between the Sonoran Desert and it's six vegetational subdivisions and the Sonoran Desert section of the Basin and Range physiographic province. The latter is smaller in geographic area and refers to geological aspects like landforms. The former is essentially a large biological community whose boundaries are functions of climate, landforms, soils, and other abiotic factors along with flora and fauna that comprise the Sonoran Desert ecosystem.
In this publication, which is overwhelmingly Clementsian in perspective and terminology, the Sonoran Desert is one desert that makes up part of the desert biome or, if but for the moment emphasis is laid on vegetation, the desert formation.
For all things Sonoran the "alpha and omega" (at least the beginning) is the life-long work of Forrest Shreve with that of Shreve in Shreve and Wiggins (1964, ps. 6-186 plus plates) the defining ecological monograph. A more recent comprehensive source (and one that still used the seven vegetational subdivisions of Shreve) is Crosswhite and Crosswhite (Chapter 5 in Bender, 1982). General descriptions and discussions of the Sonoran Desert (and also the Mojave and Great Basin Deserts) vegetation can be found in the ecological monographs of North American biotic communities (Shelford, 1963, ps. 373-387) and North American vegetation (Barbour and Billings, 1988; Barbour and Billings 2000)) and that of Californian vegetation (Barbour and Major, 1995). Unfortunately these more recent descripitions ignored the herbaceous layer of vegetation, in particular giving little or no consideration of grasses. Jaeger (1957) has been a choice starting place for general and less-advanced reading on the deserts of North America.
"In a league by itself" except for Shreve is Discovering the Desert by McGinnies (1981). McGinnies was a colleague and contemporary of Shreve and likely had more first-hand knowledge of the Sonoran Desert than any authority except for Shreve. Like Shreve and Clements, McGinnies included grasses and the herbaceous component of the vegetation and did not ignore them as did many later ecologists.
Chapter from history of Ecology: It is of both historical and biological importance to understand the "ecological perspective" of Forrest Shreve because he remains the ecologist who knew the most about the vegetation of the Sonoran Desert. Shreve laid the foundation on which most subsequent and substative ecological studies into the Sonoran Desert have been based (eg. all major contemporary descriptions of the Sonoran Desert retained the seven vegetational subdivisions of Shreve). Forest Shreve was to deserts what John Weaver was to grasslands. Shreve also worked in the other four major deserts of North America, but World War II and his deteriorating health intervened so that Shreve did not achieve "repeat successes" in these communities (Spoehr in Shreve and Wiggins, 1964, p. 5).
Like the better known, more prolific publication-producing, politically astute, and superior self-promoting Frederic Clements and famous "cactophiles" Nathaniel Britton and J.N. Rose, Forrest Shreve worked for the famous Carnegie Institution of Washington. In fact, at one time or the other all of these famous pioneer plant scientists and ecologists worked at the Carnegie Desert Botanical Laboratory near Tucson, Arizona. The fascinating story (to plant ecologists anyway) of Shreve and the intriguing politics (by academic standards) of the Desert Laboratory and related units like the Alpine Laboratory on Pikes Peak, where Clements served in the summer (and the Desert Laboratory in winter) for eight years, was told in factually detailed and lively fashion by Bowers (1988). Bowers' scholarship should be especially interesting and revealing to those versed in the conflict between the Clementsian succession-and-climax model of "dynamic vegetation" and the Gleasonian continuum or individualistic model of vegetation.
[In terms of institutional or office politics and flattering the one who foots the bill, your current author simply drew attention to the scientific name of the plant that to most serves as the iconic personification of the Sonoran Desert: Carnegiea gigantea."Carnegiea is the best know of the characteristic plants of the Sonoran Desert" (Shreve in Shreve and Wiggins, 1964, p. 147).]
Of relevance to this introduction of the Sonoran Desert (and the ecological study thereof) is the fact that Shreve-- who became synonymous with Sonoran Desert research-- was more in the camp of Henry Gleason and the individualistic view of plant communities than in the gathering of orthodox Clementsian disciples. Again the research of Bowers (1988) was to the point and she quoted from pages 21-22 of Shreves culminating publication, Vegetation of the Sonoran Desert (Shreve [1951] as Carnegie Institution of Washington Publication No. 591 and later--and more accessible-- reprinted as pages 1-186 plus plates in Shreve and Wiggins [1964]):
"The successional changes which are so important elsewhere may be read into the vegetation of the desert, but the evidence for them would hardly suggest that they are of importance in determining the relations of the communities to one another. The secular changes of surface are accompanied by the shifting of the communities over long periods, by which their locations and areas ae changed in the simplest possible manner. If a particular community is destroyed without change in the soil, the earliest stage in the return of vegetation will be appearance of young plants of the former dominants. No only do the same species reappear at the outset, but their first individuals ultimately constitute the restored community.
"It is not possible to use the term 'climax' with reference to desert vegetation. Each habitat in each subdivision of a desert area has its own climax, which must be given an elastic definition and must not be interpreted as having a genetic relation to any other climax. It is merely the particular group of species which, in somewhat definite proportions and with a fairly definite communal arrangement, is able to occupy a particular location under its present environmental conditions". (Shreve in Shreve and Wiggins, 1964, ps. 29-30).
The strains of the individualistic concept of Gleason were clear in that passage (last sentence in quotation). So also, however, was the theme of Arthur Tansley's polyclimax concept and, later, of Whittaker's climax pattern theory both of which, as described in the literature review herein, were modifications or amendments of the prevailing Clementsian model of plant succession, only one part of which was the monoclimax view. There is nothing in this explanation of vegetation or desert plant communities by Shreve that placed him in either the Gleason or Clements school. In fact, the second paragraph quoted was consistent with the range site concept which was an outgrowth of the polyclimax perspective of Tansley and of plant indicators tracing to Clements second Carnegie Institution "Sears & Sawbuck"-sized monograph (Clements, 1920).
Conversely, the first paragaraph of Shreve's writing was beyond doubt the continuum interpretation in Gleason's individualistic theory of vegetation. Bowers (1988, p. 50-51) quoted Shreve discussing "environment gradients" and she explained how Robert Whittaker later used this concept in development of gradient anaalysis. She remarked that Shreve had described Jamaican mountain vegetation as a continuum, but that Gleason received credit for the concept though his publication came out two year after the one by Shreve (Bowers, 1988, p. 54). Bowers also specified that "[g]radients were a familar concept, though". Clearly, Shreve was an individualist (in general, not ecological school-wise) who had ideas of his own, most of which remain the foundation for description of the Sonoran Desert.
Bowers (1988, p. 88) concluded that "Shreve's denial of desert succession may have been largely semantic". Bowers illustrated how some of the "processes of vegetational change" Shreve recognized were regarded as plant succession decades later. "If Shreve could have used these looser definitions, he might well have been willing to admit the existence of succession in desert environments" (Bowers, 1988, p. 88). On the other hand Bowers noted that Shreve periodically got in a few jabs at Clements (eg. Bowers, 1988, ps.33, 55) and Shreves "may have felt constrained" "... by the fact that the Carnegie Institution supported them both" (Bowers, 1988, p. 60).
Observe from the above quotation in Vegetation of the Sonoran Desert (Shreve and Wiggins, 1964, ps. 29-30) that Shreve did accept the concept of dominant species, a perspective central to the cover type concept. This concept in turn was the basis of dominance type, the form or category of type used by the Society of American Foresters and Society for Range Management for forest and rangeland cover types, respectively.
The indisuptable fact remains, however, that Shreve opposed many of Clements' central tenets (dogma in the opinions of some). Shreve certainly saw vegetation as a continuum and not as discrete, repeatable, natural units that were the mature or adult "organism" toward which all vegetation in a given climate was invariably developing. Bowers (1988, p. 61) quoted Shreve (1915) as having written that it was "... nowhere possible to pick out a group of plants which may be thought of as associates without being able to find other localitites in which the association has been dissolved". Shreve's Carnegie Institution Publication No. 217, The Vegetation of a Desert Mountain Range as Conditioned by Climatic Factors, predated the monumentally influencial Plant Succession, Carnegie Instution Publication No. 242 (Clements, 1916a) by one year. On no other point central to plant communities did Shreve and Clements have more disparate views. Clements saw discrete, natural plant communities that developed into mature organism-like assemblages of species all of which existed in a hierarchial arrangement of vegetational units; Shreve saw only individual plant communities arrayed along a continnuum of environmental gradients.
Bowers (1988, p. 135) quoted a comment by Shreve to a correspondent, "I take very little interest in the formal treatment of vegetation and the customary descriptions of successions and climaxes treated with great attention to the formal and highly artificial clasifications of which we have so many". Clements, by stark contrast, seemed to thrive on such "formal treatment of vegetation" and he certainly did benefit immeasurably from these which he "packaged" and "marketed" so successfully.
One of Clements' hierarchial units of vegetation that was most influencial with and widely used by applied ecologists like foresters and rangemen was the association. The association was central to the Clementsian concepts of climax, formation, and biome as the mature organism of vegetation and biotic community (see review of literature herein). Each climax formation had two or more major subdivisions designated as associations, each of which was defined or designated by it's one or more dominants (Weaver and Clements, 1938, p. 93).
Shreve (in Shreve and Wiggins, 1964, p. 29) accepted the idea of dominants, but he rejected associations (Shreve, 1915). It was shown in the literature reveiw herein that forest and rangeland cover types are a form of the dominance type that was derived from the association, a unit central to the English or Anglo-American Traditon or School of Plant Ecology (Shimwell, p. 53-54). Historically the association, basically the Clementsian association, and the dominance type were essentially synonymous in American and British plant ecology. (Among the various traditions or schools of Plant Ecology meaning of dominance types varied as, for instance, between the Anglo-American and the Braun-Blanquet school [Whittaker, 1980, ps.67-79].)
Without the association (without the validity of the association) the Anglo-American dominance type would be of questionable meaning or legitimacy. In other words, if the view of Shreve (1915) was correct it would be meaningless (or nearly so) to speak of cover types. This was in net effect the logical extension and practical conclusion of the Shreve-Gleason concept of plant communities as a continuum of individual groupings of plant species arranged along environmental gradients. Precisely speaking Gleason (1917, 1926) did accept plant associations but as individual assemblages, "the individualistic concept of the plant association". Of necessity (or perhaps by default) this concept, now widely endorsed by many plant ecologists, was unaccepted (or ignored) when more-or-less discrete, repeatable cover types were named, described, and published as "standards" of vegetation. Even more, there was a tendency for cover types to be climax vegetation (or disclimaxes in cases of stable communities made up of naturalized species), disclaimers of published cover types notwithstanding. It seems that Shreve lost out to Clements who so "excelled at packaging his wares" (Bowers, 1988, p. 59).
Shreve did "count coup" (sort of ) on Clements in one important and lasting regard. Shreve's vegetational subdivision of the Sonoran Desert remains the one most commonly accepted and widely used arrangement by those working therein. Shreve (in Shreve and Wiggins, 1964, p. 47- 126) based his subdivision or conceptual framework of the Sonoran Desert on three criteria: 1) geographic and physiographic features, 2) abundance of plant genera or species in the distinct geographic-physiographic areas (called "regions"), and 3) "character and organization of the communities of plants". Shreve (in Shreve and Wiggins, 1964, p. 48) presented these as "three sets of equivalent designations for the seven subdivisions" in a format with the last criteria, 3) "character ... of plants", numbered and at top of a "set" with the subdivision itself, 1) "geographic and physiographic" criteria, at bottom of the set.
Bowers (1988, p. 132) pointed out that in his vegetational subdivisions Shreve used geographical titles rather than floristic ones, and that ecologists since have followed Shreve's lead. Bowers also remarked that Shreve's criteria were primarily physiognomic. In this regard he followed such well-known preceding plant geographers as Humboldt (1807) and, more so, Grisebach (1838), Warming (1909), and, as always, Clements (1916a). Even Clements? Yes. Clements' association was the floristic subdivision of a physiognomically-developmentally derived formation. Warming (1909), in what many students interpret as the first "real" Ecology textbook, used growth forms as a basic criteria for physiognomy. (This was reviewed herein: see Range Type, Associations and Consociations, Literature Review.)
Bowers (1988, p. 120) explained that Shreve had been ciritical of the traditional growth- or life forms of Raunkiauer. Instead Shreve used Drude's classification system because it included plant physiological criteria. Herein lies a twist, and one that interwove the careers of Shreve and Clements. Drude was a follower of August Grisebach who in turn was one of Humboldt's main students (Tobey, 1980, ps. 51, 62, 90, 101), and Drude had a tremendous influence on faculty and graduate students at the University of Nebraska including Frederic Clements (Tobey, 1980, ps. 60, 62-69, 71, 83, 87-88, 90, 98-99, 101-102, 204). Though Shreve and Clements disagreed with each other-- usually indirectly or obliquely and with thinly veiled references to the views of the other (ie. flanking actions rather than frontal attacks)-- they had a common intellectual heritage.
Forrest Shreve defied ready made classification. He, having pioneered the continuum concept, was in the individualist school of vegetation, but when he mapped, described, and arranged Sonoran Desert and it's vegetation units Shreve operated at the scale of Clements' associations. Shreve's Sonoran vegetational units or large-scale plant communities (vegetational subdivisions in particular) differed little from those of Clements. Perhaps this can be partially traced to their fundamental philosophy of vegetation that had a common origin in Plant Geography back through Drude. Drude was central to the English or Anglo-American Tradition (Shimwell, 1971, 47-49) which developed into the Clements-Tansley manifestation thereof. Gleason was also a branch-- though a distant one-- from the English Tradition (Shimwell, 1971, ps. 54). Shreve was even more independent than Gleason, and it was studies of vegetation by Shreve-- not Gleason-- that were first to propose the continuum concept for which Gleason got the credit (Bowers, 1988,.ps. 28-29, 54). Again, however, Shreve and Clements had similar pre-desert backgrounds, both having done their initial research in more mesic environments. Also, both had a common perspective of vegetation through the Idealistic Tradition in Plant Geography via Drude (Tobey, 1980, ps. 87-99 passim; Bowers, 1988, 120).
Unlike Lucy Braun's (Braun, 1956) subdivision of the eastern deciduous forest into modified Clementsian associations (Whittaker, 1980, p. 68) and Weaver's (Weaver, 1954; Weaver and Albertson, 1956) treatment of prairie and plains grasslands on basis of Clementsian associations in their purest form, Shreve put maximum distance between his simple arrangement of desert vegetation and the elaborate hierarchial system of Clements that Shreve so detested. The irony was that this difference made little difference. Shreve's treatment of Sonoran Desert vegetation was still based on dominant plant species which was the same basis as Clements' associations which can be dated back through the Third International Botanical Congress and Humboldt, himself, the "inventor" of the association concept (see discussion, Range Type under Literature Review). It is also interesting that the subdivision of the Sonoran Desert on regional and physiographical scale using dominants therein (complete with a version of growth form) was amazingly similar to (though less formal than) the hierarchial vegetation classification system of George Nichols (discussed under George Nichols, Literature Review) which was, of course, similar to that of Clements (ie. yet another "formal treatment of vegetation" in which Shreve took "very little interest").
The other aspect of desert vegetation in which Shreve's ecological views prevailed (to a relative not absolute extent) over those of Clements were on plant succession. Bowers (1988, ps. 103) noted that whereas Clements emphasized the universal occurrence and importance of competition among plants as a factor in succession, Shreve minimized the role of competition in desert vegetation. "In arid regions there has been much less competition. The greatest "struggle" of the plants has not been with one another, but with the environment. Therefore the conditions tending toward the elimination of certain types and the survival and dominance of a relatively uniform one have not been operative. This fact has brought about the diversity of plant types which characterizes many semiarid and arid regions and some extremely arid ones" (Shreve in Shreve and Wiggins, 1964, p. 27). In other words, the element of "competition" that was central to the later-named "relay floristics" of the Weaver and Clements (1938, Chapter VII) textbook scheme was not very important in desert vegetation.
It should be emphasized that it was not only Clements but also some of Shreve's closest colleagues who disagreed with Shreve on this point of competition. McGinnies, who dedicated his book to Forrest Shreve, wrote (dedication notwithstanding): "On the floodplain, competition among the roots was often not as great as on the bajada. The roots of annuals growing on the bajada reach as deep as most perennials in the same habitat, and, since they occur in large numbers, competition with them must be an important causal factor contributing to the sparseness of the perennial vegetation of the bajada. The annuals also come into competition with the shallow rooted perennials through the laterals that are developed on the tap root of the annuals..." (McGinnies, 1981, p. 153).
Shreve wrote that there was "a relative scarcity of plant litter" in the desert. "Consequently this most important of the reactions of plants upon their habitats is reduced to a minimum in the desert. The long existence of a community in a given spot does little if anything to pave the way for the entrance of plants with life requirements different from those of the occupants" (Shreve in Shreve and Wiggins, 1964, ps. 28-29). There was an "almost total lack of reaction by the plant on its habitat" so that presence of a plant does "nothing to make that spot a better habitat for some other plant or some other species" (Shreve, 1942, p. 203; explained further by Bowers, 1988, p. 136). "Them's fightin' words!" Nothing ever written was more anthetical to the Clementsian "development of vegetation" model than to assert that Clements' element of "reaction"-- the factor immediately succeeding "competition" (Weaver and Clements, 1938, Chapter IX and taken from Clements, 1916a, Plant Succession)-- was inoperative in desert vegetation, Clements' "complex organism".
By way of grand conclusion Shreve wrote that "the succession concept would never have been developed in a study of the vegetation of an arid region" (Shrev, 1942, p. 204). Now Shreve had taken his conclusions almost to the point of appearing to having a personal feud with Clements. The wonder is that both men still had a social relationship that was "apparently cordial but superficial" (Bowers, 1988, p. 60). Of course Shreve was sincere and, as shown in later years, correct to a degree. For example, the concept of cyclic succession in the Chihuhuan Desert is today textbook fare (Barbour et al, 1999, ps. 274-275, an example of which was presented herein under Chihuhuan Desert, Shrublands). Emphasis should be laid upon the fact that such vegetation dynamics is interpreted as a form of succession. This again underscores Bowers (1988, p. 88) observation that currently there are "looser definitions" for succession than in the dogmatic days of Shreve and Clements.
It must also be emphasized that Shreve was the ultimate descriptive ecologist. Bowers (1988, p. 86) demonstrated that Shreve appeared to have had an aversion to quantitative methods like quadrants and transects and vegetation mensuration in general. Yet, "he painstakingly measured a variety of physiological factors". "This bias may have arisen in reaction to Clement's advocacy of qunatitative sampling" (Bowers, 1988, p. 86). Here was another irony: it was Clements' invention of the quadrant and his "quantitative method" that decades later is regarded as one of his most lasting contributions to Plant Ecology.
Beyond this, some of Shreve's meticulous measurements of abiotic factors later came into question as to their actual relevance to plant life. Bowers (1988, p. 105) cited correspondence in which Rexford Daubenmire pointed out that Shreve's herculean efforts required to measure simple soil moisture were "completely wasted" because Shreve had not also determined wilting point and field capacity of sampled soil. A defender of Shreve at cross-examination might establish that Daubenmire was a modified Clementsian and product of the Anglo-American Tradition. The other side would point out that this fact was a "rabbit trail".
As to Clements' views, it is a fact that Clements had much less experience than Shreve in the study of desert vegetation. Recall from earlier in this historical note that Clements worked at the Desert Laboratory (in winter) and at the Alpine Laboratory on Pike's Peak (in the summer), and this routine for only eight years. Shreve lived in and studied deserts for over 30 years. Numerous workers have made much of the fact that Clements' ideas of plant succession were formed from his initial work with subhumid to semiarid grasslands and that he brought this experience (with the biases that always come from previous research) when he described "succession" in the desert. The same could be noted of Shreve whose early work was with vegetation in humid Maryland and, later, in the mountain rainforest of Jamaica. Shreve reached the same conclusions about vegetation being a continuum along environmental gradients from his Jamaican studies as from later investigations in the Sonoran Desert. Perhaps both Shreve and Clements over-reacted to the aridity of the desert.
Clements realized that vegetational dynamics in the desert was different from that of mesic and hydric habitats. In Plant Indicators he specified: "No quantitative studies have been made of the actual or potential succession in the desert scrub climax" (Clements, 1920, p. 174). Clements (1920, p. 174) continued in the next paragraph: "The type of succession is peculiar to the desert". Of course, as was explained above, Shreve concluded that there was no such thing as succession in the desert and "climax" was a meaningless concept for desert vegetation (and probably all other vegetation though he stopped short of that, at least "officially").
Clements (1920, p. 174) concluded that Larrea mexicana (= L. tridentata) was the regional dominant and "the Larrea plain is to be regarded as the threefold baseline for topography, climate, and succession, toward which all the others are tending slowly but nevertheless surely". Here in one sentence was the essence of the Clementsian monoclimax in "full-flower". It was the baseline concept of Major John Wesley Powell upon which William Morris Davis based his theory of climatic control of landforms upon which Clements based his theoretical climax formation that would exist when the land surface was eroded down to a peneplain and there was one regional mature vegetation, his climax as a "complex organism". Forrest Shreve rejected this Clementsian paradigm.
In Clements' interpretation the Sonoran Desert was the western desert scrub with the Larrea-Franseria Association (Franseria dumosa= Ambrosia dumosa) while the Chihuhuan Desert was the eastern desert scrub with the Larrea-Flourensia Association. It must be pointed out that Clements (1920, ps. 170-177 ) did not include in his discussion all those units or parts (vegetational subdivisions) that Shreve later mapped and included as Sonoran Desert and that are currently accepted as parts or units (Shreve's vegetational subdivisions) of the Sonoran Desert. Clements (1920) did not describe and discuss those parts existing in Mexico such as Baja California. As such, Clements' coverage extended to only two of Shreve's seven subdivisions (Shreve in Shreve and Wiggins, 1964, ps. 6, 48-49). One of these was the Lower Colorado Valley which Shreve delinated as the Larrea-Franseria Region. This was obviously Clements' Larrea-Franseria Association which was the "Larrea plain" that was the baselevel peneplain (ie. the monoclimax, the regional climax).
The other of Shreve's seven subdivisions that fell within the area covered by Clements (1920) was the Arizona Upland that Shreve named the Cercidium-Opuntia Region. This corresponded closely with Clements' Parkinsonia-Fouquiera community (Parkinsonia microphylla= Cercidium microphylla) between which and the Larrea-Franseria Association there was a Larrea-Parkinsonia ecotone. "The dominants of the foothills and the upper bajadas approach Larrea in requirements... Here the sequence is more difficult to determine because of the irregualr topography and the confusing effect of temperature [Yes, Clements not Shreve wrote this. But Shreve wrote almost exactly the same thing in his version.] ... it may be said the Fouquiera stands nearest Larrea, Cereus [= Carnegia] comes next, and Parkinsonia micarophylla is last. ... Parkinsonia mixes with Larrea in scores of places..." (Clements, 1920, ps. 174-175).
Inescapable conclusion: for all of the personal (though hidden) antimosity and jealousy that seemed to exist between Shreve and Clements and regardless of the different styles of explanation and "oceans-apart" views on formal classification of vegetation, succession, role of competition and reactions, etc. between these two highly respected ecologists, they came up with about the same vegetational units at the scale that corresponds to vegetation cover (= dominance) types.
In his map of potential natural vegetation A.W. Kuchler (1964) used the Shreve units: K-36 (Creosote Bush-Bur Sage, Larrea-Franseria) and K-37 (Palo Verde-Cactus Shrub, Cercidium- Opuntia). The Society for Range Management (1994) used these same units for rangeland cover types: SRM 506 (Creosotebush-Bursage) and SRM 507 (Palo Verde-Cactus). To further illustrate the consistency among these various workers the Chihuhuan Desert was designated as SRM 508 (Creosotebush-Tarbush), K-53 (Trans Pecos Shrub Savanna), and Chihuhuan Desert Scrub by Brown and Lowe with the U. S. Forest Service (later published as Brown, 1994). For the Sonoran Desert (designated Sonoran Desert Scrub) Brown and Lowe (in Brown, 1994) used the seven subdivisions of Shreve when they mapped the greater southwest region of North America.
The Sonoran Desert as interpreted by Shreve (Shreve and Wiggins, 1964, p. 6, 48-49):
1. Microphyllous desert Larrea-Franseria region Lower Colorado Valley
2. Crassicaulescent desert Cercidium-Opuntia region Arizona Upland
3. Arbosuffrutescent desert Olneya-Encelia region Plains of Sonora
4. Arborescent desert Acacia-Prosopis region Foothills of Sorora
5. Sarcocaulescent desert Bursera-Jatropha region Central Gulf Coast
6. Sarcophyllous desert Agave-Franseria region Vizcaino Region
7. Arbocrassicaulescent desert Lysiloma-Machaerocereus region Magdalena Region
Shreve's life-forms were applied to and described those of the physiognomically dominant species. Examples of meanings of life-form terms included: sarcocaulescent= succulent or other forms of thick (pachycaul) trunks (stems), crassicaulescent= crassi (thick) aboveground stems, microphyllous= small leaves, and sarcophyllous= succulent or thick leaves.
Bowers (1988, ps. 130-135) summarized Shreve's seven vegetational subdivisions in which she cited recent ecologists who felt that only three of these subdivisions were actually desert: Lower Colorado Valley, Central Gulf Coast and portions of the Vizcaino Region. The other subdivisions (thereby including the Arizona Upland) were better described as "depauperate thornscrub" in the view of some workers. Even Shreve's treatment of the Sonoran Desert (the subdivisions are among his best known and most lasting contributions) was perhaps colored by his relationship with Clements (Bowers, 1988, ps. 134-135). Shreve's much greater and longer experience with deserts enabled him to provide a more precise and complex meaning of desert than that of Clements based on the latter's limited desert interlude. Bowers (1988, p. 60) wrote that "it is possible that Clements inadvertently compelled Shreve to define his position on competition, vegetation classification, succession, and so forth, particularly in relation to deserts".
This is but another example of the monumental impact of Frederic Clements. Even Bowers' biography of Forrest Shreve is partly a story (partial biography) of Clements. [It should probably be remarked in passing that Bowers (1988) appeared to your current author to have a deeper overall understanding of Clements and the Clementsian paradigm than did Tobey (1981). Bowers (1988) certaily did not review the entire career of Clements but neither did Bowers (1988) make major erroneous conclusions the way Tobey (1981) did. Students were again referred to the Literture Review hererin.]
At any rate, there was another bit of irony (and hypocrisy if truth be told) in Shreve's still-used vegetational subdivisions of the Sonoran Desert. Not only did Shreve designate large-scale units of vegetation that were very similar (identical at some points) to the more superficial units (eg. associations and consociations) of Clements, Shreve also wound up using-- in one of his most familar and important additions to desert vegetation-- a similar form of elaborate jargon and formal titles of plant communities that he critized (always indirectly) Clements for! With Shreve's seven Sonoran Desert subdivisions Clementsian coinages like prisere, subsere, clisere, subclimax, disclimax, preclimax, postclimax, ecesis, aggregation, etc. and Larrea-Franseria association or Parkinsonia-Fouquiera community were joined by the Shrevian coinages of microphyllous desert (Larrea-Franseria region), crassicaulescent desert (Cercidium-Opuntia region), ... and arbocrassicaulescent desert (Lysiloma-Machaerocereus region). Even the term "region" had a parallel in the elaborate Clementsian system in which Shreve took "very liittle interest": "The climax formation is thus a product of reaction operating with ithe limits of the climatic factors of the region concerned". From the perspective of life-forms Clements' climax was the "mature" regional vegetation "... which terminates in the highest life-form possible..." (Clements, 1916a, p. 125).
The tenet of the orthodox Clementsian "dynamic vegetation" (succession-climax) model that Shreve appearently found most objectionable was the "genetic relation" of one climax to another (Shreve in Shreve and Wiggins, 1964, p. 29). An example would be Clements' interpretation that the Cercidium-Opuntia region (large-scale community where these two genera were physiognomic dominants) or Carnegia-Encelia community was a postclimax to the Larrea-Franseria region (= Larrea-Franseria association) as shown in Weaver and Clements (1938, p. 536) or when in Plant Indicators Clements (1920, p. 174) interpreted the Larrea plain as the climax to which all Sonoran Desert vegetation was "tending slowly but nevertheless surely". Such a generalized conceptual view (a unifying theory for land forms, soil formation, and vegetation development) of "dynamic vegetation" at geologic time scale and regional spatial scale did not change the fact that both Clements and Shreve described almost the same plant communities for current physiographically distinct vegetation (eg. that of bajadas vs. valleys or Colorado Valley vs. Arizona Upland).
Aside from differences in interpretation of vegetation based on it's development (Clements's abstract theory or, perhaps more aptly, philosophy), Shreve's view of desert vegetation that distinguished him from strict Clementsians was his individualistic (vs. Clements' organismic) concept of plant communities as a continua along environmental gradients. In a particluarly important passage (quoted above) Shreve wrote that "[e]ach habitat in each subdivision has its own climax" (republished as Shreve in Shreve and Wiggins, 1964, p. 29). This was, of course, Shreve's continuum concept of vegetation (the individualistc view at which he beat Gleason to the punch), but it was (or could be interpreted as) Shreve's equivalent of the polyclimax theory of Tansley or the climax pattern theory of Whittaker.
With that one statement enclosed within a larger sentence (first wrritten in 1942) Shreve stated the range (= habitat) site concept. It was explained in the Literature Review (Range Site) that the range site unit proved acceptance of the Tansley polyclimax and rejection of the Clementsian monoclimax concept, but there was an interesting twist in this. Management of vegetation at the range site scale was probably first stated in the scientific literature by Dyksterhuis (1949), but the basis of this and the quantitative use of certain species to determine range condition had it's origin in the grazing type and plant indicator concepts stated by Clements (1920) in Plant Indicators.
In writing that "[e]ach habitat in each subdivision has it own climax" Shreve (in Shreve and Wiggins, 1964, p. 29) was speaking of Sonoran Desert vegetation within the vegetational subdivision level (ie. at the individual habitat scale-- habitat or range site-- "inside" the larger spatial scale of subdivision). The larger, the subdivision, scale was the only scale at which Clements in Plant Indicators (Clements, 1920, p. 174-175) described Sonoran Desert vegetation, and (as mentioned above) he did this for only two of the seven Sonoran Desert vegetational subdivisions. At this scale (Shreve's vegetational subdivision and Clements' climax) there was not a meaningful difference between the vegetation described and named by either ecologist in his classification system.
There was a certain humor in this "holier than thou" skirmish over sanctity of terms and "real world" fit of concepts. It was rather reminiscent of the event of building the first transcontential railroad in which for some distance both the Union Pacific and Central Pacific laid parallel tracks (rather than joining up) in order to capitalize on land grants and other financial bonanzas. In the case of Shreve and Clements there was a publication bonanza but no act of Congress that forced a Promontory Point meeting of rails. Instead the tracks of Vegetation Science ran asymptomatically parallel. They still do.
In the following presentation, Sonoran Desert vegetation was shown at varying spatial scales ranging from microsite to range site (scale of soil series or association and local physiography) up to scale approaching that of landscape units. These varying scales of the greater Sonoran Desert ecosystem were described in the context of range cover types (and the spatial scale thereof) to stay within the subject of this publication. The photographs were used to represent range dominance types as mapped and/or described by various authorities and to explain various things about Sonoran Desert range ecology. The photographs were not intended to depict representative examples of smaller ecological or vegetational units like range sites or microsites although some of these units of scale were obvious in the photographs. Different spatial scales, aspects, slopes, etc. were easily distinguised in some slides (eg. upper bajada, lower bajada, and valley or north slope and south slope within the overall basin and range physiography). Differences in corresponding vegetation were often dramatic.
If Sonoran Desert vegetation is a continuum along environmental gradients as Shreve-- the undisputed definitive Sonoran authority-- argued, it could be further argued that units of vegetation at smaller scales (eg. range site) are largely meaningless (ie. each is a unique assemblage of species determined primarily by a unique combination of abiotic factors existing along any number of intersecting gradients). At the scale of range cover (= dominance) type there was almost unanimity-- certainly close agreement-- among the published sources.
For vegetation units within Shreve's vegetational subdivisions, the series (Brown, 1994) were used (eg.Creosotebush-White Bursage series and Saltbush series in Lower Colorado River Valley subdivision; Paloverde-Cacti-Mixed Scrub series and Jojoba-Mixed Scrub series in Arizona Upland subdivision). The Brown (1994) series of the Mojave Desert (Mohave Desertscrub) were also used. For some examples, the range sites of Humphrey (1960) were also shown (when it could be determined that these range sites applied).
Given the inherently arbitrary distinction of deserts (especially the border between the Mojave and Sonoran) and the often confusing designation of a "Colorado Desert" (Jaeager, 1957, p. 85) some examples of vegetation were included under both Mojave and Colorado Deserts and Sonoran Desert in this publication. Readers are free to draw their own "property lines".
1.Sonoran Desert— Saguaro (Carnegiea gigantea= Cereus giganteus ), jojoba (Simmondsia chinensis), Palo-verde (Parkinsonia = Cercidium spp.), ironwood (Olneya tesota) and bursage (Ambrosia deltoidea, A. dumosa = Fanseria deltoidea, F. dumosa) with very little herbaceous understory. Like the Chihuhuan, the Sanoran Desert is a hot arid scrub type. Cochise County, Arizona. December. FRES No. 30 (Desert Shrub Ecosystem). K-37 (Paloverde-Cactus Shrub). SRM 507 (PaloVerde-Cactus).
2.Sonoran Desert— Shown here is the basin and range physiography and the dominance of the succulent life form. In addition to the species in the previous scene plants here include beavertail prickleypear (Opuntia engelmannii) and one of the teddy bear or jumping chollas (perhaps O. fulgida). Creosote bush is also a component of this desert but it is not as exclusive a dominant as in the Chihuhuan Desert. FRES No. 30 (Desert Shrub Ecosystem). K-37 (Paloverde-Cactus Shrub). SRM 507 (Palo Verde-Cactus)..
3.Sonoran Desert— Composite shot of both the typical arborescent shrub and succulent components. Velvet mesquite (Prosopis juliflora) that is infested with mistletoe (Phoradendron californicum) with a lower layer of barrel cactus (Ferocactus spp., Mammillaria spp.) and numerous composites.Grasses are relatively rare in the communities shown, but there are scattered clumps of bush muhly (Muhlenbergia porteri) and threeawns, especially red threeawn (Aristida longiseta). Cochise County, Arizona. December. FRES No. 30 (Desert Shrub Ecosystem). K-37 (Paloverde-Cactus Shrub). SRM 507 (Palo Verde-Cactus).
4. Jojoba or goatnut (Simmondsia chinensis)- This rather unique species has been placed in either of two families (Simmondsiaceae or Buxaceae) by different taxonomists. Judd (1962, p. 93) stated that "California jojoba is one of the most important browse plants in the Southwest". The Forest Service (1940, p. B148) rated jojoba as good to very good winter browse and fair summer browse for all livestock classes. Jojoba is monecious.
Sonoran Desert, Cochise County, Arizona. December.
5. Leaves of jojoba- Mojave Desert, Riverside County, California. June.
6. White brittlebush or incienso (Encelia farinosa)- This is another woody composite (and of the same tribe as the preceding species). The alternate, ash-gray leaves of this much-branched low shrub make identification of this desert denizen an easy matter. Colorado Desert. San Bernardino County, California. June.
7. Incienso or white brittlebush in bloom- The farmer "makes hay while the sun shines", but desert plants bloom "if and when it rains". Brittlebush was shown here blooming in the Sonoran Desert in winter (January). The general precipitation pattern of the Sonoran Desert is biseasonal with the major moist periods being winter and summer. (The Sonoran Desert is located between the Chihuhuan Desert which gets most of it's moisture in summer and the Great Basin Desert which gets most of it's precipitation in winter. The Sonoran Desert receives some moisture from the storm tracks and precipitation patterns of each of the neighboring deserts.)
Pinal County, Arizona. January.
8. Inflorescences of white brittlebush or incienso- White and yellow are the two predominant colors of the huge composite family. Brittlebush is readily identified (though not readily by it's flowers), but that is not generally the case for many of the composite forbs. It is common practice among rangemen and foresters when asked the identity of a blooming composite that they do not know to reply with the tongue-in-cheek answer, "Some DYC (or DWC)" (codifying Damned Yellow Composite or Damned White Composite). Viewers see a bright DYC blooming in winter in the Sonoran Desert.
Pinal County, Arizona. January.
9.Colorado Desert— Another hot desert but a much smaller one is named after the river draining into it. Here is the creosote bush-bursage (mostly Ambrosia dumosa) section of a scrub composed of both arborescent and succulent shrubs. The herbaceous understory is an intermingled stand of the naturalized Mediterranean red brome (Bromus rubra) and the native perennial big galleta (Hilaria rigida). Note the immense alluvial fan at foot of the mountain range. San Bernardino County, California. June. FRES No. 30 (Desert Shrub Ecosystem). K-36 (Creosotebush-Bursage). SRM 506 (Creosotebush-Bursage).
10.Colorado Desert scene with basin and range topography and desert pavement— Creosotebush and teddy bear or jumping cholla (Opuntia bigelovii). Joshua Tree National Monument, Pinto Basin, California. June. FRES No. 30 (Desert Shrub Ecosystem). K-36 (Creosotebush-Bursage). SRM 506 (Creosotebush-Bursage) variant.
11. Creosotebush-white bursage (= burroweed) flats in Colorado Desert- This desert basin was co-dominated by two of the most common scrub species of the Colorado Desert. These were featured in the immediate foreground: creosotebush (left) and bursage or burrow bush (right). This landscape was a good example of the physiography characteristic of the Basin and Range province (a basin in foreground; a mountain range in background). Herbaceous species were largely absent from this browse range. The dried herbage was red bromegrass.
The palatability of creosotebush to larger herbivores like ruminant livestock and wildlife and horses is zero "for all intents and purposes", but white bursage is often a valuable browse plant. Dayton, 1931, p. 154) cited observations that reported white burrow bush to be the most palatable shrub in the region for horses plus being nutritious to cattle and sheep. Stubbendieck et al. (1992) also stated that burrow bush was "preferred by horses and donkeys".
Pinto Basin, Joshua Tree National Monument, Riverside County, California. June. FRES No. 30 (Desert Shrub Ecosystem). K-36 (Creosotebush-Bursage). SRM 506 (Creosotebush-Bursage).
12. White bursage, burroweed, or burro bush (Ambrosia dumosa= Franseria dumosa= F. albicaulis)- A view of an individual plant of white bursage from the basin shown immediately above. Pinto Basin, Joshua Tree National Monument, Riverside County, California. June.
13. Leaves and the small fruits of white bursage- Dayton (1931, p. 154) reported that these small fruits were quite valuable for fattening range livestock. White bursage is a member of one of the largest tribes of the Compositae. There are many shrubby as well as hearbaceous composites on the ranges of North America, including all the deserts.
14. Desert wash vegetation- The Colorado Desert has the physiography of the general Basin and Range province of which it is one part. This landscape has pronounced dendritic drainages and such smaller land forms as alluvian fans, benches, arroyos (desert gullies), and stream beds. Many of the latter are ephemeral water courses that are dry much or most of the year. They have often been described as desert washes. Increased quantity of available water, even for brief periods, in these washes supports vegetation that differs from that of adjoining plant communities in species composition, structure, etc. These desert wash communities could be interpreted as riparian vegetation though some might argue that such a perspective is more imagination than interpretation. The general dominant of wash vegetation in the Colorado Desert is the leguminous shrub or small tree commonly known as smoketree (Dalea spinosa= Parosela spinosa). Common associate species include palo verde or green tree (Cerdicium floridum= Parkinsonia floridum), desert ironwood (Olneya tesota), and Fremont cottonwood (Populus fremontii).
Vegetation shown in this slide was a smoketree consociation, but jojoba (Simmondsia californica) as the associate species was also plentiful (by desert standards). Two jojoba plants can be seen here (the dark green, tall shrub in left center and the spreading shrub in far right background). Jojoba as a range browse plant was discussed in two time-honored USDA publications (Dayton, 1931, ps. 94-95; Forest Service, 1940, B148). The current author can personally attest to the value of jojoba as cover for California quail (Callipepla californicus). He "bird-dogged" a nice covey from one of the pictured jojoba plants back to the other. As seen in the photograph herbaceous species were absent from this wash.
Joshua Tree National Monument, Riverside County, California. June. FRES No. 30 (Desert Shrub Ecosystem).Subunit of K-36 (Creosotebush-Bursage). The water course variant or subdivision of SRM 211 (Creosote Bush Scrub).
15. Smoketree (Dalea spinosa= Parosela spinosa) growing in a desert wash. Joshua Tree National Monument, Riverside County California. June.
16. Inflorescences of smoketree- Smoketree is a papilionaceous legume, a member of the Papilionoideae subfamily. The species of this subfamily are often thought of as the "true" legumes because they are the ones with the papilionaceous flowers that are characterized by five petals: the largest and unpaired petal known as the banner or standard, two keels (partially fused), and two wings. Some taxonomists have concluded that this subfamily is the "real" bean or legume family (Leguminosae= Fabaceae) and that the other subfamilies (Mimosoideae) and Caesalpinioideae) should be elevated to the distinct and separate mimosa family (Mimosaceae) and senna or caesalpinia family (Caesalpiniaceae). There is general agreement that it is only members of the papilionaceous group that host nitrogen-fixing bacteria in sufficient populations to add much nitrogen to the soil. Thus papilionaceous legumes are more or less synonymous with nodulated legumes.
Both of these two slides were of the same tree. Joshua Tree National Monument, Riverside County, California. June.The Lower Colorado Valley is the largest and most "desert-like" subdivision of the Sonoran Desert. Creosotebush is the dominant over the vast desert "super-region" of North America from the Chihuhuan through the Great Basin Deserts. Creosotebush is especially singularly prominent over the Lower Colorado portion of the Sonoran Desert.
17. Creosotebush flat (exterior view)- Example of the "Larrea plain" described by Clements (1920, p. 174) and the intermont plains of the Lower Colorado Valley subdivision of Shreve (Shreve in Shreve and Wiggins, 1964, ps. 49-50, 57-58). This is a creosotebush consociation with essentially no white bursage present (author did not find a single plant of bursage). Understorey was dominated by red brome (Bromus rubens) with fluffgrass (Tridens pulchellus= Erioneuron pulchellum) an associate or local do-cominant. There were periodic patches of Arabian or Mediterranea grass (Schismus arabicus). Fluffgrass is a native perennial, but it's stolons and their daughter plants are of such short life as to appear annual-like. Red brome is one of the least productive and -palatable Bromus species (although it will be eaten when young). Arabian grass was described by Hitchcock and Chase (1950) as "an excellent forage grass in winter".
The community shown here was obviously winter range, with the usefulness of that varying from year-to-year depending primarily on winter moisture. In describing creosotebush communities like the one shown here Humphrey (1960, p. 55) remarked that unless they "... may support a crop of winter or summer annuals in season, they could not even be classed as range".
This range community was part of the "Mojave Desert-Sonoran Desert transition", a segment of the Lower Colorado River Valley subdivision that was part of the "tenuous border with the Mojave Desert" (Brown, 1994, p. 190). It could logically be included with the Mojave Desert and, in point of fact, vegetation shown here made a case for the less common, more informal, and less accepted classification of this part of the southwestern desert region as the "Colorado Desert". Brown (1994, p. 157) remarked that the Colorado Desert was "Shreve's Lower Colorado subdivision of the Sonoran Desert". Kuchler (1966) designated this vegetation in lower California as unit #35, Creosote bush (Larrea), in contrast to unit #36, Creosote Bush- Bur Sage (Larrea-Franseria), farther east into Arizona.
Readers attention was directed to the large nest of California harvester ants (Pogonomyrmex claifornicus) in left center foreground. The other mounds were locations of former creosotebush plants (one shown closer in succeeding slide).
Vegetation in this photograph was in an intermont basin of the Sonoran Desert section of the Basin and Range physiographic province, specifically a bolson which is an undrained basin (Fenneman, 1931, ps. 357-377).
Bureau of Land Management, San Bernardino County, California. June. FRES No. 30 (Desert Shrub Ecosystem). K-35 (Creosote bush). SRM 211 (Creosote Bush Scrub). Brown (1994, p. 162-163) included this as the Creosotebush series of Mohave Desertscrub while the scrub community co-dominated by creosotebush and white bursage was designated as the Creosotebush-White Bursage series of the Sonoran Desertscrub. The Kuchler (1966) vegetation map did not delinate deserts so the Mojave and Sonoran were not differentiated.
18. Interior of a creosotebush desert flat- Two views inside the creosotebush scrub range community presented in the immediately preceding slide. These slides showed the regular or uniform dispersion pattern typical of many arid shrublands-- the deserts or desert scrub-- (see Barbour and Major, 1995, ps. 840-841, 875). The understorey was a single herbaceous layer of annual red brome and perennial fluffgrass with periodic patches of the annual Arabian or Mediterranean grass. Both annual grasses were Eurasian natives of anthropogenic introduction. The grasses and a few other species of winter annuals or spring ephemerals had already disintegrated under action of dessicating winds and herbivory (recall harvester ant colony in first slide and note the deer trails in these two photographs: right midground in first slide and central midground of second slide).
The first slide included a mound with remaining woody stems where a creosotebush once grew. This showed the ring growth pattern or form of this sometimes clonal species. Some creosotebush clonal rings have been dated to be several thousand years of age with some rings having several satellite shrubs per clone making distinction of individual "plants" difficult. (Barbour and Major, 1995, p. 837-838). Creosotebush is obviously a xerophyte but it actually has many mesophytic-appearing features (eg. it has C3 metabolism; Barbour et al., 1999, ps. 426, 550 and Barbour and Major, 1995, ps. 881-882). The major adaptation of creosotebush to desert habitats appears to be ability of it's protoplasm to withstand desiccation (Barbour et al., 1999, p. 628).
Bureau of Land Management, San Bernardino County, California. June. FRES N0. 30 (Desert Shrub Ecosystem). K-35 (Creosotebush). SRM 211 (Creosote Bush Scrub). Brown (1994, p. 162-163) designated this as Creosotebush series of Mohave Desertscrub. Distinct from the dominance type with creosotebush and white bursage as co-dominats.
19. Ground-level of creosotebush scrub community- The grass-dominated herb layer (now largely disintegrated) was beneath a creosotebush shrub layer. Except for the sometimes-present cryptogamic layer this vegetation had a simple two-layer sructure. The creosotebush had a clonal ring growth pattern, but this was not as well-defined as in some cases (eg. mound in first slide above preceding caption).
This site was in the Sonoran Desert section of the Basin and Range physiographic province and could be interpreted as either part of the Lower Colorado subdivision of the Sonoran Desert (ie. "Colorado Desert") or as a part of the Mojave Desert. It was best described as the Creosotebush Section of the Mohave Desertscrub (Brown, 1994, ps. 162-163), but it was typical of much of the Sonoran Desert as well, delinations being as mentioned arbitrary.
Bureau of Land Management, San Bernardino County, California. June. FRES No. 30 (Desert Shrub Ecosystem). K-35 (Creosotebush). SRM 211 (Creosote Bush Scrub).
20. Desert saltbush (= allscale)-creosotebush range in Lower Colorado Valley- In what is probably the most comprehensive treatment of vegetation in southwestern North America, Brown and Turner (in Brown, 1994, ps. 194-197) recognized and described a Saltbush series of the Lower Colorado River Valley vegetational lsubdivision of the Sonoran Desert. This was consistent with early (and still outstanding) work by Humphrey (1960, p. 54) who placed both desert saltbush and creosotebush range types as well as "paloverde-triangleleaf bursage sites" (Humphrey's use of "site") and "desert wash" in the "southern desert shrub" (Humphrey, 1960, ps. 52-59)..
Note: It was explained above that recent workers in Brown (1994) added details and subunits to the traditionally recognized deserts (and other biomes in southwestern North America). The subunits were designated as "series". Series was a hierarchial level within a digitized classification syustem for biotic communities (Brown, 1994, ps. 303-315 passim). The series unit (Brown, 1994, p. 306) was taken from several sources including the Daubenmire "climax series" based on "major dominants in climax communities" and also the "series' or "cover types" going back to the Society of American Foresters (1954) as well as the natural vegetation approach of Franklin and Dryness (1973). Obviously series is either the traditional or slightly modified dominance type and therefore corresponds closely to forest and rangeland cover types. Application of the "climax series" (and along with biotic communities [= biomes]) to vegetation of the Sonoran Desert was yet another irony to the Clements-Shreve argument discussed in the introduction to the Sonoran Desert. And again, as was more often the case than not, Shreve lost out. By using climax dominance types in a system built on the "biome approach" (Brown, 1994, p. 8-12) the individualistic concept of communities introduced by Shreve and Gleason was, once again, either ignored or made to "suck hind tit" to the organismic concept of Clements. "The biome as a complex organism" (Clements, 1939, ps. 21-25) and biome, formation, and climax were synonyms "when used in the biotic sense" (Clements, 1939, p. 20) vs. "It is not possible to use the term 'climax' with reference to desert vegetation" (Shreve in Shreve and Wiggins, 1964, p. 29).
In their treatment of the Sonoran Desert, Turner and Brown (in Brown, 1994, ps. 193-200) included five series within the Lower Colorado Valley vegetational subdivision of Forrest Shreve. The only one of these series that received much recognition by those who previously mapped and described Sonoran Desert vegetation was the creosotebush-bursage type. The saltbush series refers to desert saltbush that is also known as allscale, cow lettuce, and cattle spinach (Atriplex polycarpa). Desert saltbush was described as "highly palatable" for livestock and big-game and "excellent cover and nesting habitat for quail" by Sampson and Jespersen (1963, p. 66). Humphrey (1960, p. 54) described desert saltbush range and concluded that desert saltbush was "the most palatable and valuable forage plant in most of the area where it grows" (Humphrey, 1960, p. 114). Given the value of this dominant range plant and it's dominance as a "climax series" the saltbush series should be accorded a separate SRM rangeland cover type designation in future publications (ie. Desert Saltbush-Creosotebush).
In the range community shown in this and the next photograph creosotebush was the associate species overall and co-dominant locally (in patches). Western honey mesquite (Prosopis glandulosa var. torreyana= P. juliflora var. torreyana) was also present (eg. left center).
Relict area except as understorey modified by annual exotics. Maricopa County, Arizona. Early estival aspect, June. FRES No. 30 (Desert Shrub Ecosystem). K-35 (Creosotebush) is closest thing but not the same. Likewise closest rangeland cover type was variant of SRM 211 (Creosote Bush Serub) or perhaps of SRM 506 (Creosotebush-Bursage); neither one was descriptive or precise enough. Desert Saltbush (allscale) series of Lower Colorado River Valley subdivision of Sonoran Desertscrub (Brown, 1994).
21. Allscale-creosotebush form of Sonoran Desert scrub- At local scale of this photograph allscale or desert saltbush and creosotebush shared dominance (the latter was at least a strong associate) on this basin within the Lower Colorado Valley of the Sonoran Desert. This was the climax vegetation and an outstanding desert browse range. It also afforded excellent habitat for the California quail (Callipepla californicus).
The herbaceous understorey was dominated by red brome, but cheatgrass (Bromus tectorum) and Arabian grass were well represented. The one conspicuous forb was London rocket (Sisymbrium irio).These Eurasian annuals were not part of the climax, but-- as is the case for the California annual grassland-- they should be regarded as part of the now-potential vegetation (though naturalized and not natural). It was not clear from the literature to what extent native annual and ephemeral species were components of the virgin vegetation, or even if vegetation of the Sonoran Desert (and other deserts) was studied before Eurasian species that had been introduced from the onset of Spanish exploration were already naturalized. From a Range Management perspective, annual herbaceous species generally provide some feed for range animals and afford some soil protection on intershrub spaces. To the extent that these naturalized Mediterranean plants do not in a substantiative way adversely impact range ecosystem structure and function (eg. do not reduce populations or cover of climax and valuable species like allscale, accelerate soil erosion, or induce range retrogression measured as downward range trend) they are either more-or-less "neutral" or somewhat beneficial. Eurasian annuals did not appear to have the major competition and reaction roles of similar species in the shrublands of the Great Basin Desert or adjoining bunchgrass-shrub steppe.
Turner and Brown (in Brown, 1994, p. 194) concluded that the Desert Saltbush series formerly occupied a total area second only to the Creosotebush-White Bursage series. The desert saltbush "climax series" or cover (= dominance) type was the most widespread community in the Gila Valley of Arizona and Coachella Valley of California before it was converted to field crops. Turner and Brown (in Brown, 1994, Figure 119 caption) summarized: "Few unaltered stands of this community remain: most have become irrigated cropland and the rest have been disturbed by livestock with the resulting establishment of such exotic annual species as Mediterranean grass (Schismus arabicus and S. barbatus), red brome (Bromus rubens), and London rocket (Sisymbrium irio)". Humphrey (1960, p. 54) wrote: "Stands of this shrub [desert saltbush] were originally much more widespread in the Salt River Valley on lands that are now under irrigation". "Trails Plowed Under", Charlie M. Russell.
The degree to which entry of Eurasian annual weeds altered the range vegetation presented here was not known. Neither was it known whether the disturbance-induced alternation of the vegetation was beneficial or detrimental when it was used as a grazing (= range) type.
Maricopa County, Arizona. Early estival aspect, June. FRES No. 30 (Desert Shrub Ecosystem). K-36 (Creosotebush-Bursage). Variant of SRM 201 (Creosote Bush Scrub) or perhaps 506 (Creosotebush-Bursage); neither of these was adequate. There should be an SRM rangeland cover type consistent with the desert saltbush type of Humphrey (1960, p. 54) and the Desert Saltbush series of Brown (1994, ps. 194-197): Desert Saltbush-Creosotebush. Saltbush (Desert) series of Lower Colorado River Valley subdivision of Sonoran Desertscrub (Brown, 1994).
22. Interior of desert saltbush range- Desert saltbush (also allscale, cattle spinach) is an extremely valuable browse plant in the Lower Colorado Valley of the Sonoran Desert (the part often designated the Colorado Desert). Allscale is a climax species of the Sonoran Desert and deserves wise use management to insure it's protection and continued existence on desert grazing lands. Humphrey (1960, p. 114) stated: "Desert saltbush is one of the better browse species in the dsert shrub of southern Arizona. Although, in general, it is the most palatable and valuable forage plant in most of the area where it grows, the arid climate results in a low carrying capacity for these ranges."
Most Atriplex species are dioecious. The two sexes of allscale were easily distinguishable in this slide: female plant at full bloom in left midground; rest are male plants.
Relict area in Maricopa County, Arizona. June, early estival aspect. FRES No. 30 (Desert Shrub Ecosystem). K-35 (Creosotebush), but at mapping scale so large as to include another smaller unit of potential natural vegetation. Variant of SRM 201 (Creosote Bush Scrub) or perhaps of SRM 506 (Creosotebush-Bursage), but these are not really correct. Should be an SRM rangeland cover type of Desert Saltbush-Creosotebush. Recognized as a range type by Humphrey (1960, p. 54). Desert Saltbush series of Lower Colorado River Valley subdivision of Sonoran Desertscrub (Brown, 1994).
23. Female plant of desert saltbush or allscale in full bloom- Allscale is one of the most prized browse plants of the Sonoran and Mojave Deserts. Desert saltbush is a climax shrub and physiogonmic dominant of a desert scrub community of the Lower Colorado Valley subdivision of the Sonoran Desert that is second in size to the more commonly described creosotebush and creosotebush-white bursage cover types.
This attractive lady was doing "her thing" during the worst drought (severe to exceptional) in history of Arizona weather records. The native range species will survive and leave their genetic legacy in progeny if given a chance. This specimen was protected in a relict area. Maricopa County, Arizona. June.
24. Flowering leader of a female allscale or desert saltbush (Atriplex polycarpa)- Female inflorescence and leaves on branch of desert saltbush-- in midst of worst drought in Arizona since territorial days. Drought is probably the most severe of all disturbances affecting rangeland (excepting the impacts of meteors of which Arizona has also had it's allocation) and still this native shrub was sexually reproducing.
25. Vegetation on upper bajada of Lower Colorado River Valley subdivision of Sonoran Desert- Shreve (in Shreve and Wiggins, 1964, ps. 61-63) explained that ascent in elevation from lower bajada to upper bajada within the Lower Colorado Valley subdivision was accompanied by the addition of foothill paloverde (Cercidium microphyllum) or blue paloverde (C. floridum); appearance of giant saguaro (Carnegia gigantea); and increases of ocotillo or coachwhip (Fouquieria splendens), cane or snake cholla (Opuntia echinocarpa= Cylindropuntia echinocarpa), and California barrel cactus (Ferocactus acanthodes= Echinocactus acanthodes) with a commensurate relative decrease in dominance by creosotebush and white bursage (Franseria dumosa= Ambrosia dumosa). Brown (1994, p. 194) gave an example of an ecotone between the Creosotebush-White Bursage series of the Lower Colorado River Valley subdivision and the Paloverde-Cacti-Mixed Shrub series of the Arizona Upland subdivision.
Those descriptions fit the vegetation shown here except that triangleleaf bursage (Ambrosia deltoides= Franseria deltoides) was an associate of creosotebush in a lower shrub layer that formed a general middle layer of vegetation. Foothill paloverde comprised the upper shrub layer which with creosotebush and bursage formed a general middle layer of vegetation. The lower or herbaceous layer had already disentagrated but it was dominated by the Mediteraanean annual grass, red brome. Saguaro comprised the tree layer.
Pinal County, Arizona. June. FRES No. 30 (Desert Shrub Ecosystem). K-36 (Creosotebush- Bursage). Arborescent variant of SRM 506 (Creosotebush-Bursage).
26. Transition Sonoran Desert scrub- Ecotonal vegetation between Lower Colorado River Valley subdivision and Arizona Uplant subdivision of Sonoran Desert. This general range plant community was described by Shreve (in Shreve and Wiggins, 1965, ps. 63-64) as an upper bajada in the Lower Colorado Valley subdivision. Foothill paloverde formed a line at footslope of the hills while creosotebush dominated the lower slope of this bajada above a typical desert basin. The herbaceous understorey was dominated by the Eurasian annual grass, red brome. There were traces of cheatgrass and also (though even less) of Arabian grass.
The leafless, smaller, and less abundant shrubs were individuals of triangleleaf bursage (ie. an associate to creosotebush). This is the typical drought-deciduous condition, and the vegetation was suffereing through a drought rated as severe to extraordinary. Bursage was more obvious in the second of these two photographs (lower left-hand side).
Pinal County, Arizona. June. FRES No. 30 (Desert Shrub Ecosystem). K-36 (Creosotebush-Bursage). Arborescent variant of 506 (Creosotebush-Bursage).
27. Interior of range vegetation on an upper bajada of Lower Colorado Valley subdivision of Sonoran Desert- "Close-up look" of a transition between Lower Colorado Valley and Arizona Upland subdivisions of Sonoran Desert. This vegetation was prominently zonational as well as transitional. A creosotebush consociation (with triangleleaf bursage as associate) that was featured prominently in the foreground gave way to another consociation-- an exclusive community dominated by a single species-- of foothill paloverde, immediately higher in elevation (farther up the hills). Red brome comprised a nearly "pure" herbaceous understorey at lower levels of this bajada. Any winter or ephemeral spring forbs had long since disentegrated. Rangeland vegetation of the composition described by Shreve (in Shreve and Wiggins, 1964, ps. 63-64) as upper bajada of Lower Colorado Valley of Sonoran Desert.
Note: the geologic term bajada was and still is used quite frequently in descriptions of the Sonoran Desert. Much of this usage appears to trace back to Forrest Shreve, Dr. Sonoran Desert. Allaby (1998) defined bajada: "An extensive gently sloping plain of unconsolidated rock debris resting against the foot of a mountain front in a semi-arid environment. Typically it is made up of a number of coalescing alluvial fans laid down by ephemeral streams as their gradients lessen on leaving the mountain zone. Material is also supplied by the weathering of the mountain front.".
The Arizona Upland is the second largest vegetational subdivision of the Sonoran Desert. It is one of the best developed-- most complex and diverse-- vegetation-wise and is probably the best-known due to it's picturesque species especially cactus like the saguaro. This is largely a function of more annual precipitation, more favorable distribution of precipitation, and generally less xeric soil conditions. Due to location of the Carnegie Desert Botanical Laboratory within the Tucson Mountains (at which Forrest Shreve and other pioneer desert ecologists were headquartered) the Arizona Upland is likely the best studied of the Sonoran Desert subdivisions.
The photographs presented below were of Arizona Upland vegetation in the Tucson Mountains of the Sonoran Desert section of the Basin and Range physiographic province. Photographs were often arranged in sets of "photo-quadrants" to show details of the vegetation in local aggregations or groups of species and different age classes. This arrangement seemed to the author to be consistent with Shreve's individualistic view of vegetation as a continuum along habitat gradients. Alternatively it could be seen as providing "snapshots" of a diverse climax vegetation arranged along range sites and microhabitats (ie. a polyclimax perspective).
Topography ("the lay of the land") was one of the major or predominant abiotic factors determining features of scrub vegetation that was recognized by pioneer desert ecologists. Shreve called this "physiographic control of vegetation": "So great is the importance to desert plants of the texture, depth, and surface character of the soil that in every area of adequate size there are differences in the vegetation which are closely correlated with these conditions. The same relations of vegetation to soil are presented over and over again in hundreds of valleys and intermont plains. There are hills or mountain slopes with rock in place and pockets of soil, there are pediments, upper bajadas with shallow, coarse soil, lower bajadas or plains with deeper soil of uniform but rather coarse texture, and flood plains or playas with deep fine soil. The physiographic features of the basin determine the physical character and distribution of the soils, which in turn determine the vegetation." (Shreve in Shreve and Wiggins, 1964, p. 37).
That quote by Shreve could certainly be interpreted by Gleasonians as reflecting the individualistic view of plant communities, but it could just as readily be viewed as being the Tansian polyclimax concept that was used by Clementsians like Dyksterhuis (1949) to blend Tansley's polyclimax with the basic Clementsian paradigm of grazing types and indicator plants (Clements, 1920) to formulate the range site concept. It was also consistent with the climax pattern concept later proposed by Whittaker (1953) that was more specific and amounted to a refined blend of the Clementsian and Tansian concepts."The same relations of vegetation to soil are presented over and over again in hundreds of valleys and intermont plains" rings "over and over again" of a pattern of range sites arrayed along catenas of repeating patterns of soil series. Either way, photographs were arranged below on basis of physiographic location, in particular as to lower bajada, upper bajada, and hilltop.
28. Various views of lower bajada portions of the Arizona Upland of the Sonoran Desert- As indicated by it's designation, the Arizona Upland is found above the lower parts of the Basin and Range province with a distribution along the higher mountain ranges. There are basins, bajadas, and mountains within the Arizona Upland vegetational subdivision and this topographic variation contributes to amazing diversity in physiognomy, structure, composition, and development of the vegetation in it. Differences in habitat (soils, soil water, temperature, wind currents, steepness, aspect, etc.) due to topographic features are pronounced even between the lower and upper bajada. Hence this has been a common basis for distinction of Arizona Upland vegetation. A good summary of such is Crosswhite and Crosswhite (in Bender, 1982, ps. 217-221).
In this series of four slides (and many of the succeeding photographs) differences in vegetation due to location on bajada and aspect (eg. north slope vs. south slope) as well as those reflecting edaphic features (eg. sandy vs. gravelly soils) were often pronounced-- even striking-- and, as described by Shreve in numerous publications, within relatively short distances (over small spatial scale).Vegetation in these photographs was on a lower bajada above the Santa Cruz River Valley. Soils were generally better developed with more favorable moisture conditions.
Creosotebush was present in each of these "photo-plots" with this regional dominant being conspicuous in the first and fourth photographs. Creosotebush plants were typically smaller than those in the more xeric Lower Colorado Valley subdivision. Saguaro were ever-present, and at younger age classes indicating good regeneration. This was featured prominently in the first two "quadrants" of this set. In the first photograph ocotillo (still retaining it's dry season-deciduous leaves in a severe drought) was growing at far-right foreground. Vegetation presented in the second photograph had more triangleleaf bursage than cresotebush. Most of this bursage had shed it's drought-deciduous leaves, but several plants had retained their leaves. These appeared as short plants with light gray leaves (left-center of photograph). The cactus featured in the second photograph (center foreground and scattered throughout) and in the center of the third photograph was desert pricklypear (Opuntia phaeacantha var. discatha= O. discantha).
The third slide of this foursome showed the diversity of cacti in this lower bajada scrub community. Desert botanists and ecologists like Shreve distinguished between the cylindropuntias and the platyopuntias of the Cactaceae. The former were given the taxonomic rank of genus by some taxonomists (eg. Cylindropuntia fulgida for chain-fruit cholla; Cylindropuntia ramosissima for branched pencil cholla). Cylindropuntia was later put ("lumped") back in Opuntia (eg. Opuntia fulgida; Opuntia ramossissima). Shreve used these two nouns, which at the time of his studies were used to designate "sections" of Opuntia, as "life forms" (eg. in Shreve and Wiggins, 1964, p. 45). In this third photograph the platyopuntia life form was represented by desert pricklypear and the cylindropuntia was represented by chain-fruit cholla (left foreground). The saguaro represented the columnar type of life form according to Shreve's scheme (Shreve in Shreve and Wiggins, 1964, ps. 42-44). The barrel type was not visible in the third photograph, but was presented elsewhere as represented by the genus Ferocactus. All of Shreve's cactus life forms were included in his general life form of stem succulents (Shreve and Wiggins, 1964, p. 44). The smaller, leafless, and drab-appearing shrubs scattered throughout fore- and midground was triangleleaf bursage, an "always there" dominant species.
The range vegetation portrayed in the fourth slide included plenty of creosotebush, but ocotillo and triangleleaf bursage were also present along with foothill paloverde (see midground) and saguaro, the other physiognomic dominants.
Viewers should make special note of the lack of an herbaceous layer, and the absence of herbaceous species even as individual plants. If annual or ephemeral species had been present they had disintegrated and vanished. ("For the wind passeth over it, and it is gone; and the place thereof shall know it no more." Psalms 103:16 KJV.)
Saguaro National Park, Pima County, Arizona. June, early estival aspect (awaiting summer monsoon). FRES No. 30. K-37 (Paloverde-Cactus Shrub). SRM 507 (Palo Verde-Cactus). Paloverde-Cactus-Mixed Scrub series of Arizona Upland subdivision, Sonoran Desertscrub (Brown, 1994).
29. Presented below were photographs showing species composition, life form, and structure of range vegetation on lower bajadas in the Paloverde-Cactus-Mixed Scrub series of Arizona Upland subdivision, Sonoran Desertscrub (Brown, 1994). FRES No. 30. K-37 (Paloverde-Cactus Shrub). SRM 507 (Palo Verde-Cactus). Deep-Soil Palo-Verde--Triangle Bur-Sage Range Site (Humphrey, 1960, p. 56).These All "photo-plots" were on Saguaro National Park, Pima County, Arizona. Late vernal aspect, June.
29a. Diversity of species and life forms in Arizona Upland of Sonoran Desert- Creostoebush, the regional defining dominant, was present in abundance throughout this "quadrant". Shreve (Shreve and Wiggins, 1964, p. 42, 46) described the life form of creosotebush as evergreen (perennial leaves), true (woody, nonsucculent) shrub. Old individuals of saguaro, the columnar type life form of stem succulents, marked the "plot" with picturesque distinction. Saguaro were joined with foothill paloverde (the defining dominant of the Arizona Upland vegetational subdivision), the individual tree of which was featured in far-left foreground and was parasitized by desert mistletoe (this parasitic species was featured below). Paloverde was placed in the life form of trees and shrubs with green stems and small drought-deciduous leaves by Shreve (Shreve and Wiggins, 1964, p, 42-43, 47). Chain-fruit cholla, the cylindropuntia life form of stem succulents, was visible in front of the paloverde; immediately to the right (center foreground) was a fishhook or candy barrel cactus (Ferocactus wislizenii = Echinocactus wislizenii), the barrel type life form of stem succulents. Short, multi-stemmed, leafless shrubs were individuals of triangleleaf bursage whose life form Shreve (Shreve and Wiggins, 1964, p. 45) described as nonsucculent, woody, leafy perennial.No herbaceous species were present.
29b. Diverse species compostion of Arizona Upland vegetational subdivision- The scrub vegetation of this "photo-plot" on a lower bajada was similar to that in the preceding photograph. Fishhook barrel cactus (right foreground), creosotebush (foreground), chainfruit cholla, saguaro, foothill paloverde (one immediately to left of foremost saguaro), and triangleleaf bursage (left foreground in front of cholla) were all present. Note that the soil was sandy but free of gravel. The small brown patches on the otherwise bare soil was remnants of red brome, the most common naturalized (from Eurasia) annual grass in this area.
29c. "Calendar quality" example of structure and composition of Sonoran Desert (Arizona Upland) range scrub- The blooming matriarch saguaro was surrounded by "rings" of plant populations. The species immediately around the saguaro was triangleleaf bursage (and one creosotebush) while the "circle behind that of bursage was chair-fruit cholla. The crown of a foothill paloverde was visible at right margin.
29d. "Synopsis view" of the Paloverde-Cactus-Mixed Scrub series of Arizona Upland, Sonoran Desert- On this lower bajada looking to hilltops foothill palover (left and right margins) joined saguaro to form an upper woody layer to this succulent scrub range. Paloverde on left was parasitized by desert mistletoe (Phoradendron californicum) apparently to point of losing the struggle. In front of this was the wooden remains of an otherwise decayed saguaro. Triangleleaf bursage formed a shrub layer in this range community to which a succulent layer was addeed by desert pricklypear.
29e. Succulent, woody shrub, tree, and herbaceous (more-or-less) components of Arizona Upland Sonoran scrub- On this "photo-quadrant" along the lower bajada physiographic unit, species diversity of the Cactaceae was obvious (as was basis for the common designation of the Sonoran Desert as a form of succulent scrub). Desert pricklypear (left foreground and right-center midground), chain-fruit cholla (right margin midground), saguaro (including a "youngster" in front of background desert pricklypear), and a cholla variety of the species Opuntia acanthocarpa represented the Cactaceae. Foothill paloverde (left margin and background) was a large shrub and of the life form of a true (nonsucculent) woody plant with green-barked stems and drought-deciduous small leaves (Shreve in Shreve and Wiggins, 1964, ps. 42-43, 47). Creosotebush, the defining dominant of the Chihuhuan and Mojave as well as Sonoran Desert, was obvious (in fore- and midground). Soil typical of lower bajadas was visible. On it were scattered basal parts of red brome (all that remained of the most common annual herbaceous species, and that a Eurasian grass). An herbaceous layer was, nonetheless, part of this desert range.
29f. Exterior view of Arizona Upland Sonoran Desert range- Overall physiognomy of the Paloverde-Cactus-Mixed Scrub series on lower bajada. Foothill paloverde, saguaro, and triangleleaf bursage (short, leafless, multi-stemmed shrubs in foreground) were obviously the dominants of this portion of the desert community. Other cactus species were desert pricklypear and chainfruit cholla. Relatively few remnants of annual or ephemeral species (and those mostly red brome). The severe drought that plagued this area for four years was most likely a factor in scarcity of herbaceous species, but this area had been overgrazed for many years (more on that at end of this section) and, perhaps most important, apparently there were often very few herbaceous species in the understorey of virgin Sonoran Desert range, especially in the paloverde-traiangleleaf bursage type (or site) of the Arizona Upland (Shreve, 1951, ps. 11-12; Humphrey, 1960b, ps. 56, 57).
30. The following set of photographs were taken on bajadas just below hilltops, the crests of the mountain ranges. These "photo-plots" showed species composition, life form, and structure of range vegetation on the upper bajada of the Paloverde-Cactus-Mixed Scrub series of Arizona Upland vegetational subdivision, Sonoran Desert (Brown, 1994). FRES No. 30. K-37 (Paloverde-Cactus Shrub). SRM 507 (Palo Verde Cactus). Shallow Soil Palo-Verde--Triangle Bur-Sage Range Site (Humphrey, 1960, p. 57). All "photo-quadrants" were in Tucson Mountains on Saguaro National Monument, Pima County, Arizona (June).
Sonoran Desert scub on upper bajada in Tucson Mountains- Edaphic conditions become less favorable for plant life as soils of the valleys and lower bajada grade into those of the upper bajada and, finally, rocky mountain tops. Soils are generally less developed and have lower moisture contents in ascending the land from valley to hilltop. This is not an absolute however as soils vary in reponse to many factors. Perhaps more importantly, precipitation may increase with increasing elevation. Likewise, temperatures, especially nightly temperatures, are sometimes warmer at higher elevations than at lower elevations (ie. upper bajada vs. river valleys, basins, and even lower bajada). This is of utmost importance in distribution of frost-intolerant species like saguaro.
In some instances, there is but slight variation in desert vegetation due to compensating arrays of abiotic (even biotic) factors. That phenomenon of "variation within sameness" in range vegetation was shown in this set of photographs from the Tucson Mountains.
30a. Transect from upper bajada to hilltop- This was a classic mix of saguaro, foothill paloverde (line of green, several-stemmed shrubs at base of hill), desert prickly pear, and triangleleaf bursage (short, leafless, multi-stemmed shrubs in foreground). It was essentially the same "mixture" as that of the lower bajada presented in the immediately preceding slides. There was no herbaceous vegetation. If as Shreve stated (Shreve and Wiggins, 1964, p. 37), and quoted above, there are differences in soil due to physiographic differences why did the less xeric foothill paloverde and saguaro persist in such relative abundance at this elevation? Let's climb up and see.
30b. Greenness of foothill paloverde and saguaro- Inside the "green line" of paloverde seen in the preceding slide. At this distance the explanation for abundance of this green-barked legume and saguaro was clear. These species grew along a small wash (drainage as an ephemeral stream) that provided more moisture than surrounding land of the same elevation, slope, aspect, etc.This was an example of local conditions producing different vegetation in rather close proximity.
30c. Biodiversity in a dry land- The gravelly soil on this upper bajada supported a species-rich range community. (Surface texture of this soil should be compared to that of the lower bajada visible in the preceding set of photographs.). Several of Shreve's life forms were represented by cactus and other species : columnar type by saguaro, platyopuntia by desert pricklypear, shrubs with green-stems and small, drought-deciduous leaves by foothill paloverde, and nonsucclulent, woody, leafy perennial by triangleleaf bursage. There were several age classes of saguaro ranging from dead, bare-wood trunks to only-a-couple-of-feet-tall young ones (note the short, young saguaro in center foreground, and with log of an ancient, fallen "grandparent" adjacent to it). For all the species diversity there were no herbaceous species.
This spot was on an alluvial fan coming down out of the drainage from the hills above.
30d. More biodiversity in a dry land but a little different composition- This part of an upper bajada was another alluvial fan but with a soil distinctly different from that seen in the preceding slide. Another difference was addition of ocotillo to the plant community. Foothill paloverde and saguaro were still present as would be expected for dominant species. There were saguaros of various age classes (at least sizes, which presumedly correlated somewhat with age on the same site). Triangleleaf bursage was also present as a dominant (as was the case on the alluvial fan in the previous photograph). There were no herbaceous plants.
30e. Soils and slopes made the difference- In this pair of photographs there were two different soils and two different slope-aspects. The photograph on the left showed vegetation on a west-facing slope and with a coarse-textured, caliche-strewn soil. The photograph on the right showed vegetation on an east-facing slope and with a finner-textured soil. Vegetation on the left was dominated by creosotebush and triangleleaf bursage, but it also included chain-fruit cholla throughout with foothill paloverde and saguaro higher on the hill. Vegetation on the right included saguaro and footshil paloverde throughout, but also jojoba and ocotillo which were absent from vegetation on the west slope. Desert pricklypear was a major component on the east-facing slope with finner-textured soil in contrast to chain-fruit cholla on the west slope with gravelly soil. Photographs were taken on the same day and within four miles of each other. Absence of any herbaceous species was noteworthy.
Humphrey (1960, 57) observed that on what he designated as the "shallow-soil palo verde-triangle bur-sage range site" there is usually a greater diversity of plant species on shallow soils that "contain appreciable amounts of caliche". In this pair of photographs was an example of the range site concept and illustration of the fact that species composition and structure of range plant communities vary within the larger spatial unit of range cover type.
30f. Old denizens of the desert scrub- The two physiognomic dominants of the Arizona Upland of the Sonoran Desert are foothill paloverde, the undisputed dominant species (and life form) of this vegetational subdivision, and giant saguaro, at least an aspect dominant (dominant from perspective of crown or upper vegetational layer) and the other dominant life form (columnar type of stem succulents).
This is a standard (and a classic) textbook example of the nurse plant syndrome (eg. Barbour et al., 1999, ps. 153-155). Paloverde (at this location in the Tucson Mountains, Cercidium microphyllum), the dominant species, is the most important nurse plant species for saguaro. This has traditionally been interpreted as a form of commensalism (an interaction that is beneficial for one "party": saguaro in this example) that might turn into competition (presumedly for soil moisture) as the cactus grows larger. Recall from an earlier quote that Shreve minimized the role of competition (Shreve and Wiggins, 1964, 27).
31. The Paloverde-Cactus-Mixed Scrub is the largest (most area) series in the Arizona Upland, but the Jojoba-Mixed Scrub series is another vegetational unit of this subdivision (Brown, 1994, 202-203). Given the sparse herbaceous understorey of the Arizona Upland and the value of jojoba as browse this unit (series) was featured next. Jojoba does best where precipitation (especially winter precipitation) is greater than over most of the Sonoran region. As such, jojoba is most common at higher elevations (ie. it is more abundant on the upper than on the lower bajadas) and closer to the Arizona chaparral at northern extremes .
Examples of Jojoba-Mixed Scrub series (Brown, 1994). FRES No. 30 (Desert Shrub Ecosystem). K-37 (Paloverde-Cactus Shrub). Variant of SRM 507 (Palo Verde-Cactus).
31a. Jojoba joined-up with more typical Sonoran scrub species- Foothill paloverde (the subdivisional dominan), major species of cactus (predominantly desert pricklypear and chain-fruit cholla along with regenerating, young saguaro), triangleleaf bursage, and ocotillo that are common throughout the Arizona Upland were joined by jojoba to provide this example of the Jojoba-Mixed Scrub series. Shreve (Shreve and Wiggins, 1964 ps. 42, 46) regarded jojoba and creosotebush as in the same life form that he described as evergreen (perennial leaves), true (nonsucculent; woody) shrub. Creosotebush was absent from the local range community shown here. Jojoba apparently partially took it's place. This was another example of slightly different vegetation due to local variations in environments. Shreve interpreted this as individual assemblages of plants along a vegetational continuum that corresponded to environmental gradients.
Saguaro National Park, Pima County, Arizona. Late vernal aspect, June.
31b. Jojoba on Sonoran Desert range- Jojoba was shown at two locations and two microsites on the upper bajada landform of the Arizona Upland vegetational subdivision. In the first photograph the Jojoba-Mixed Scrub series (Brown, 1994) was on the rocky soil of a fairly level, benchlike microsite. In the second photograph the Jojoba-Mixed Scrub series was on a wash (ie. wash vegetation) and at higher elevation than that of the first range community. Careful scruity revealed that jojoba grew in close proximity to ocotillo on both microsites. Saguaro, foothill paloverde, and cactus species were common to both sites. A microsite is a local site, the smallest recognizable unit of habitat. The environment or "microclimate" immediately around each individual plant is that plant's microsite, but the boundaries of this microhabitat are never clear-cut. This is particularily true in instances where plants are not in an obviously "self-contained" microenvironment (eg. not under shade of a larger plant or confined to the crevice of a sheltered rock).
The unit of microsite is obviously smaller and more restricted than a range site. The range site is the smallest unit of habitat at the range plant community level. Range sites have historically been identified, described, and delineated in conjuction with the soil mapping unit (usually soil series or soil association). One such range site(unnamed and not described) was shown in the first photograph. The second photograph included at least two range sites: 1) that of the wash and 2) that of the hilltop. The range site is the smallest unit or spatial size of range that has the potential to produce a range plant community that is different from all others (ie. the smallest, most distinctive taxonomic-like unit of range).
The immediate question to be answered was, "How different to these plant communities have to be to be different?' In the second slide the wash vegetation had jojoba as a key member whereas the hilltop vegetation had much less jojoba. Ocotillo was present in the wash vegetation and absent from that of the hilltop range site. The composition (species proportions or percentages based on relative abundance such as foliar cover) of foothill paloverde and saguro appeared to quite similar on both range sites.
The more fundamental question to be answered was, "Were any differences that existed in vegetation due to repeatable, predictable units of habitat (eg. range site) or were vegetational differences just unique groups of species and ages of plants occurring along a continuum and brought together by environmental gradients and, maybe, chance?" Shreves and Gleasonians would argue for the latter view while Clementsians would attest to the former. Both camps could agree (to a degree) with the microsite concept.
Saguaro National Park, Pima County, Arizona. Late vernal aspect, June.
31c. Floristic richness of the Arizona Upland- This botanically diverse range vegetation was primarily in the Arizona Upland vegetational subdivision of the Sonoran Desert, but it was ecotonal between that unit, the Lower Colorado Valley subdivision, semidesert grassland, and some species or features of the Arizona chaparral. All four of these vegetational units are in the Basin and Range physiographic province.The latter two are in the Mexican Highland section while the first two are in the Sonoran Desert section of this province. Obviously vegetation does not correspond directly with either physiographic, edaphic, climatic, or biotic features singularly but with all collectively and synergistically.
The transitional vegetation of this range was dominated by foothill paloverde and saguaro, the dominants of the Arizona Upland (at least paloverde is the subdivisional dominant with saguaro-- along with other cacti-- more typically an associate ), but jojoba (bush with entire crown visible in left foreground), velvet mesquite (Prosopis juliflora var. velutina= P. glandulosa var. velutina; to left and right-rear of jojoba), and creosotebush were also "major players" in the plant community. Other shrubs included wait-a-minute bush (Mimosa biucifera) and skunkbush sumac (Rhus trilobata= R. aromatica). Annual grasses were locally abundant in the understorey and included red brome, Mediterranean grass (Schismus barbatus), wild oats (Avena fatua), and Orcutt lovegrass (Eragrostis orcuttiana). Paloverde were "loaded" with fruit.
This ecotone was in the Jojoba-Mixed Scrub series. Presence of jojoba here at upper elevational limits of the Sonoran Desert and where it was ecotonal with Arizona chaparral was consistent with the description by Brown (1994, p. 203).
Maricopa County, Arizona. Late vernal aspect, June.
31d. Desert friends- The three defining species or physiognomic dominants of the Jojoba-Mixed Scrub series: jojoba, foothill paloverde, and saguaro. Saguaro National Park, Pima County, Arizona. Late vernal aspect, June.
32. Wash vegetation in Arizona Upland subdivision of Sonoran desert- On this austere landscape on the Mexican Highland section of the Basin and Range province the range vegetation differed from that of the Tucson Mountains presented above. While foothill paloverde was dominant and saguaro was abundant there were several species that were plentiful at this location which are less common throughout most of the Arizona Upland subdivision of the Sonoran Desert. These included Mormon tea or joint-fir (Ephedra trifurca), the tall, darker green plants in center foreground, and rubber rabbitbrush (Chrysothamnus nauseosus), green plant immediately to left of Ephedra.
Yavapai County, Arizona. June. FRES No. 30 (Desert Shrub Ecosystem). K-37 (Paloverde-Cactus Shrub). Variant of SRM 507 (Palo Verde-Cactus). Paloverde-Cactus-Mixed Shrub series (Brown, 1994).
These photographs of range vegetation in the Arizona Upland vegetational subdivision of the Sonoran Desert should end where they began by quoting Shreve: "From the floristic viewpoint the Arizona Upland may well be designated the Cercidium-Opuntia region, as these two genera play a more important part in its vegetation than do any others (except Larrea) and are more important here than in any other subdivision of the Sonoran Desert. It is from the important role of the succulents, including Carnegia, that the Arizona Upland is called crassicaluescent, or stem-succulent, desert." (Shreve and Wiggins, 1964, ps. 50-51).
Presented below were several range plant species that are of importance in the Sonoran Desert due to dominance, widespread distribution, value as indicator species, use as forage or browse plants, scientific interest, aesthetics, etc.
33. Creosotebush (Larrea tridentata= L. divaricata)- If there was one overall dominant species of the Sonoran Desert it would unquestionably be creosotebush. It is the defining dominant of the Lower Colorado Valley (= "Colorado Desert") and Arizona Upland vegetational subdivisions of the Sonoran Desert (Shreve in Shreve and Wiggins, 1964, ps. 49-50). "Larrea is one of the most abundant and widespread shrubs of the Sonoran Desert, and it is equally prominent in the vegetation of the Chihuhuan Desert, the Mojave Desert, and a small part of the Great Basin Desert" (Shreve in Shreve and Wiggins, 1964. p. 165). Creosotebush is the quientessential range plant of the North American deserts.
Creosotebush is also called "greasewood" (especially in west Texas), but this is very confusing and as close to an "incorrect" common name as is possible because greasewood is the preferred common name for Sarcobatus vermiculatus.
The individual plant displayed here was of the Sonoran Desert ecotype (Brown, 1994, ps. 172-173). It was growing as an associate on the Desert Saltbush series of the Lower Colorado Valley vegetational subdivision. Maricopa County, Arizona. June
34. Leader of Sonoran Desert creosotebush- Leaves, fruit, and bark of the Sonoran Desert ecotypic form of creosotebush. Maricopa County, Arizona. June.
35. Foothill or littleleaf paloverde (Cercidium microphyllum)- The physiognomic domiant of the Arizona Upland subdivision of the Sonoran Desert. Creosotebush is the overall (as in "averaged across") dominant of the Lower Colorado Valley and Arizona Upland vegetational subdivisions of the Sonoran Desert (ie. the regional dominant), but foothill paloverde is "king of the mountain" within the latter subdivision. It's larger size and dominance of the upper vegetational layer of the Arizona Upland define littleleaf paloverde as the dominant. This member of the mimosa subfamily of the Leguminosae has green-barked stems and is drought-deciduous where "drought" is generic for dry soil conditions and includes "seasonal droughts" (climatically dry seasons). Littleleaf paloverde differs from the perennial leaf (evergreen-leaf) feature of creosotebush and jojoba. Shreve (in Shreve and Wiggins, 1964, ps. 42-43, 46, 47) placed these three species along with ocotillo in the woody shrubs and trees life form category of his life form dichotomous key, but he distinguished paloverde and ocotillo from jojoba and creosotebush on basis of drought-deciduous leaves versus perennial (= evergreen) leaves, respectively.
The green-bark and dry season-shedding of leaves is an obvious adaptation that allows photosynthesis with minimal water loss via transpiration under dry edaphic and atmospheric conditions. The green stem and drought-deciduous leaf combination was shown in the photograph of the foothill paloverde leader at right.
Saguaro National Park, Pima County, Arizona. June (severe to exceptional drought and before onset of typical summer monsoonal rains).
36. Littleleaf or foothill paloverde in "full-fruit"- These examples show the legume or pulse kind of fruit that distinguishes all legumes. There are three Fabaceae (= Leguminisae) subfamilies only one of which has the five-petal papilionaceous corolla, but all have the legume fruit. The compound leaves of the two plants shown had been shed recently as the seasonally dry spring advanced (accompanied with a severe to exceptional drought).
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
37. Giant saguaro (Carnegia gigantea= Cereus gigantea)- The one plant species of the Sonoran Desert that captures the popular imagination and captivates the attention of desert tourists and that is universally accepted as the icon of the Sonoran Desert (if not that vague designation of "desert Southwest") is the giant saguaro. "Save the Saguaro" joins "Save the Redwoods" (and the "Whales", the "Condors", and the "Wolves"). Scientifically speaking creosotebush, the regional dominant, and littleleaf paloverde, Arizona Upland dominant, "outrank" the saguaro in their ecological role and influence on structure and function of Sonoran Desert vegetation and ecosystem, but the lay public is anything and everything else but scientific. It could only be Saguaro National Park. Paloverde National Park is every bit as poetic (if not more so), but that would not not pass muster. When a plant is chosen to commenrate the robber baron whose wealth underwrote (at least for a while) the Carnegie Desert Botanical Laboratory and whose likeness adorned the title page of monographs by sagurao-sized authors like Shreve and Clements it could only by the sentinel-appearing, "guardian of the desert", biggest cactus in these United States of America.
The saguaro does serve as a subject by which to teach some fundamental lessons in desert biology, including ecological concepts. In that context-- not popular culture or Walt Disney and public television personification of the plant-- this series of photographs was presented for those readers choosing to study further the harsh but intriguing Sonoran Desert ecosystem.
Immense specimens of saguaro like the magnificant, blooming matriarch shown here are old by desert plant standards. This one was probably well into it's second century of life. It is popularly thought that the first "arms" (limbs or branches) of the saguaro typically do not appear until the plant is about 75 years old. Size is variable however and does not correrlate closely with age. Saguaro are usually slow-growing, adding only about 10 cm per year to the trunk. A good concise introduction to biology of the saguaro is in Shreve and Wiggins (1964, ps. 147-148).
Saguaro National Park, Pima County, Arizona. June.
38. Upper trunk of saguaro showing the branching pattern of old cactuses- The saguaro is the ultimate expression of the stem succulent (columnar style) life form of Shreve (in Shreve and Wiggins, 1964, ps. 42-44). Secondary branches are rare and always small. Apical dominance is obviously quite strong in the columnar forms of the cactii. Leaves of the saguaro are interpreted as having been reduced to spines as adaptation via natural selection to water scarcity.
The fluted trunk is capable of swelling (and then shrinking) with storage of water (and it's subsequent use) by the plant. The fluted pattern of the trunk also provides shade to and reduces temperature of some of tissue. Shreve (in Shreve and Wiggins, 1964, 147) estimated that 75-95% of the green weight of saguaro was water (which is no greater than in many common forages in the green-- live-- state). Note beginning of a new branch on the central stem in the second photograph.
Saguaro National Park, Pima County, Arizona. June.
39. Baby branches on saguaro- Branch buds in early stages of development were forming the first branches off the trunk of a saguaro. Maricopa County, Arizona. June.
40. Woody tissue of saguaro- Saguaro-- like all, at least most, Cactaceae members-- does form woody material. This is, however, covered by nonwoody suculent tissue while the woody tissue surrounds a fiberous pith (vertical slide on right). Shreve (in Shreve and Wiggins, 1964, ps. 42-45) characterized the stem succulent life form as succulent and distinguished this from the nonsucculent woody perennials of typical shrubs and tress. Harlow et al. (1979, p. 424) included the Cactaceae in their standard Dendrology text in which they described cactus as "woody plants" some of which "produce tree forms" having columnar, fluted, and fleshy stems.The Society of American Foresters (Helms, 1998) defined wood so as to include material produced by "trees and other woody plants".
The first photograph was of the weathered, partly decayed trunk and fallen limbs of a saguaro. Loss of succulent tissue left the woody tisssue intact. Obviously there is enough woody material in saguaro to portray the entire morphology of the plant following decay of succulent or fleshy tissue.
The second (from left) photograph was of a saguaro that appeared to the photographer to have been struck by lightening, frequent bolts and flashes of which commonly acompany life-giving (and life-taking) summer thundershowers. Inside the woody tissue of the shattered trunk was the wood-like pith in which substantial quantities of water were stored. Advocates of prescribed fire on range (and fire enthusiasts in general) should take delight in evidence of natural sources of ignition on this rangeland cover type. Hot desert breezes provide adequate oxygen, but whether there is adequate fuel to complete the fire triangle is situation-specific. Apparently fuel was limited in the case covered by the photograph., Fire-loving rangemen (like your author) should that even one-third of the equation resulted in browse. The shrub by the lightening-struck saguaro was jojoba. Hurray!
Steenbergh and Lowe (1977, ps. 30, 189) mentioned lightening kill of saguaro although this was a less common cause of death than that by catastrophic freezes.
Third photograph was the woody trunk base of a saguaro. A piece of the saguaro epidermal tissue with spines (cactus "hide and hair") was in front of the woody trunk. Viewers make note of all the organic matter (detritus) on the desert floor, and absence of the same from herbaceous species..
Saguaro National Park, Pima County, Arizona. June.
41. Sex in the saguaro- In the worst drought in history of Arizona weather records this survivor was in the process of producing genetic progeny and passing on it's heritage of deoxyribonucleic acid (DNA) to future generations. Individual saguaro flowers are solitary but appear close together on the apices of shoots (including branches). The green, oblong structures (with dead, brown tissue at their tips) were young fruit (and their recent flowers). Details of the cactus flower and fruit were described immediately below.
Saguaro National Park, Pima County, Arizona. June.
42. Sexual reproduction in saguaro, State Flower of Arizona- Sexually reproductive apex of saguaro branch with ripe fruit recently eaten by dove (covered below) and flower newly opened. The saguaro inflorescence has a hypanthium (a saucer- or cup-shaped expanded tip of the floral axis that produces flower organs like sepals, petals, and stamens from its upper margins). Saguaro is in the Cereoideae (= Cacteae) subfamily of Cctaceae whose members have the hypanthium and rudimentary leaves but lack glochids (the tiny hairs or bristles in the aerole). The many stamen arranged in a circular were readily seen in the second photograph.
Saguaro National Park, Pima County, Arizona. June.
43. Unripe saguaro fruit- This single immature fruit topped the single stem of a young saguaro. It contained numerous seeds (see below). Maricopa County, Arizona. June.
44. Ripe and unripe saguaro fruit- When the saguaro fruits ripen they become a striking, dark-rose color. Early explorers could see the red pigmentation (but not further details) atop the tall shoots so they mistakedly assumed and recorded that saguaro flowers were red in color. (Why they could not find flowers and fruits neaar the ground as did this photographer must remain a mystery.) The white-winged dove (Zenaida asiatica) can readily distinguish mature fruit and helps itself to the bounty. The rose-colored fruit with the notched or V-shaped indentation (left-center fruit) was fed on by a white-winged dove.
Maricopa County, Arizona. June.
45. White-winged dove feeding on saguaro fruit at dusk- Most desert animals are active in the cooler times in the hot Sonoran Desert. White-winged dove feed actively at dawn and dusk. When saguaro fruits ripen dove have a feast. One was helping herself to rare desert bounty. Maricopa County, Arizona. June.
46. Trash left over from a dove picnic- Saguaro fruit detached and knocked to ground by feeding of white-winged dove. Saguaro National Park, Pima County, Arizona. June.
47. White-winged dove was cupid- Close-up view of saguaro fruit that had been eaten and knocked off the cactus by dove. Feeding by dove allowed saguaro seed to come into contact with the desert soil. In helping themselves to the sexual (vs. asexual or vegetative) offspring of the saguaro white-winged dove play an essential role in completing the reproductive process and passing on the cactus' DNA. Dove become the sowers of seed. The process is actually not that simple. Dove also make the seed (imbedded in the "core" of the fruit) more available to granivores (seed-eating herbivores) like ants which remove the seed embryo and store the germless seeds (Shreve and Wiggins, 1964, p. 147). Other herbivores like rodents and coyotes (Canis latrans) eat the fruity core or pulp and disperse the seed. Species of birds that do not grind the seed aerially disperse them to the ground (most of which is bare and a ready made seedbed). What a soap opera!
Saguaro National Park, Pima County, Arizona.
48. Windthrow in the desert- "Real" trees are not the only plants subject to the phenomenon of windthrow, windfall, or blowdown (trees felled or broken off by wind [Helms, 1998]). Shreve (in Shreve and Wiggins, 1964, p. 149) made the following observation: "The principal natural cause of injury [of saguaro] is the breaking of branches by high winds. The broken surface heals quickly in dry weather, but in a rainy period it usually leads to infection and the rapid death of the entire plant. The principal cause of death is overturning by wind or by the removal of soil and stones from the base of the tree, or because of the softening of the soil by prolonged rain." [Editorial note: in this passage Shreve viewed saguaro-- at least one with branches-- as a "tree".]
Fortunately for the casualty shown here, it was the dry season (and a severe drought) so this injured patriarch likely lived to tell the tale. Fortunately for the photographer, the aeolian defoliation took place a few moments before he and his Nikon arrived on the scene (more fortunately a second branch did not crash as he daringly recorded the Kodak moment for your continuing education.)
Saguaro National Park, Pima County, Arizona. June.
49. Fishhook or candy barrel cactus (Ferocactus wislizenii= Echinocactus wislizenii)- Shreve (in Shreve and Wiggins, 1964, 42-43, 44) classified this species as in the barrel type of stem succulent (unbranched), but Shreve did not accord this species much coverage by dent of it's limited role, abundance, etc. in the Sonoran Desert. Like saguaro, however, fishhook barrel cactus has long been regarded as an indicator of the Arizona Upland vegetation subdivision. It is also one of the more "popular" plants with biped desert visitors. Ferocactus species are placed in the Cereoideae subfamily of Cactaceae along with most other species except for those in the Opuntioideae subfamily.
Cause of the leaning barrel of the larger individual shown here was unknown, but likely due to erosion of soil in some torrential downpour or movement due to top-heavy plant weight in wet soil (obviously a most infrequent condition). Refer to discussion of this situation by Shreve and Wiggins (1964, p. 149) as quoted in immediately preceding caption.
Dead-looking ("looks are deceiving") shrubs all around these cactuses were triangleleaf bursage.
Saguaro National Park, Pima County, Arizona. June.
50. Exquisite barrel pattern- Detail of the fishhook and other spines on the surface of a shoot of fishhook barrel cactus. The "thorns" afford not only armor against herbivores but provide shade and insulation against cool desert nights and hot desert days. Saguaro National Park. June.
51. Coville barrel cactus (Ferocactus emoryi= Echinocactus emoryi= E. covillei)- Another well-known and widely distributed barrel cactus. Two individual plants. Pima County, Arizona. June.
Specimens of the following seven species of Ferocactus (= Echinocactus) are native to southern and western protions of the Sonoran Desert. They were growing in the cactus garden of the Arizona-Sonoran Desert Museum, Pima County, Arizona. Under these somewhat horticultural (at minimum, transplanted) conditions ground materials surrounding the cactuses could not be regarded as part of the species natural habitat. They were included to present viewers with a taste of the biodiversity within just one genus of cactus.
52. California fire barrel cactus (F. cylindriaceus)-
53. Biznaga (F diguetii)- Native to Baja California.
54. Biznaga (F. johnstonianus)- Native to Baja California.
55. Baja fire barrel cactus (F. gracilis var. or subsp. gracilis)- Native to Baja California.
56. Biznaga (F. gatesii= F. gracilis subsp. gatesii)- Native to Baja California.
57. Twisted barrel cactus (F. herraerae)- Native to western Sonora, Mexico.
58. Chain-fruit cholla (Opuntia fulgida)- Chain-fruit cholla is one of the dominant Opuntia species in the Arizona Upland vegetational subdivision. It is often associated with foothill (= littleleaf) paloverde and saguaro with which it forms a middle shrub layer between their upper or arborescent layer and the lower shrub layer dominated by triangleleaf bursage.
Shreve (in Shreve and Wiggins, 1964, ps. 42-44, 45) labeled the cholla group of Opuntia species the cylindropuntia life form. At one time the cylindrical-segmented group of the Opuntioideae subfamily, the chollas, were placed in the genus Cylindropuntia.
Saguaro National Park, Pima County, Arizona. June.
59. "... thy breasts shall be as clusters of the vine..."--Song of Solomon 7:8 (KJV)- This may not have been exactly what King Solomon had in mind, but there is no doubt that these fruits of chain-fruit cholla make up a cluster. The cylindrically-segmented stem that supported the cluster was also shown. Saguaro National Park, Pima County, Arizona. June.
60. Chain-fruit cholla fruit and inflorescence- Greenhorns should not let the unique arrangement and attractive flower of this cholla species 'lure them in". Experienced cowhands know this as one of the "jumping chollas". The slightest brush or rub against the fruit or stem segments results in the organ of unwitting affection becoming attached to horse or rider. This is a very effective means of propagule (both sexual and asexual) dispersal but much to discomfort of man and beast. Furthermore, members of the Opuntioideae possess glochids, the smallest spines or pubescence in or around the aeroles, which are hardest to get out of skin.
Opuntioid cactus do not have a hypanthium. This can be seen by comparison of this Opuntia flower to that of the saguaro shown above. The inflorescence shown here had not completely opened. Saguaro National Park, Pima County, Arizona. June.
61. Wren house- Nest of cactus wren (Campylorhynchus brunneicapillus), State Bird of Arizona. Brown (1994, p. 203) remarked that the Paloverde-Cactus-Mixed Scrub series of the Arizona Upland subdivision was "particularily noted for its rich birdlife". To illustrate that fact the ill-kept, disheveled nest of the cactus wren was presented. The chain-fruit cholla is one of the preferred building locations of this species of arid avifauna.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
62. Cactus wren at home- This male cactus wren was guarding one of several nests (some are used for resting or, perhaps, to give his mate a choice of accomodations) in chain-fruit chollas. The cactus wren (at least the male) is a raucous, bad-mannered, ill-bred bird if there ever was one. He is as unruly as his nest. Even the male is a dull-colored bird, but there is never a dull moment while he is around to add a grating, raspy, racket to the desert aura.
Arizona-Sonoran Desert Museum, Pima County, Arizona.
63. Thurber cholla (Opuntia thurberi)- This is another species of Shreve's cylindropuntia life form. It is one of numerous species of cholla in the Sonoran Desert. The two examples shown here were both in the Paloverde-Cactus-Mixed Scrub series of the Arizona Upland vegetational subdivision where they added species diversity to a middle shrub layer. This stratum of vegetation was between the upper arborescent layer formed by littleleaf paloverede and saguaro and a lower shrub layer dominated by the halfshrub, triangleleaf bursage.
Saguaro National Park, Pima County, Arizona. June.
64. Maze of succulent stems- Branching pattern of Thurber cholla with immature fruit. Upper photograph, Saguaro National Park; lower photograph, Arizona-Sonora Desert Museum. June.
65. Desert pricklypear (Opuntia discata= O. phaeacantha var. discata)- Desert pricklypear is co-dominant with chain-fruit cholla in a middle layer of woody species between the arborescent layer of foothill paloverde and saguaro and a lower woody layer dominated by triangleleaf bursage. Shreve (in Shreve and Wiggins, 1964, ps. 42-44, 45) described the platyopuntias as having "flat joints of round, oval or phriform outline". He placed them in life form distinct from the cylindropuntias.
Saguaro National Park, Pima County Arizona. June.
66. Purple pricklypear (O. violacea)- One of many species, subspecies, and varieties of Opuntia in the Sonoran Desert. Maricopa County, Arizona. June.
67. Transition form of Sonoran Desert- Example of vegetation in an ecotone between the Lower Colorado Valley and Arizona Upland vegetational subdivisions was shown here. The local dominnant species was organ-pipe cactus (Lemaireocereus thruberi var. thurberi= Cereus thurberi= Stenocereus thurberi= Pilocereus thurberi, Marshallocereus thurberi; this thing has as many synonymns as stems).
Organpipe Cactus National Monument, Pima County, Arizona. June.
68. Organpipe cactus- This many-stemmed, columnar cactus is not a major species other than in a fairly restricted area around southwestern Arizona and down into Sonora. Organ-pipe National Monument was set aside primarily to preserve this rather novel cactus. Incidentally, "thereby hangs a tale". Most of the "crown jewel" National Parks and Monuments-- as viewed by the general public-- are those having spectacular scenery, unique geologic formations, or some novel feature. Thus there is Yosemite and it's mountains and valleys, Yellowstone with it's Grand Canyon of the Yellowstone and Old Faithful, Grand Tetons, Grand Canyon of the Colorado, even Devil's Postpile, but no Tallgrass or Great Plains National Parks. When there is a National Park or Monument set aside for a plant it is a photogenic, star-quality, megafloral species like redwood, saguaro, Joshua tree, or organ-pipe cactus. This political phenomenon is known as monumentalism, setting aside as "national treasures" those "natural wonders" that are monument-like (eg. specific mountains that in the public eye are natural equivalents of Mount Rushmore, or the Washington, Jeffrson, or Lincoln Memorials). If it is an endless sea of big bluestem and Indiangrass, plow it up and plant corn; if it is a cactus that resembles a pipe-organ in a catheral, desigate it a "national monument".
Cactus garden, Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
69. Organ-pipe cactus- This "critter" looks like a cross between a cylindropuntia, a cholla, and a saguaro. It has cylinderal segmented stems of the cholla but the more columnar and fluted stem of the nonjointed saguaro. Organ-pipe is in the Cereoideae subfamily of the Cactacea that includes the saguaro and barrel cactus. Cactus garden, Arizona-Sonoran Desert Museum. Pima County, Arizona. June.
This "sample" of succulents from only one family illustrted the familar designation of the Sonoran Desert as a "succulent desert" in contrast to the Chihuhuan, Mojave, and Great Basin Deserts that are more obviously dominated by the true nonsucculent (woody) plants or nonsucculent woody perennials (Shreve's life form discriptions: Shreve and Wiggins, 1964, ps. 42-43, 45). Even in the Sonoran Desert the nonsucculent woody species "embrace the plants of greatest abundance in the make-up of the vegetation as well as the majority of the members of the flora" (Shreve and Wiggins, 1964, p. 45). Other plants commonly regarded as succulents by both scientists and laymen include the Agave species (eg. A palmeri) that Shreve identified as having succulent leaves but which he called caudical plants not succulents per se. Shreve limited succulent plants primarily to the Cactaceae (Shreve in Shreve and Wiggins, 1964, p. 41-43). The common usage of "cactus and succulents" is thus inconsistent and confusing with regard to the Shreve system which -- along with about everything else "Shrevian"-- should be regarded as the definitive aujthority.
70. Jojoba or goatnut (Simmondsia chiensis)- Importance of this woody perennial shrub is due to it's dominance in the Jojoba-Mixed Scrub series of the Arizona Uplan and as a source of palatable browse. (This was explained periodically throughout this discussion of the Sonoran Desert.) Family designation of jojoba has been (and remains) a matter of debate with some authors placing it in the Buxaceae (box family) and others putting it in it's own family (Simmondsiaceae).
Jojoba is dioecious species as was discussed immediately below. The specimen shown here was a large male.
Saguaro National Park, Pima County, Arizona. June.
71. Details of male jojoba- Leader of male jojoba with leaves and small inflorescences. Saguaro National Park, Pima County, Arizona. June.
72. Details of female jojoba- Leader of female jojoba with leaves and lots of fruit. Saguaro National Park, Pima County, Arizona.
The Compositae or Asteraceae (sunflower or aster family) is one of the larget plant families in the world. One of the first lessons in a college course devoted to Range Plants is the importance of the composite family from the standpoint of 1) numbers of species of forbs and shrubs and 2) dominance of many range plant communities (including at the cover type level) by major species of composites. That lesson is as appropriate for range vegetation of the Sonoran Desert as for any other. Included below are some of the more common and/or important composites of the Sonoran Desert.
73. White brittlebush or incienso (Encelia farinosa)- This is another woody composite (and of the same tribe as the preceding species). The alternate, ash-gray leaves of this much-branched low shrub make identification of this desert denizen an easy matter. Colorado Desert. San Bernardino County, California. June.
74. Incienso or white brittlebush in bloom- The farmer "makes hay while the sun shines", but desert plants bloom "if and when it rains". Brittlebush was shown here blooming in the Sonoran Desert in winter (January). The general precipitation pattern of the Sonoran Desert is biseasonal with the major moist periods being winter and summer. (The Sonoran Desert is located between the Chihuhuan Desert which gets most of it's moisture in summer and the Great Basin Desert which gets most of it's precipitation in winter. The Sonoran Desert receives some moisture from the storm tracks and precipitation patterns of each of the neighboring deserts.)
Pinal County, Arizona. January.
75. Inflorescences of white brittlebush or incienso- White and yellow are the two predominant colors of the huge composite family. Brittlebush is readily identified (though not readily by it's flowers), but that is not generally the case for many of the composite forbs. It is common practice among rangemen and foresters when asked the identity of a blooming composite that they do not know to reply with the tongue-in-cheek answer, "Some DYC (or DWC)" (codifying Damned Yellow Composite or Damned White Composite). Viewers see a bright DYC blooming in winter in the Sonoran Desert.
Pinal County, Arizona. January.
76. Triangleleaf bursage (Ambrosia deltoidea= Franseria deltoidea)- Triangleleaf bursage is the most common composite on many of the bajadas of the Arizona Upland vegetational subdivision of the Sonoran Desert. It is the dominant-- often, almost the exclusive-- species of the lower shrub layer. Shreve (in Shreve and Wiggins, 1964, ps. 42, 45) put both triangleleaf and white bursages in his life form of nonsucculent woody perennial (also nonsucculent leafy perennials) but as "low bushes , wood soft" in contrast to "shrubs and trees, wood hard". The latter are the true shrubs and trees whereas the former are suffrutescent or suffruticose-- whichever is the most descriptive intermediate gradation-- shurbs ("semishrubs" was Shreve's choice of terms; "halfshrubs" is the traditional rangeman's term).
Triangleleaf bursage is the constant companion to littleleaf paloverde and saguaro; it, too, is a physiognomic dominant and forms a conspicuous layer of vegetation. These bursage plants were photographed in late spring of a severe to exceptional drought and prior to summer monsoonal showers. These specimens were thus in dry season-dormancy had already shed most of their leaves.
Saguaro National Park, Pima County, Arizona. June.
77. Shoot apex of triangleleaf bursage- Shoots of bursage in dry season-dormancy but with some remnant leaves and the dried remains of flowers and fruits. Ambrosia (= Franseria) species are monoecious with flowers of both sexes forming an involucre. The involucre of the pistillate head has one to four cells that become beaklike to form a bur.
Saguaro National Park, Pima County, Arizona. June.
78. Canyon ragweed (Ambrosia ambrosoides= Franseria ambrosioides)- Another important understorey composite of the Sonoran Desert is canyon ragweed. It grows as an associate species with triangleleaf bursage though in less xeric microsites (it is often found in slight depressions). Canyon ragweed is not a dominant like triangleleaf bursage and it does not usually grow in dense colonies tending, rather, to be solitary in occurrence. Canyon ragweed blooms with adequate soil moisture and temperature in winter or spring and has dry season-dormancy.
Saguaro National Par, Pima County, Arizona. June.
79. Shoot apices of canyon ragweed- The plant in the left photograph was thriving in the shade of a huisache plant. It had not entered dry season-dormany, but was prominently bearing the large, anything-but-desertlike leaves of it's species. The plant in the right photograph appeared mostly dormant but had retained a few green leaves. This plant was bearing abundant fruit from the previous growing season. The bur-like fruit of this genus was described in the caption for the preceding species.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
80.Desert marigold (Baileya multiradiata)- This is another common composite that is often abundant enough to produce a brillant yellow understorey in the multi-layered Sonoran Desert. Desert marigold is an associate to the dominant composites like triangleleaf bursage, but it frequently forms exclusive populations thereby becoming a local dominant. It is one of the species that blooms in the summer in contrast to those like triangleleaf bursage and brittlebush that flower in winter and spring.
Arizona-Sonoran Desert Museum, Pima County, Arizona.June.
81. Desert zinnia (Zinnia acerosa)- Another composite that dares to bloom later in the dry season of the Sonoran Desert is this eye-catching (by absence of about anything else) member of the lower layer of vegetation. It is one of the relatively few forbs (vs. semishrubs or halfshrubs which are suffrutescent or suffruticose plants) with widespread distribution in the foothill paloverde-saguaro-cholla scrub of the Arizona Upland.
Arizona-Sonoran Desert Muiseum, Pima County, Arizona. June.
The other big family of woody range plants in the Sonoran Desert (as in other range types) is the Leguminosae. The legume family was herein interpreted in the traditional division of the three subfamilies of Mimosoideae (mimosa, acacia, mesquite), Caesalpinioideae (paloverde, retama, senna), and Papilionoideae (papilionaceous, nodulated legumes).
Foothill (= littleleaf) paloverde, the dominant of the Arizona Upland vegetational subdivision, was presented above and second only to creosotebush due to it's dominance of the Arizona Upland subdivision of the Sonoran Desert.
A case could be made that the second-most important Sonoran Desert legume is mesquite (Prosopis julifora, P. glandulosa). Such a claim would be based on the widespread distribution and the increased cover and density of mesquite as a result of human influence (reduced incidence of fire, overgrazing, industrial activities like mining and commerce and, possibly, climatic shifts). Authorities have traditionally recognized two mesquite taxa either as two species or two integrating varietal forms of the same species: 1) western honey mesquite (P. glandulosa var. torreyana= P. juliflora var. torreyana) and velvet mesquite (P. velutina= P. juliflora var. velutina).
82. Western honey mesquite- A relatively large honey mesquite grew with allscale or desert saltbush and creosotebush on a Saltbush series of the Lower Colorado Valley vegetational subdivision of the Sonoran Desert. Relict area, Maricopa County, Arizona. June.
83. Western honey mesquite- Branches with leaves and legumes on the tree shown in the preceding slide. Maricopa County, Arizona. June.
84. Velvet mesquite- Even at this distance the short but relatively dense pubescence on leaves was visible on those of the lower branches of this specimen of velvet mesquite. Verde County, Arizona. June.
85. Leaves and inflorescences of velvet mesquite- The more subdued or less shinny leaves distinguish velvet mesquite from western honey mesquite. The infloresence of this member of the mimosa subfamily are distinctively different from those of the other two subfamilies shown below. Pima County, Arizona. June.
86. Velvet mesquite- Leaves and legumes of velvet mesquite. McGinnies (1981, ps. 56, 59) listed velvet mesquite as one of "the most abundant species" in the Arizona Upland subdivision. Pima County, Arizona. June.
87. Desert senna (Cassia covesillo)- One of the herbaceous legumes of the Arizona Upland subdivision of the Sonoran Deserte is this member of the Caesalpinioideae subfamily of the legume family. Real perennial forbs (not halfshrubs or Shreve's "semishrubs") are not common in the Sonoran Desert, especially those that flower in between the main periods of precipitation in winter and summer. Many of the desert species have a wide span of time over which they can bloom and produce fruit. Desert senna appears to be one with an even wider potential flowering period. When it comes to sexual reproduction most desert plants are opportunistic. They have to be.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
88. Shoot apex of desert senna- In this "all-in-one" shot viewers can see the inflorescences, new legumes, and last year's dry and dehisced legumes as well as the typical compound leaf of leguminous species. Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
89. Coral bean or chilcote (Erythrina flabelliformis)- This shrub to slender tree is a papilionaceous (Papilionoideae subfamily) legume. It is also a poisonous plant (mostly the seeds). Coral bean is very sensitive to cold temperatures and usually grows among large rocks or boulder piles which retain daytime heat that results in a more favorable microhabitat (microsite) allowing survival of this attractive and delicate desert range plant. This is another species with opportunistic timing of flowering, but it typically blooms during the hot, dry season. Here is an attractive example of the concept of ecological niche. Each species has it's own combination of abiotic and biotic factors that define it's habitat and role in the ecosystem (ie. it's ecological niche).
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
90. Lotebush or gray-thorn (Zizphus lycioides= Condalia lycioides= Condaliopsis lycioides)- Blooming shoot (left photograph) and detail of inflorescences (right photograph) of a member of the Rhamnaceae (buckthorn family). This widely distributed shrub is by no means limited to the Sonoran Desert or even the Sonoran and Chihuhuan Deserts. It's range extends from the Rolling Red Plains of Texas to the so-called Colorado Desert of southern California and south into Chihuhua and Sonora. Undoubtly there are many ecotypes of the wide-ranging species. It was included as an example of ecotypic variation and to illustrate that deserts-- like all biomes-- are combinations of species some of which rather unlikely members of a given range plant community. Lotebush was described by McGinnies (1981, ps. 56, 59-50) as one of "the most abundant species" of the Arizona Upland.
Maricopa County, Arizona. June.
91. Squaw-bush or Warnock's condalia (Condalia warnockii= C. spathulata)- A range plant with dainty foliage that calls both the Chihuhuan and Sonoran Deserts it's home. This delicate, duffusely branched shrub can attain heights of 10 feet. Like lotebush in the preceding photograph squaw-bush is in the buckthorn family (Rhamnaceae).
Pima County, Arizona. June.
92. Yellow trumpet bush, or trumpet flower (Tecoma stans)- Even quick observation of these showy desert flowers should tip-off the botanically astute student that yellow trumpet-bush is in the Bignoniaceae (bignonia or catalpa family). It's flowers closely resemble those of catalpa, trumpet creeper, and desert catalpa or desert willow, the latter being another showy flowered desert dweller. Also like these species, Tecoma stans became a popular horticultural plant, especially for xeriscaping. In fact, plants shown here were in a landscaped urban planting in Phoenix, Arizona where they added the subtle beauty of native species while conserving water, the most precious natural resource in the desert. June.
93. Desert mistletoe (Phoradendron californicum)- Desert mistletoe parasidizing foothill (= littleleaf) paloverde. Parasitism is one of several forms of interactions between two species. Parasitism is positive to the parasite and negative to the prey (parasitized species). It is obvious from the green color of the mistletoe that it has some chlorohyll and is capable of some photosynthesis. Nonetheless, mistletoe does have some adverse impact on the host paloverde. The example seen here was a fairly severe case.
Saguaro National Park, Pima County, Arizona. June.
94. A single plant of desert mistletoe- This desert mistletoe had also infested foothill paloverde although the degree of infestation was less than in the preceding case. Parasitism is one of several forms or categories of species interaction including amensalism, commensalism, competition, predation, mutualism, and some like herbivory that appear to have attributes of several interactions and thus defy ready categorization. These various interactions were covered in standard textbooks (eg. Barbour et al., 1999, chapters 6 and 7). An example of mutualism in the Arizona Upland was white-winged dove feeding on saguaro fruit as discussed above. Pollination of saguaro would be another expression of mutualism. Now entered parasitism.
The fact that desert mistletoe parasitizes the chief physiognomic dominant, and the arborsecent layer, of this range type might seem to confirm a logical conclusion that parasitism has an impact (ie. this interaction is not a neutral event or process in this desert range ecosystem). Students in the Life Sciences learn early on, however, that biological systems and organisms are not always "logical". Forrest Shreve was quoted earlier as postulating that "[t]he greatest 'struggle' of the plants has not been with one another, but with the environment" (in Shreve and Wiggins, 1964, p. 27). Most ecologists would point out that "the plants" are part of the environment (biotic not abiotic, but environment or habitat nonetheless).
It should be specified that Shreve had placed emphasis on competition in that passage. Such clarified, the fact remins that one of the Shreve's central tenets regarding ecology of North Amerinca deserts (not just the Sonoran Desert) was that interacations among species were of less moment than influence of chemical-physical environment on plants. To Shreve it was the abiotic more than the biotic factors and impacts that were operative in desert plant communities.
Such a view was pretty much "diametrically opposed" to the view of Clements to whom all these species interactions-- what Clements called coactions-- (and in conjuction with competition and reaction) that where the driving force of plant succession and development of vegetation. For example, Clements' wife Edith (also a Ph.D. in Botany) was an authority on the coaction of pollination. Plant Ecology contained a sizeable section on pollination-- complete with line drawings by Dr. Edith Clements-- including competition for pollination visitors (Weaver and Clements, 1938, ps. 256-267).
By comparison, Shreve concluded that it followed from the limited role of coactions-- along with competition and reaction-- (Shreve's earlier conclusion) that "successional changes" were unimportant (and perhaps did not actually exist) and "[i]t is not possible to use the term 'climax' with reference to desert vegetation" (Shreve in Shreve and Wiggins, 1964, p. 29). Shreve did elaborate on role of shade from trees and large shrubs on subordinate plants (Shreve in Shreve and Wiggins, 1964, ps. 38-39). This is the nurse plant phenomenon illustrated previously.
In final analysis, the role and influence of desert mistletoe on species and plant community seems to have received very little study. Obviously the extent of parastism shown here had not affected the role of littleleaf paloverde as the dominant of the Arizona Upland. Desert mistletoe made it's contribution to biological diversity of the Sonoran Desert and added an aesthetically interesting aspect to this xeric range type (and provided subject matter valuable in itself as well as an example to illustrate basic ecologic principles).
For his part, Shreve (with whom we begin and end with) apparently saw such limited role or ecological significance to desert mistletoe that he did not even include this species in his life form classification. Likewise, Shreve did not refer to it in Vegetation of the Sonoran Desert (in Shreve and Wiggins, 1964) leaving Wiggins to treat desert mistletoe in Flora of the Sonoran Desert including the statement: "Parasitic on Prosopis, Condalea, Olneya, Acacia, and other angiospermous desert shrubs..." (in Shreve and Wiggins, 1964, p. 400).
Saguaro National Park, Pima County, Arizona. June.
95. Site of infection- Shoot base of desert mistletoe where it entered a shoot of littleleaf paloverde. This is where the mistletoe haustoria penetrated the wood of the littleleaf paloverde casuing swelling of the legume branch.
Saguaro National Park, Pima County, Arizona. June.
96. Boojum or cirio (Fouquieria columnaris= Idria columnaris)- This member of the Fouquieriaceae (ocotillo family) is certainly one of the most wierd-looking plants in North America if not on Earth. Shreve (in Shreve and Wiggins, 1964, p. 162) described cirio in his usual terse writing: "Idria ranks without rival as the most bizarre plant of the Sonoran Desert. In fact, it is one of the most striking woody plants in the flora of North America." Shreve included the boojum as a woody (nonsucculent) plant and labeled it's life form as sarcophyte, stout-stemmed drought-deciduous trees (in Shreve and Wiggins, 1964, ps. 42, 46; structure of stem shown in Plate 31 and 32).
Boojum grows mostly in the Vizcaino vegetational subdivision of the Sonoran Desert (often deisgnated as the "Vizcaino Desert") on the Baja California peninsula where it is a dominant in three of the seven series of this subdivision (Brown, 1994, ps. 208-209). Cirio also grows in the Central Gulf Coast vegetational subdivision (Shreve in Shreve and Wiggins, 1964, map 12, p. 164; Brown, 1994, p. 215).
The specimens presented here were grown outside the species' range for educational purposes in the botanical garden of the Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
97. Detail of boojum or cirion- The trunk and branching pattern of one of the strangest (appearance, habitat, and ecological role) plants on North America range. Botanical garden of Arizona-Sonoran Desert Museum, Pima County, Arizona.
Woody species, including the halfshrubs or Shreve's "semishrubs" (suffrutescent and suffruticose plants), so dominate the Sonoran Desert that herbaceous species often get proverbial "short shrift" in all but the most knowledgable and detailed coverage. This is true "in spades" for the grasses which are not showy and do not make colorful prints for "wild flower books". Besides, grasses are mostly of interest to stockmen (or so it would seem to many who buy-- or publish-- pretty "wild flower" field guides and to some of whom stock-raisers are a lower caste of resource exploiters). Even scientific works on vegetation and plant-animal communities of deserts often limit coverage to physiognomy and general community structure at large spatial scale. Non-dominant herbaceous species of the understorey and microsites all too often get lost. For example, in the list of plant taxa (mostly species but some genera and varieties) for the Sonoran Desertscrub (all subdivisions of Sonoran Desert) Brown (1994, ps. 331-332) had 182 entries of which grasses totaled six (6= 3.3% of total). Three of these six were Mediterranean annuals and one of the three native perennials was a listing of the genus (Aristida spp.).
Shreve did not fall into this pattern as much as did recent workers. Instead Shreve devoted a separate chapter to perennial vegetation and to ephemeral herbaceous vegetation (chapter 3 and 4, respectively, in Shreve and Wiggins, 1964). In addition, Shreve included major perennial grasses as part of the perennial vegetation according these species the life form of shoot base and root crown perennial under the general heading of grasses that included only perennial species (in Shreve and Wiggins, ps. 41-43). He specified that perennial grasses "show vegetative activity in both summer and winter, although flowering is confined to the former season" (in Shreve and Wiggins, 1964, p. 41).
Shreve included annuals (strictly seasonal) and facultative perennials under the heading of ephemerals. He divided the strictly seasonal life form into winter ephemerals and summer ephemerals (Shreve in Shreve and Wiggins, 1964, ps. 41-43).
However, even Shreve omitted the Mediterranean annual grasses that are herbaceous understorey dominants which Brown (1994, ps. 331-332) listed in his limited list. In Flora of the Sonoran Desert Wiggins (in Shreve and Wiggins, 1964) did include all known vascular plants, including members of the Gramineae, but this was floral not vegetational coverage.
Discussion of the Sonoran Desert in the massive monograph of California vegetation (Burke in Barbour and Major, 1995, ps. 869-889) also ignored grassses "for all intents and purposes". In fact, all herbaceous plants were ignored under the discussion of growth forms except for three sentences (Burk in Barbour and Major, 1995, ps. 880-884). Herbaceous species fared no better coverage in the monograph of North American vegetation (MacMahon in Barbour and Billings, 1988, ps. 242-248; MacMahon in Barbour and Billings, 2000, ps. 295-303). Turner et al. (1995) produced a large ecological atlas-encyclopedia of 339 plant species of the Sonoran Desert. Though described as a "comprehensive work" (Turner et al. 1995, dust jacket) this useful (and lovely) labor-of-love (and obvious passion) reference was devoted to woody plants and shrubs and included only one species of grass (big galleta).
Kuchler (1964, p. 43) listed 27 species for the Paloverde-Cactus Shrub and did not include a single herbaceous species.
Those having an interest in the range vegetation of the Sonoran Desert and possessing an agrostological bent of mind typically find the graminaceous offerings in the ecological literature "ephemeral".
One outstanding exception to the general tendency of ignoring herbaceous species, and especially the grasses, was McGinnies (1981). It was remarked above that W.G. McGinnies was perhaps the scientist and author who after Forrest Shreve had the most on-the-ground and longest-tenured experience in the Sonoran Desert. Throughout his classic Discovering the Desert (McGinnies, 1981) included the grasses and forbs in his discussion of Sonoran Desert vegetation rather than "excomminating" them from desert plant communities as did most contemporary authors (those of, say, late Twentieth Century).
Another exception was Humphrey (1960b) who included numerous grasses-- annual and perennial-- along with shrubs in his description and general discussion of range in three Arizona counties. Admittedly Humphrey (1960b) emphasized forage production (the founders of our profession never forgot what their profession was about; they "stuck to the knitting"). Humphrey's objective was not ecological description of vegetation per se. Nevertheless, Humphrey (1960b)-- like Shreve-- was very much aware that there was an herbaceous understorey in the Sonoran Desert vegetation, and that this range herbage was a forage resource to be wisely used and carefully managed.
Like Shreve, McGinnies, and Clements, Humphrey worked (for 13 months) at the Carnegie Botanical Desert Museum. In fact, Humphrey was hired by Shreve (Bowers, 1988, ps. 96-97).) It was noteworthy that everyone of these accomplished ecologists recognized presence and importance of the herbaceous layer of Sonoran Desert vegetation. It must be emphasized, however, that some of the major range plant communities in the Sonoran Desert apparently had limited and, in some cases, little if any herbaceous species, including grasses. This was apparently the case for the "pure stand" form of creosotebush (creosotebush flats= Larrea plain, much of SRM 211 [creosotebush scrub], K-35), desert saltbush range, paloverde-triangleleaf bursage, and paloverde-white bursage (latter two placed together as SRM 507 [Palo Verde-Cactus], K-36 [Cresostebush-Bursage]) (Humphrey, 1960b, ps. 53-58).
Humphrey was unequivocal about this, as for example: "Ranges where palo-verde and triangle bur-sage are dominant produce little grass and have a low carrying capacity. Not only do they grow little perennial grass today; they apparently never grew much more." (Humphrey, 1960b, p.56). Humphrey went on to specify that paloverde and triangleleaf bursage were "indicators of a range that will not grow much grass and cosnsequently will always have little value as grazing land". He then listed bush muhly, tobosagrass, slim tridens and Texas timothy (= wolftail) as some grasses that do occur on some of this range (Humphrey, 1960b, p. 56). Humphrey estimated that carrying capacity for these types of Sonoran Desert range at "about three to six head per section yearlong" (Humphrey, 1960b, p. 53).
Close reading of Humphrey (1960b) in this passage and throughout his guide revealed that he considered primarily native perennial grasses and gave little thought to naturalized annual species. For example, Humphrey (1960a, 1960b) gave more coverage to introduced perennials, including weakly adapted ones like weeping lovegrass (Eragrostis curvula), than to red brome which naturalized to become part of the potential vegetation. While production of forage by annual species is always a "feast or famine", "boom or bust" proposition it is more dependable than reliance on weakly adapted, reseeded perennials. Feed production and soil protection by naturalized Mediterranean grasses and forbs must be factored into grazing capacity of ranges, if but on an "as-available", tentative basis. It is possible that human introduction of Eurasian species followed by their subsequent naturalization created another layer of vegetation-- hence, another dimension to the range forage resource-- beyond the original or virgin vegetation. Examples of this could include addition of a partial herbaceous layer in the creosotebush plains and desert saltbush ranges of the Lower Colorado Valley subdivision of the Sonoran Desert, range types that apparently had little such herbage in the natural vegetation (Humphrey, 1960b, ps. 54-55).
It seemed appropriate to end this discussion with a quote from Shreve (1951, p. 11-12): "The role played by grasses in the desert vegetation of lower elevations is a minor one depend on the occurrence of a few locally abundant species".
As evidence that some herbaceous plants, including grasses (perennial and annual), do grow in the Sonoran Desert thereby providing some feed and making some contributions to the meager grazing capacity your author provided the following photographs.
98. Herbaceous species in the Sonoran Desert- Native perennial grasses and forbs formed an herbaceous layer in the vegetation of Arizona Upland, Jojoba-Mixed Scrub series, in the Tucson Mountains. Desert senna, a papilionaceous legume (covered separately above), and tanglehead (Heteropogon contortus), a panicoid perennial grass of the bluestem tribe, (immediately to right of senna), and desert marigold (also covered previously) formed an herbaceous stratum in vegetation dominated by succulent and woody (nonsucculent) plants. The woody (including succulent perennials with soft wood) dominants were clearly present: littleleaf paolverde (at both far right and far left background), saguaro, ocotillo (center background), and triangleleaf bursage (leafless gray bunches in left background in front of the far-left palobverde). Also present was a diverse and well-developed herbaceous layer.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
99. Lower shrub and herbaceous layer(s) of relict vegetation in Arizona Upland subdivision, Sonoran Desert- On a location long protected from livestock grazing (discussed below) perennial grasses, threeawn (Aristida glabrata, A. havardii), grew among creosotebush (center background), the regional dominant of the Sonoran Desert, and triangleleaf bursage (plant with gray leaves), one of the dominant shrub species of the Arizona Upland of the Sonoran Desert.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
100. Composition of a lower layer of vegetation made up of herbaceous and shrub species - Textbook example of a the life forms and lower layers in relict Arizona Upland vegetation. Huisache (shrub or small tree at far left, along photo margin) was part of a woody arborescent-like layer and was an example of the winter-deciduous woody plant life form. Creosotebush (shrub with light green leaves at right photo margin-- behind cholla--and background), regional dominant species, made up most of a woody layer below that of large shrubs and small trees. Creosotebush was an example of the perennial-leafed nonsucculent (woody) shrubs and trees (evergreen trees and true shrubs) life form. Fishhook or candy barrel cactus was part of the same layer as creosotebush but was of the unbranched, leafless, elongated stem succulent (soft) (barrel type of stem succulent) life form. Behind the fishhook barrel cactus triangleleaf bursage represented the low bushes, soft wood, of nonsucculent (woody) plants (nonsucculent leafy perennials) life form. The cholla cactus in far right corner was an example of branched, leafless, cylindrical stem segment, elongated stem succulent (soft) (cylindropuntia) life form. The cespitose grass in left foreground was Santa Rita threeawn (A. glabrata), an example of shoot base and root crown perennial (perennial) life form. The underlined life form descriptions were from Shreve (in Shreve and Wiggins, 1964, ps. 41-47).
This photograph emphasized two features of this rangeland cover type. First, the layers and life forms of the Arizona Upland vegetational subdivision of Sonoran Desert seemed remarkable given the harsh abiotic environment of shallow, poorly developed soils; paucity of precipitation, and extreme summer heat. Secondly, for this immediate part of the publication was the presence of an herbaceous layer dominated by perennial grasses.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June (severe to exceptional drought; prior to summer rains).
101. Santa Rita threeawn (Aristida glabrata= A. californica var. glabrata)- Population of one of the common species of perennial grass in Arizona Upland vgetational subdivision of the Sonoran Desert. Aristida species are strictly cespitose with exclusively intravaginated shoots, tillers. There are both annual and perennial Aristida species in the Chihuhuan and Sonoran Deserts.
Viewers' attentions were directed to the bare ground even in this location that had been protected from disturbances like heavy livestock grazing for a number of years and that supported populations of native perennial grasses.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
102. Santa Rita threeawn- Two specimens of one of the many confusing species of Aristida found in parts of the Arizona Upland subdivision of the Sonoran Desert and adjoining vegetational areas such as the semidesert grassland. As with about every taxonomic unit within Aristida there is a general "mess" regarding nomenclature of what most rangemen know as Santa Rita threeawn. Hitchcock and Chase (1951, p. 564-565) and Kearney and Peeples (1960, p. 120) distinguished between A, glabrata and A. california. Gould (1951, p. 234-235) also distinguished between A. glabrata and A. californica specifying that these two species were "generally similar". Humphrey (1960a, p. 15; 1960b, p. 73) followed previous workers and identified Santa Rita threeawn as A. glabrata. Allred (in Barkworth et al. 2003, ps. 319-322) united these taxa as A. californica var. glabrata (vs. A. californica var. californica).
Humphrey (1960a, p.s 15-16; 1960b, p. 73) noted that although this species (or variety as case may be) was "hard and wiry" it was "grazed readily" especially after other grass species had dried out. Humphrey (1960a; 1960b) recommended utilization that left about one-fourth of the flowering culms following grazing.
Arizona-Sonoran Desert Museum, Pima County, Arizona. June (drought, rated as severe to exceptional, before summer rains).
103. Red threeawn (A. longiseta)- A specimen of red threeawn was shown as an example of an Aristida species that is widespread over parts of the eastern and northern partions of the Sonoran Desert and adjoining areas.
Red threeawn has been divided into two varieties: A. longiseta var. longiseta and A. longiseta var. robusta.The latter was described as being larger with leaves less crowed at base and panicles denser and longer (Gould, 1951, p. 244; Wiggins in Shreve and Wiggins, 1964, p. 265). Gould (1951, p. 244) reported "a puzzling intermediate form between A. longiseta and A. purpurea" that appeared "generally closer to A. purpurea" near Tucson, Arizona. Other workers interpreted red threeawn as A. purpurea var. longiseta (Powell 2000, p. 252). Allred (in Barkworth et al., 2003, ps. 330-335) viewed A. purpurea as "composed of several integrating varieties" (seven in his view). A purpurea is generally viewed as one of the most variable complexes within the North American Gramineae, and it is apparently getting bigger. It seemed to this author that in the instance of dividing A. longiseta into varieties it was spliting lemma awns too finely and in the case of "lumping" former species into a "super species" it was weaving together culms of several distinct, long-recognized species. Viewers are free to flip a coin or play the "true or false" game and guess.
The important fact is that several species (or varieties) of perennial grass are components of the understorey of the Arizona Upland subdivision of Sonoran Desert.
Yavapai County, Arizona. June (exceptional drought conditions).
104. Havard threeawn (A. havardii or, according to some authorities, A. barbata) or maybe poverty threeawn (A. divaricata) but, wait, it might be A. hamulosa- Another guessing game. Here is the next case of confusion that only an agrostologist could love. Gould (1960, ps. 236-237, 239) used A. havardii and A. barbata as synonyms while distinguishing A. divaricata which he interpreted as "very closely related to" A. barbata and "also closely related" to A. hamulosa. Powell (2000, ps. 248-249) cited various sources alleging that A. havardii was A. barbata (synonyms) and that there were intermediate specimens between A. havardii and A. divaricata while still yet other taxonomists noted a resemblance of A. divaricata and A. hamulosa. As this caption was written, the latest interpretation was that of Allred (in Barker et al., ps. 323-325) who recognized both A. havardii and A. divaricata that intergrade while A. barbata was listed as a synonym of A havardii and A. hamulosa was given as a synonym for A. ternipes var. gentilis which had previously been viewed as A. ternipes var. hamulosa (Powell, 2000, p. 245). Gould (1951, p. 238) previously viewed A. ternipes as "closely related to" A. orcuttiana. Sherlock Holmes mysteries were less confusing.
Anyway two plants of another perennial species of Aristida, that is unless the A. purpurea complex takes in this taxon (or these taxa) also. Two things seemed certain to this photographer: there were obviously two different taxonomic threeawns and they were part of an herbaceous understorey beneath the dominant shrubs as seen in the next photograph.
105. Two kinds of perennial threeawn in the understory of Arizona Upland subdivision, Sonoran Desert- There were two distinct plants of Havard or poverty (or some other species of) threeawn in the center of this photograph while the panicles of a plant of Santa Rita threeawn "lapped over" from the left margin. Plants of both Aristida species made up part of an herbaceous understorey under creosotebush, the defining dominant of the Lower Colorado Valley and Arizona Upland subdivisions of the Sonoran Desert. Creosotebush was a major component of another layer in this floristically rich and life form-diverse vegetation.
Dominance within the herbaceous layer and relative abundance of Aristida species was not discussed by Shreve or recent workers cited above. As so often before, however, Clements "rode to the rescue" in his brief description of the Sonoran Desert (compared to that of Shreve in earlier discussions). Clements (1920, p. 177) concluded his discussion of the Western Desert Scrub writing: "[t]here are a number of perennial grasses, some of which have entered from the desert plains in contact with the scrub at its upper limit, and others which may be regarded as relicts of a former savannah condition of certain portions of the desert scrub. Such are Muhlenbergia porteri, Aristida divaricata, and Bouteloua rothrockii in particular".
In his description of the Sonoran Desert (Western Desert Scrub) Clements provided several lists of species that he attributed to Professor J.J. Thornber. John James Thornber was Professor of Botany at University of Arizona on whom Shreve and others depended for identification and nomenclature of desert plants, but these investigators-- Shreve in particular-- were not completely satisfied with his assistance (Bowers, 1988, ps. 45, 63). Shreve provided his own lists of species (in Shreve and Wiggins, 1964, for eg. ps. 58, 60,62, 66, 68 for Lower Colorado Valley subdivision) which, given Shreve's many more years of direct experience in the Sonoran Desert, should be regarded as much more reliable. Unfortunately, Shreve's lists included relatively few grasses (none of the naturalized Mediterranean annuals for example).
Shreve did use as an example of the perennial grass life form the species of Aristida ternipes. Powell (2000, ps. 244-246) explained that other agrostologists (Trent, 1985, in particular) felt that the traditional species of A. ternipes, A. hamulosa, A. schiedeana, A. divaricata, A. havardii, A pansa, and A. dissita could be distinguished as distinct taxa but that certain of them "could be linked by forms with intermediate characters" (Powell, p. 245). This was especially the case for the "species pair" of A. ternipes and A. hamulosa that are possibly conspecific and that were recently merged and distinguished as two varieties (Allred in Barker et al., 2003, ps.323) as was elaborated on above. Gould (1951, ps. 238-239) explained that A. ternipes was "closely related" to A orcuttiana and "very similar" to A. hamulosa. In one of the most useful range guides for this general area, Humphrey (1960a, ps. 73-74) recognized both spidergrass (A. ternipes) and poverty threeawn (A. divaricata) and noted that these two species resembled each other "in most respects including general appearance, palatability, and growth habits". He also described Santa Rita threeawn (A. glabrata), but did not mention the more widely distributed A. longiseta (Humphrey, 1960b, ps. 73-74). He did, however, discuss A. longiseta in his more comprehensive work (Humphrey, 1960a, ps.16-17) in which he described it's occurrence as "[r]ather widespread and locally abundant". The former is more common in the three counties covered by Humphrey (1960b), but the latter is found in Pima County, Arizona (Humphrey, 1960a, p. 17; Kearney and Peebles, 1960, ps. 119-120). Taxonomy of Aristida remains problematic.
Brown (1994, p.332) avoided this conundrum by simply putting "Aristida spp." in the species list for Sonoran Desertscrub. McGinnies (1981, p. 228) had previously resorted to this same apparent but nothing-solved "solution" whereas he provided species names to most other grasses (more than any other author except Humphrey [1960b] that the current author could find).
It was concluded by the current author that one to perhaps three closely related species of Aristida are some of the climax dominants that contribute to an herbaceous layer in range vegetation of the Arizona Upland subdivision of the Sonoran Desert. That is, however, barring continued growth of the octopus A. purpurea complex which has a propensity to grow new varietal legs at a need-for-publication growth rate. At this rate the Sonoran Desert may end up with two threeawn species (A. ternipes, and A. divaricata or A. havardii), or maybe only one threeawn species (A. purpurea) and a jillion varieties of it.
As this was written the most current treatment of this Aristida "complex" (it gives rangemen who are not Aristida authorities a complex alright) was Allred (in Barkworth et al. ps. 323-325 and 330-335 for A. purpurea group).
Arizona-Sonoran Desert Museum, Pima County, Arizona. June (severe drought conditions).
106. Tanglehead (Heteropogon contortus)- This member of the Andropogoneae (bluestem tribe) grew in the understorey of Arizona Upland range vegetation of the Jojoba-Mixed Scrub series. Tanglehead was associated with desert senna and desert marigold in the herbaceous layer immediately beneath a lower shrub layer dominated by creosotebush and triangleleaf bursage with littleleaf paloverde and saguaro forming an arboresecent-like upper layer and with jojoba as an intermediate-sized shrub. (A photograph of that herbacous layer in the Arizona Upland, Jojoba-Mixed Scrub, community was presented in this sequence prior to slides of individual species.)
Tanglehead is a palatable perennial bunchgrass with "a world-wide distribution" (Hitchcock and Chase, 1950, p.779). Presence of this panicoid grass among several eragrostoid grasses yielded a mixture of Gramineae taxa typical of the arid, subtropical climatic pattern of southwestern North America. Remnants of these grasses in areas that were protected from severe disturbances like overgrazing by livestock attested to existence of an herbaceous layer in Arizona Upland vegetation of the Sonoran Desert.
Humphrey (1960a, ps. 53-55; 1960b, ps. 89-90) explained that due to coarseness or rank growth habit tanglehead has "moderately low palatability". "It usually persist longer on heavily grazed ranges than most of the associated perennial grasses." Early spring growth of tanglehead, however, makes it valuable for cattle and horses. Humphrey (1960b, p. 93-94) summarized feed value of tanglehead "as fairly good forage when green". This interpretation seemed to concur-- though not totally --with that of Hitchcock and Chase (1950, p. 779). Powell (2000, p. 338) rated tanglehead as "fair to good forage for livestock" prior to maturity after which it becomes "too coarse". Tanglehead serves as an indicator species in that heavy use of tanglehead indicates overuse of "most of the other forage plants (Humphrey, 1960a, p. 55; 1960b, p. 90).
McGinnies (1981, p. 51, 91) reported that tanglehead, along with feather grass (scientific name not given) and muhly (Muhlenbergia affinis, M. rigida), was common on upper desert slopes especially the more bare areas.
Tanglehead is an obligate apomict. Economic importance of tanglehead was illustrated by the fact that the Tucson Plant Materials center released "Rocker", an accession of this warm-season native grass.
Arizona-Sonroan Desert Museum, Pima County, Arizona. June (in drought rated as severe).
107. Inflorescence of tanglehead- Spikelets of tanglehead hung conspicuously from the panicle of this native perennial grass that was thriving in the understorey of Arizona Upland desert vegetation during the worst drought in Arizona history. A pointed callus below the caryopsis can be mechanically injurious, especially to sheep (Powell, 2000, p. 338).
Arizona-Sonoran Desert Museum, Pima County, Arizona. June.
108. Bush muhly (Muhlenbergia porteri)- This is unquestionabaly one of the most important climax perennial grasses in both the Chihuhuan and Sonoran Deserts. This native bunchgrass has a range from southern Mexico to Colorado, Utah, and Nevada and extending eastward to the Great Plains and even rarely into the Edwards Plateau of Texas. Bush muhly grows in sandy habitats in the Chihuhuan Desert, but also it also grows on rocky slopes in the Sonoran Desert. These rather depauperate specimens with their extra-dense clumps-- an atypical habit-- grew in a caliche pile on a calcareous soil in west Texas under severe drought conditions (which explained their stunted growth).
Shreve (1951, p. 13) described bush muhly in the Sonoran Desert in his typical terse coverage: "Under the original conditions of the desert lowlands, Muhlembergia porteri was abundant. This grass grows in the open in rounged bushlike clumps but is more abundant as a semiclimbing plant in cresote bushes. The stems are weak but when supported by stout bushes sometimes reach a height of 4 feet. This grass is very palatable, which accounts for its being much less abundant than formerly". Gould (1951, p. 202) also described bush muhly as "highly palatable", but had a somewhat different view of its association with shrubs: "Bush muhly originally existed in extensive stands on the open range lands of southern Arizona but now occurs for the most part in the protection of shrubs and subshrubs and is seldom locally abundant". Here "protection of shrubs" referred to shelter from excessive livestock grazing because animals are reluctant to graze close to woody, spiny, plants; it was not so much an allusion to the nurse plant phenomenon.
Humphrey (1960a, p. 68; 1960b, p. 99-100) wrote: "Bush muhly was formerly one of the most abundant and important grasses of southern Arizona", but that currently it was largely limited to microhabitats "growing under protection of shrubs". Humphrey further noted that bush muhly provides green feed all of most of the year given sufficient moisture. He recommended allowing bush muhly to set seed in summer (the main growing season) every second or third year and deferment in July and August on depleted ranges to allow restoration of bush muhly (Humphrey, 1960a, p. 69; 1960b, p. 100).
Emphasis on bush muhly by these workers was consistent with that Clements (1920, p. 177) who put this species first on his list of the most important grasses in the Sonoran Desert. Shreve (in Shreve and Wiggins, 1964) did not mention bush muhly; in his flora portion, Wiggins (in Shreve and Wiggins, 1964, 256-257) described the species withour comment as to ecological or economic importance.
An example of bush muhly growing (and outcompeting) creosotebush in the Chihuhuan Desert was presented in this publication in photographs and captions of Chihuhuan Desert, Shrublands.
Midland County, Texas. April, dormant in severe drought so still in hibernal aspect.
109. Panicles of bush muhly- These lacy, purple-colored infloresecences of bush muhly are a "dead giveaway" of this species from a considerable distance. Midland County, Texas. April, but severe drought so still dormant in winter phase.
110. Spikelets of bush muhly- The exaggerated teardrop-shape of the caryopsis with adherring lemma and palea is a prominent characteristic of this species. Midland County, Texas. April.
111. Orcutt lovegrass (Eragrostis orcuttiana), now intrpreted as a subspecies of Mexican lovegrass (E. mexicana subsp. virescens)- This species or subspecies served as an illustration of Shreve's summer ephemeral (strictly seasonal) life form. This specimen was growing on an upper bajada in the Arizona Upland subdivision in a wash community with large plants of rubber rabbitbrush and Mormon tea as well as the dominant foothill paloverde and saguaro. This annual Eragrostis species was flowering in early summer.
The combination of sexual and the pronounced pattern of asexual reproduction in a summer annual appeared atypical but uniquely adapted for survival in the extreme habitat of a flow of sandy soil. The plants shown here were shallow-rooted (in dry sand) and sent out stolons with new daughter plants originating along this runner. The "mother" or "parent" (older, larger) grass clumps were blooming (and positively identified as E. orcuttiana in Hitchcock and Chase [1950]) and also reproducing vegetatively. Such a "double-barrel" approach to perpetuation seemed to be a highly evolved adaptation for survival in an arid habitat. A genetic individual was cloning itself, but allowing recombination of genes for on-going evolution.
Yavapai County, Arizona. June.
Summary of grass species photographed in the Arizona Upland vegetational subdivision: based on presence of eragrostoid and panicoid (both annual and perennial) grasses and review of the classical literature, it seemed a valid conclusion that there is an herbaceous layer in the Sonoran Desert vegetation described as rangeland cover type SRM 507 (Palo Verde-Cactus). Native perennial and annual grasses, along with naturalized Eurasian annual grasses, are frequently or occasionally part of the Sonoran Desert vegetation. Some of these same grass species also occur on range dominance types that are conterminous with (or in close proximity to) Arizona Upland Sonoran Desertscrub (eg. semidesert grassland and Chihuhuan Desert). These grasses-- both native and naturalized Mediterranean species-- are part of the potential natural vegetation of all these range cover types as well as that of the vegetation units mapped and described by Kuchler (1966, 1971) and Brown (1994).
Recent descriptions of the natural vegetation of the Arizona Upland subdivision have in effect ignored the herbaceous layer. Descriptions dealt almost exclusively with woody plants (including succulents) that dominate the upper or open canopy, middle, and lower layers (ie. upperstorey and mid-stories of trees and shrubs, including cactus and succulents). There has been either very little or no description and discussion of the herbaceous layer which this author concluded to be part of the structure of climax or potential natural vegetation. For example, the Society for Range Management description of the Palo Verde-Cactus cover type made mention only of herbaceous plants that were annual grasses and forbs (most of these were naturalized Eurasian species). These naturalized Mediterranean species should be interpreted as part of the now-potential (or climax) vegetation. They may have impacts both positive (increase forage production or diversity of animal diets, increase biological diversity) and negative (compete with natives for limited resources, increase incidence of unnatural fire regimes). Eurasian species are not, however, the only grasses and forbs in the Arizona Upland vegetation. In fact, they are not necessarily the dominant, most common, or defining herbs. Descriptions of herbaceous species in Arizona Upland range vegetation should not stop with naturalized exotics.
The classic and seminal literature on Sonoran Desert vegetation, most notably that written by Forrest Shreve, gave more detailed description of the herbaceous layer and species comprising it-- both perennial and annual-- than recent works. This should be rectified. The herbaceous part of Sonoran Desert range, however limited (eg. sparse in several of the natural range communities in the Lower Colorado Valley and Arizona Upland), is the defining part from the perspective of the vegetation as range. This herbaceous understorey is the key part of Sonoran Desert vegetation from the standpoint of using these forage and resources by scientific range management.
112. Cow pasture?- This may not seem much like cattle range but try telling that to those hearty stockgrowers who used the Sonoran Desert range ecosystem as native pasture as recently as 1984. From arrival of railroads in Arizona Territory around 1880 until 1958 cattle grazing occurred on the Rincon Mountain Section of Saguaro National Monument and some cattle grazing continued on parts of the monument until 1984 (Shelton, 1985). Unfortunately the use made of this natural desert pasture was not always based on scientific and time-tested practices of professional range management. Some grazing and related management practices were not consistent with sustained yield and wise use of such natural resources as the range vegetation and soils. Even ranching resources like financial investment in cattle seemed to be frequently managed other than wisely and rationally.
Saguaro National Park, Pima County, Arizona. June.
113. "Open Range"?- No, technically this was not legally open range. Open range literally meant that land was open to the public for anyone to turn out livestock on. Usually such range was part of the unregulated Public Domain (land that was still part of Federal holdings because such land had not been deeded into private ownership; public not private property).
Open range does not mean unfenced. That is the meaning of road signs such as this one that warn motorists to watch for livestock that can wander onto the road because adjoining range is not fenced. (Often livestock have the right of way when public roads are marked with "open range" signs.) When this sign was put in, land on both sides of the road (that eventually became Saguaro National Park) was grazed by privately owned cattle under some agreement, probably dating back to when the rangeland was administered by the U.S. Forest Service. In keeping with precedent of "grazing rights" (legally, grazing privileges) cattle grazing on this range continued long after it had been designated Saguaro National Monument (later, Park). (The desert range in this photograph was still unfenced, but it was no longer grazed by cattle.)
Use of the forage resources on Saguaro National Monument for beef cattle production was certainly not that of the unregulated open range when the land was part of the Public Domain (when range was a grazing commons and free for the taking). Neither was it overall proper use of the range. In the judgment of numerous observers generally it was not sound husbandry of ranching resources nor good stewardship of the land given the state of the art of scientific range management. Instead the general practice was overgrazing with range deterioration the outcome.
The author of the present publication was not privy to details of stocking rates, timing and length of grazing seasons, distribution of animals, etc., but damage done to the range vegetation, specific species like saguaro, soils, etc. was documented and reported by Steenbergh and Lowe (1977, ps. 26, 168-169, 180, 181, 188, 197). These investigators stated that parts of Saguaro National Monument had "been subjected to intensive grazing by cattle for more than three-quarters of a century"and that this "had an unquestionably detrimental effect on the germination, establishment, and survival of young saguaros" (Steenberg and Lowe, 1977, p. 168). In general, high concentrations of shade-seeking cattle resulted in trampling damage to young plants, including saguaro, under trees or large shrubs that served as nurse plants. There was also some direct trampling of young saguaro, but mainly overuse of forage by improper cattle grazing modified microclimate so as to reduce regeneration of saguaro as well as of grasses. Steenbergh and Lowe (1977, ps.172-179) presented paired photographs to demonstrate their conclusions.
Saguaro National Park, Pima County, Arizona. June.
114. This sign was not for beautification (neither was the fence)- Eventually most of Saguaro National Monument was fenced and all of the cattle removed. The sign remained as an artifact so to speak (ie. nobody volunteered to take it down, least of all the county road department). The small white metal strips on the fence proclaimed " US- NPS" (United States- National Park Service). Those strips were not for beauty, vanity, or pride. Removal of range cattle (and keeping them out) was a major management act of "planning and directing range use" (in this case as range for native animals). Elimination of cattle grazing was deemed a necessity for restoration of Sonoran Desert range vegetation.
By no means was improper management of range resources and the desert ecosystem of Saguaro National Monument (Park after 1994) limited to that of overgrazing. Abuse by unregulated firewood cutting (which destroyed valuable nurse plants, and probably dated as far back as improper grazing) was also noted by Steenbergh and Lowe (1977, p. 168). Fire exclusion-suppression by the U.S. Forest Service and the National Park Service may have adversely impacted the Tucson Mountains ecosystem, including the Saguaro National Park (Steenbergh and Lowe, 1977, ps. 169, 180; Shelton, 1985, p. 84). Overgrazing may have reduced incidence of lightening-set fires which could have benefitted survival of some species like saguaro (Steenbergh and Lowe, 1977, p. 180). Conversely there has generally been an increase in native woody species, most notably mesquite, to noxious population densities and cover. This was due to many factors including shifts in climate, fire cessation, rodent activity, and overgrazing (not necessarily limited to cattle).
Other activities adversely affecting vegetation of the Arizona Upland, including that within Saguaro National Park, were road-building, vandalism, and concentrated visitor use (Steenbergh and Lowe, 1977, ps. 182-187). One special form of destruction of Sonoran Desert plants has been their removal from the range-- both legal and illegal-- for sale to house-owners, especially urban dwellers, for ornamental purposes. Steenbergh and Lowe (1977, ps.167, 182-1830) discussed adverse impacts of this activity on vegetation and the desert environement in general on Sagurao National Monument
115. Sonoran Desert range ecosystem (lower bajada)- Viewing the Sonoran Desert as range (which was the thrust of this publication) and not as picnic grounds, crime scenes of cactus pirates, or the sin of urban sprawl meant that emphasis was on grazing management and influence of grazing, especially by tractable livestock. This author suspected that some of the sparsity of herbaceous understorey from Arizona Upland range was due to improper management of livestock and/or cessation of fire. Seemingly greater populations of native perennial grasses and legumes on land of the Arizona-Sonoran Desert Museum than on the Saguaro National Park (long subjected to long-term intensive grazing under extensive management practices) suggested to the author of this publication that there had been reduction in these herbaceous species partly due to grazing by domestic animals.
Some evidence for this idea was provided by the findings of Steenbergh and Lowe, 1977). For instance, they included a photograph of vegetation inside an exclosure protected from cattle grazing for 11 years which through natural regeneration had developed a dense grass cover of sideoats grama (Bouteolua curtipendula), slender grama (B. filiformis), bush muhly (Muhlenbergia porterii), and Arizona cottontop (Triache californica) (Steenbergh and Lowe, 1977, p. 177).
None of this was intended to imply that Sonoran Desert range should not be grazed by cattle or other livestock, but only that such grazing must be consistent with sound range management that has major emphasis on proper stocking rates, proper distribution of grazing, and proper season of grazing. Consideration should also be placed on choice of the proper kind of livestock and/or wildlife. It could be that smaller species like sheep would be easier managed or less apt to produce disturbances that induce damage than larger species like cattle and horses. Or perhaps the converse is the case. Perhaps wildlife would be more effiicient and profitable commodities to produce (ie. game rather then livestock range). In the instance of Saguaro National Park it was only logical management to eliminate domestic animals and replace them with the natural increase of native animals for restoration of the Sonoran Desert range ecosystem to the extent possible.
The main goals for national parks should be research and education and as demonstrations to citizens of what the land was like before it was converted to cropland, suburbs, interstate highways, and shopping malls. This was being achieved through the principles and practices of range management.
ALWAYS REMEMBER THE FOUR CARDINAL PRINCIPLES OF RANGE MANAGEMENT:
Proper Degree of Use (stocking rate consistent with grazing capacity of the range)
Proper Distribution of Grazing Use
Proper Season of Grazing Use
Proper Kind and/or Class of Range Animal.
Vegetation on this lower bajada of the Arizona Upland subdivision, Sonoran Desert consisted of saguaro, triangleleaf bursage (bare, low bushes in woody understorey), Thurber cholla, desert pricklypear, ocotillo, fishhook barrel cactus, and littleleaf paloverde, but no herbaceous species (not even remnants of Eurasian annuals).
Saguaro National Park, Pima County, Arizona. June.
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