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Defined and Described
Range Site vs. Habitat Type
Range Site: A Unit of Habitat
Theories of Climax
Range Site vs. Micosite
Range site has traditionally been defined precisely by the Society for Range Management as:
--"an area of land having a combination of edaphic, climatic, topographic and natural biotic factors that is significantly different from adjacent areas" (Huss, 1964),
--"a distinctive kind of rangeland, which in the absence of abnormal disturbance and physical site deterioration, has the potential to support a native plant community typified by an association of species different from that of other sites" (Kothmann, 1974), or
--"an area of rangeland which has the potential to produce and sustain distinctive kinds and amounts of vegetation to result in a characteristic plant community under its particular combination of environmental factors, particularly climate, soils, and associated native biota" (Jacoby, 1989).
Range site is conspicuous by its absence from all major principles or introductory Range Management textbooks (going back to the first by Arthur W. Sampson) except Bell (1973). Heady and Child (1994, p. 138, 143) wrote the terms range site and ecological site but did not define them specifically except to cite Shiflet (1973): "A unit of land supporting or capable of supporting a distinctive climax vegetation". This vague description obviously could include any unit from as large as a biome (or formation) down in size to a plant association and finally as small as an actual range site.
Historically the Soil Conservation Service (SCS) relied heavily on range sites and has provided the most specifics of any organization or individual in this regard. SCS devoted over six pages in its 1976 National Range Handbook to the use, determination, permanence, naming, describing, etc, of range sites (SCS, 1976, units 302 through 304). SCS defined range site as:
--"a distinctive kind of rangeland that differs from other kinds of rangeland in its ability to produce a characteristic natural plant community".
Over many years SCS range conservationists and specialists wrote hundreds of range site descriptions. It is logical that the one Range Management textbook that treated the range site concept was written by a man who spent 30 years as a range specialist with the SCS.
In December, 1982 the Bureau of Land Management (BLM) officially adopted the SCS Range Site Inventory method as outlined in the 1976 SCS National Range Handbook BLM, 1990).
The current National Range and Pasture Handbook of the Natural Resources Conservation Service (1997) devoted about 56 pages to "ecological sites for native grazing land". Rangeland ecological site was defined as:
"a distinctive kind of land with specific physical characteristics that differs from other kinds of land in it its ability to produce a distinctive kind and amount of vegetation" (NRCS, 1997, Section 1, 3.1-1).
The criteria and basic meaning of range site has remained remarkably consistent over the years, at least among groups subscribing to the basic definition that essentially traces back to the Soil Conservation Service.
Site has traditionally had a less specific meaning in Forestry. In one of the earlier references for Forestry terms the Society of American Foresters (SAF) defined site as:
"an area, considered as to its ecological factors with reference to capacity to produce forest or other vegetation; the combination of biotic, climatic, and soil conditions of an area" (Munns, 1950).
Since then the SAF (Ford-Robertson, 1971) defined site as a synonym for habitat:
"an area considered in terms of its environment, particularly as this determines the type and quality of the vegetation the area can carry".
The current SAF definition is identical to the latter except the words "…in which a plant or stand grows…" followed the word "area" (Helms, 1998).
These are similar to the earlier yet (and even more general) silvicultural-oriented definition in which "site" was synonymous with "environment":
-- "The environment of a forest is very commonly referred to by the term site. Site as used by foresters is the exact equivalent of habitat as used by ecologists. It is equivalent to the term locality as used by European writers on silviculture… In addition to its earlier meaning it has now become a scientific term applied to the combination of climatic and soil conditions affecting a plant, which are incidental to the place in which the plant lives. …From the standpoint of slivics site may be considered as including everything relating to the factors operating in a geographically definite locality so far as these factors influence forest vegetation. Thus site is very complex, being the result of the interactions of many varying factors… [T]he term is applied to areas in which all the environmental conditions are essentially uniform throughout" (Toumey and Korstian, 1947, ps. 7-8).
Although this description includes climate, it also specifies soil. While soil could be considered at scales above series (eg. association) or even larger scale (eg. great group), inclusion of the edaphic component in conjuction with climate and the qualifier of uniform environmental conditions clearly connotes a view of site similar to range site and not of larger, heterogeneous vegetation units like vegetation types. Clearly, neither climate nor any other single factor is the principal determinate of forest community.
Smith (1986, p. 263) in defining site (again in a silvicultural context) distinguished site from habitat: "While the term site is the traditional one denoting the total environment of a place, habitat more fully connotes the idea that the place is one in which trees and other living organisms subsist and interact". Then in discussing site identification and classification Smith (1986, p. 264) remarked: "The first consideration in site classification is the regional climate". The initial emphasis on regional climate suggest regional not local vegetation (ie. forest type or associations and not vegetation equivalent to the level of range sites). In fact, immediately thereafter Smith (1986, ps. 264-268) used different forest cover types to illustrate site thereby mixing vegetation type and site.
Smith then cited Spurr and Barnes (1980) as the reference for site classification. Spurr and Barnes (1980, ps. 297-335) discussed several site classification systems including the Landscape Hierarchy classification scheme of Daubenmire (1968, ps. 259-263). It is this system that has habitat type:
--"All parts of the landscape that support, or are capable of supporting, what seems desirable to consider as the same kind of relatively stable phytocoenosis (homogenous as to dominants in all layers) in the absence of disturbance, comprise one habitat type … Ordinarily one habitat type is highly discontinuous, with intervening areas occupied by other types differentiated by either soil or microclimate". The habitat type is "… the lowest level in an ecosystem classification that aims to reflect potentialities…among layered structure, floristics, dominance, edaphic factors, microclimate, and succession". The potential vegetation is usually climax vegetation (climatic, edaphic, or topographic climaxes but usually only one or two). Plant communities growing on "scattered units of one habitat type usually embrace a wide variety of seral stages as a result of disturbances at varied times in the past", but each seral community "… can be related dynamically to some one primary climax…". Use of potential vegetation irrespective of current successional stage allows "… the closest possible correlation among vegetation, microclimate, and soil" (Daubenmire, 1968, p. 260).
Spurr and Barnes (1980, p. 313) cited Daubenmire (1968) and then succinctly defined habitat type as "literally, the type of climax vegetation on a particular habitat or site". Unfortunately, in their brevity they garbled the two vegetation units of site and vegetation type. Still, one gets the general idea that habitat type is similar to, if not the same as, site.
Habitat type has been controversial in the Range Management profession. ("What else is new?") While the Daubenmire habitat type clearly fits most major criteria of range site notable range authorities disputed the oft-expressed assertion that these two terms are synonyms. That range (= ecological) site and habitat type are "similar" is the current interpretation of the Society for Range Management. After restricting habitat type to the vegetation unit of plant association and specifying the Daubenmire conception, the SRM stated: "Habitat type is similar in concept to ecological (= range) site. The difference depends mainly on how specifically plant associations are defined" (Bedell 1998).
This is official, but that alone does not make it correct. Anderson (1983) alleged that range site and habitat type are different because the former "has management implications" while the latter does not. Likewise, range (= ecological) site "is a management tool" while a habitat type "is primarily an academic tool", but "… at any specific location on the landscape, an ecological site and a habitat type are likely to be synonymous" (Anderson, 1983).
The enduring Dyksterhuis entered the fray noting that habitat includes biotic as well as abiotic factors. He concluded "… that at best the habitat type approach, with its list of species, is a cumbersome method of designating habitats" (Dyksterhuis, 1983).
A year later Daubenmire (1984) responded to the conclusions expressed by Anderson (1983), but seemingly by trying to write his own restrictive interpretation of range site by asserting that the term "…suggest a single-purpose objective, i.e., management of land for producing livestock forage". In point of fact, as clearly recorded above in quoting all SRM definitions of range site, the term never was defined in the narrow sense of "livestock forage". Obviously, the word "range" has always meant land with native vegetation serving as feed resources for grazing/browsing animals, but this always included wildlife the same as livestock. By contrast Daubenmire (1984) pointed out that habitat type had "… implications for a variety of land management objectives such as livestock, wildlife and timber production, for predicting disease hazard, for indicating hydrologic cycles, etc." All these commodities and processes are common to range and are not excluded by grazing. Range as defined by the SRM was always compatable with multiple uses, and given that it is the profession of Range Management represented and promoted by the Society for Range Management, the adjective "range" would seem preferable to that of "habitat". This seemed especially obvious given the attempt by Daubenmire (1984) to strengthen his view by presenting a commodity-restricted definition of "range" that was never identified or used by the Society for Range Management.
Next, biologist G.R. Hoffman (1984) weighed in on the side of habitat type and like Daubenmire stated that these vegetation units do have management implications. "The habitat type concept has been tested in some management-related studies and has been shown to be sound." Mason (1985) proposed "… that range site concept is essentially the same as the habitat type concept", plus "[w]ith very minor modifications they could be exactly the same".
There seems to be a consensus among rangemen that range site and habitat type are, if not essentially the same, quite "similar in concept" and perhaps in range practices. Again, the SRM range term glossary (Bedell, 1998) said as much.
In conclusion, though, that still does not prove that these two conceptually "similar" terms are interchangeable. The most specific observation was perhaps that of Hall (1985). He noted that the habitat types of Daubenmire and Hoffman are floristic classifications of plant communities (climax plant associations, in Daubenmire's words and as noted by SRM) and, as these proponents themselves emphasized, that "topography, soil, and climate are not part of the classification…". This latter point is proven by examples where because plant communities are the same "the same habitat type can occur on low precipitation north slopes and higher precipitation south slopes" (Hall, 1985).
It is plant community, and not physical/chemical site factors, that delineate habitat types. By contrast, it is the abiotic components that delineate, that identify, range sites. The unique plant communities growing on range sites are determined by soils, relief, aspect, etc. The range or forest community that is characteristic for a range site is but a reflection of that unique combination of abiotic factors, but it is not the unique vegetation as such, at least not the vegetation alone, that determines the range site. In the case of habitat type, it is solely the vegetation that distinguishes one habitat type from another. "The difference between habitat type and ecological site/range site is the inclusion of abiotic factors in making the classification for the latter" (Hall, 1985).
Pfister et al. (1977, p.1) stated that habitat type (as it came to be adopted by the U.S. Forest Service, which probably was what brought it into conflict with range site whether such be admitted or not) was introduced by Daubenmire (1952). Daubenmire's classic ecological monograph of forest vegetation is highly recommended for its: 1) detailed descriptions of forests throughout a region, 2) application of the plant association concept to vegetation classification (discussed in the Range Types section below), and 3) its "practical applications" to "forest, range, and wildlife management" (Daubenmire, 1952, ps. 324-327). Most importantly for current discussion, this seminal paper in which Daubenmire introduced his version of habitat type seemed to provide the answer to an argument concerning the unit that arose 30 years later.
In this descriptive vegetation classification, Daubenmire used two "phytosociologic entities" or vegetation units. The larger and basic unit was the association, specifically the climax association, defined to embrace
"all unions that are superimposed on the same area, and each distinctive combination of vascular unions is ordinarily considered a separate association. Such an aggregation of union ['s', sic] constitutes a phytocoenosis".
The other entity was
"The union (or synusia) is considered the smallest structural unit in the organization of vegetation, each union consisting of a population of one species, or of several species that are closely similar in ecology (ie. in microenvironmental requirements) as indicated by similarity of local environmental amplitude, phenology, and frequently by similarity of life-form as well." Species making up a union vary "and for the most part seem a result of historical factors or chance dissemination rather than of present variation in environmental factors" (Daubenmire, 1952, p. 302).
Climax associations were the basis of vegetation classification. "Association is a concept embodying those characters of all actual stands among which differences in species compostion are attributable to historical events or chance dissemination rather than to inherent differences in environments. Widely separated dissimilar stands may eventually be shown to be connected by an intermediate series exhibiting continuous variation that is correlated with gradual macroclimatic gradient." An association is designated and named by the species of a dominant union (the dominant tree species) and species of a subordinate union (the dominant understory species). The climax association "extends into distinctly diffferent climatic regions, and in so doing occupies very diverse soils and topography by way of compensation" but "each stand contains essentially the same galaxy of subordinate species…". This is due to "… intrinsic characters of the habitat or upon microenvironmental conditions created by the dominant trees". As such, "diversity with respect to ecologic role must be subordinated in biogeographic classification" because "all stands of an association have a high degree of biotic similarity" (Daubaenmire, 1952, p. 303).
Next Daubenmire provided a key specific criterion:
"In the interest of clarity, it is desirable to make a distinction between vegetation and the area it occupies. The collective area which one association occupies, or will come to occupy as succession advances, is called a habitat type." Daubenmire went on to explain that varied habitat factors compensated such that "the ecologic sums of the different sets of conditions are essentially equivalent" within the habitat type of a climax association. As such, "the fundamental characters of the habitat type are not permanently affected by disturbance" (Daubenmire, 1952, p. 303).
In this passage, in the subordination of "ecologic role" (ie. habitat), and in the greater importance of "chance dissemination" than "variation in environmental factors" Daubenmire clearly established that it was the climax vegetation and not the habitat that distinguished the association. As discussed below this is consistent with definition of plant association. However, the area occuped by one association is its habitat type so that these two things are just two aspects of the same parcel(s) of land. Most importantly, it was the vegetation that was described and not the habitat. Actually it was the subordinate unions within one zone comprised of one dominant tree species that distinguished one association from another and, by extension, one habitat type from another (Daubenmire, 1952, ps. 306-319, 323). As such, habitat type was in actuality a unit of vegetation and not a unit of the environment, the habitat, in which the vegetation grew. In fact, the habitat ("environmental factors") was less important than "chance dissemination" in determining species composition of the habitat type! Again, as habitat type is "the collective area which one association occupies", association equals habitat type. Daubenmire (1952, p. 323) cited previous workers that referred to this as the "vegetation-indicator approach" or the "vegetation-indicator concept" (ie. it is the vegetation that indicates what habitat exist).
This is exactly the point that Hall (1985) made when, as quoted above, he noted that habitat types were merely "floristic classifications". Recent descriptions of habitat types by the U.S. Forest Service gave some reference to habitat factors by including short general sections on soil, climate, fire history, range management, etc. (Pfister et al., 1977; Mueggler and Stewart, 1980). These habitat types are much more inclusive of environmental factors than were the original ones of Daubenmire (1952). Even these expanded habitat type descriptions are obviously defined first by plant community and then habitat variables were added to the defining vegetation.
Another factor or aspect of habitat type to be considered when comparing it to range site is size or, more accurately, scale. In his original association/habitat type unit, Daubenmire (1952, ps. 302-303) viewed stands within an association to be at scale of macroclimate ("gradual macroclimatic gradient", "macroclimatic and geologic variations" [Daubenmire, 1952, ps. 302, 303]). In fact, " the same climax type extends into distinctly different climatic regions, and in so doing occupies very diverse soils and topography by way of compensation" (p. 303). The union, on the other hand, is comprised of species that have similar "microenvironmental requirements" ("similarity of local environmental amplitude").
Four facts emerged as relevant to the issue of scale from Daubenmire's units of vegetation and their size relative to environmental features like climate, topography, and soils:
(Obviously the concept of local environment, microenvironment, or microsite could conceivably be reduced to an infinitely small space, as small as that occupied by one plant, say a cubic meter, or even to the space of a root or single root hair. It is equally obvious that microenvironment so defined autecologically would reduce habitat below that of vegetation or the synecological scale. Such restrictions [reductions] on microenvironment of the union would be an absurd strained rationalization to rule out union in order to try to make association (= habitat type) correlate with the scale of microenvironment after the fact. This circular ex post facto reasoning would be meaningless because realistically microenvironment as applied in vegetation classification (which was what the Daubenmire paper was about) must be at the scale of plant communities by definition. This plant community microenvironment is the scale of the smallest Daubenmire unit, the union, not the association.)
Range site is the smallest unit of range (but it is not a unit of vegetation per se) and it exists at the smallest scale of vegetation habitat.
Ergo, as habitat type exists at the scale of associations that can extend across climatic regions, there is no way that habitat type can be equivalent in size or scale to range site, which is the smallest unit of range. If any Daubenmire unit is the equivalent of range site, it has to be the union, the smallest unit of vegetation in Daubenmire classification. There is simply no way that habitat type = range site because these two units are of two different scales in both absolute and relative size.
When Glossary of Terms Used in Range Management (Bedell, 1998) stated that habitat type and range site are "similar in concept" depending "mainly on how specifically plant associations are defined" it mistakenly drew a comparison to the wrong vegetation unit. It should have used the Daubenmire union not the association. This is the issue of scale.
The other issue was the fact that habitat type is not a unit of habitat at all but a unit of vegetation, a "phytosociological entity". The range site is a unit of habitat that is distinct in being able to support a unique plant community; the habitat type is a unit of vegetation that bears some relation to habitat, but not much because even "chance dissemination" is more influencial in determining vegetation than is habitat. With range site, it is habitat that is used to distinguish vegetation; with habitat type it is vegetation that is used to distinguish (sort of that is) habitat.
A more precise statement in the Glossary would be: "Habitat type and range site are dissimilar in concept. The difference depends mainly on how specifically Daubenmire unions are defined".
Conclusion: range site and habitat type are not the same. "They ain't even close." By ignoring abiotic factors, other than indirectly as reflected through plant communities like associations, habitat types are strictly and solely units of vegetation. As such they do not fit into any ordered organization, any hierarchy, of ecosystems or land units that include climatic, edaphic, topographic, pyric, etc. factors. Habitat type as a unit of vegetation could logically be one level (= a unit) of plant community hierarchy much as plant formation, association, and alliance are units of vegetation. Habitat type could not be interpreted legitimately as an ecosystem unit— a unit or level in an ecosystem hierarchy such as those of the U.S. Forest Service or Bailey (1996, p. 23-26). This is because habitat types are strictly vegetation—the flora biota—and include no abiotic factors except those that were described after the vegetation was selected (ie. habitat is not used in the selection of habitat types). Habitat types cannot be part of a land or landscape hierarchy or classification system because, again, there is no land, landform, soil, climate, etc. component except "after the fact" (after the unit was selected on the basis of dominant species in the vegetation). Instead, habitat type is strictly a plant community at some level of vegetation. It is incorrect to include the word "landscape" in "a classification scheme" entitled "A Landscape Hierarchy" (Daubenmire 1968, ps. 259-263) that is not landscape. The "Landscape Hierarchy progresses from smallest unit or lowest hierarchial level of habitat type up through vegetation zone to vegetation province to vegetation region". Daubenmire (1968) incorrectly used "landscape". His is not a "landscape" but rather a vegetation hierarchy. Neither can it be an ecosystem hierarchy because none of his units of plant community organization were even partly selected on basis of abiotic components of ecosystems or, for that matter, even animal biota. By ignoring the physical/chemical factors—the abiotic agents—of biotic communities in the selection process, habitat type ignores or, at very least, de-emphasizes the ecosystem concept, a bedrock of modern Ecology.
Habitat type does provide a useful unit of vegetation classification, at least in the Daubenmire Vegetation Hierarchy. Habitat type has been particularity useful for describing units of forest vegetation, especially as used by the U. S. Forest Service.
The Forest Service substantially expanded the habitat type as employed by Daubenmire. The statements in the preceding paragraph apply most specifically (but not exclusively) to habitat type as introduced by Daubenmire (1952) and defined/described by Daubenmire (1968). This could be designated the "Daubenmire habitat type" which is in effect the climax plant association because a single habitat type is all land ("the collective area") occupied by a single association (Daubenmire, 1952, 303). In fact, in this original monograph Daubenmire (1952) designated his units of forest vegetation as associations and not as habitat types. Habitat type was barely mentioned and habitat features were indicated only indirectly by vegetation and none of the association descriptions included other than passing reference to features like soils, slope, or exposure.
The Daubenmire habitat type as modified (descriptions expanded and more inclusive of actual environmental variables) by the Forest Service, the "Forest Service habitat type" as distinguished from the Daubenmire association (later, habitat type), is the unit (of vegetation) that is more useful. The above conclusions regarding habitat type are less appropriate for the Forest Service-expanded unit. Consistent with arguments advanced by advocates of the habitat type in the 1980s, the expanded habitat type descriptions do include some actual habitat information (again, after habitat types were distinguished or selected based on vegetation).
The "Forest Service habitat type" does resemble, for some criteria, the range site as developed by the Soil Conservation Service. This is obvious for example in the descriptions of grassland and shrubland types in Montana (Mueggler and Stewart, 1980).
Indeed, these units could be designated accurately as "Forest Service ecological sites". These included considerably less detail on soils and geologic material, but sections devoted to grazing responses and general range practices were very similar to those equivalent parts of standard SCS range sites that have been used for years.
There is still a fundamental difference between these two units. Range site bases vegetation on habitat, the abiotic environment primarily, and is thus a unit of habitat classification. Habitat type bases habitat on vegetation and is a unit of vegetation classification. The former starts with habitat and "builds" vegetation descriptions from it; the latter starts with vegetation (dominant species) and develops habitat descriptions from the plant community. As explained in the Range Types section below dominant species have always been used to designate plant associations, and habitat types as used by Daubenmire are units of potential associations. Again, when he first introduced this vegetation classification, Daubenmire (1952) called his vegetation units associations and not habitat types. It seemed that for practical purposes habitat types were (are) associations by another name.
The distinct difference between habitat type and range site and the impossibility of interpreting these two as synonyms is discussed further under the Range Type section below. There it is shown that Daubenmire (1968, p. 32) in his Synecology textbook reinterated that habitat type is all the area that can support (has potential to support) one climax plant association.
Important point: range site (and the slightly different forest site) is a unit of land and not solely vegetation. This is obviously essential for the concept to have any utility because range (rangeland and forest range) is the land and not just the vegetation. In fact, "land" includes all natural resources involved in production of the forage and/or browse crop. In the ecosystem concept, this includes light which, though a feature or factor of atmosphere and not land, is affected by the upperstory vegetation as light is filtered to the understory on forest range sites, especially transitory forest ranges. Nonetheless, the range site is not a unit of vegetation; it is a unit of land supporting vegetation (ie. range site is a unit of land including the vegetation it supports). Range site is a unit of native grazing land, but not a unit that is limited to the vegetation. (Habitat type is a unit of vegetation and only or, at least, primarily vegetation because no other aspect or factor is used in its selection.) As such: range site does not fit into a hierarchy of vegetation. Range site is not a subunit of the plant community or even of the plant and animal community. Range site is not a subset of the biome or a subdivision within the plant association or the range or forest cover (= dominance= vegetation) type. All these larger or broader hierarchial levels are limited to the biotic community in case of biome or formation and the plant community in the instance of the plant association. Range site is both smaller (more restricted) and more encompassing than these upper levels of community organization because: 1) range site is the smallest unit of a unique plant community (smallest hierarchial unit that includes vegetation) but 2) directly inclusive of abiotic factors, especially soil, topography (slope and aspect), and landform. Range site goes beyond (includes factors besides) vegetation as the smallest unit theoretically capable of unique management. Thus, range site is not comparable to a range biome (say, grassland or desert) or a range cover type (say, semidesert grassland or mixed prairie grassland). Range site is not, strictly speaking, a logical reduction of range vegetation types which again are units of vegetation only. Range sites fall within range cover types and can be visualized as residing therein, but these most unique kinds of range are land or habitat units (these include the vegetation, but also soil and other natural resources). Range sites are not vegetation units; they are not part of vegetation classification.
In conclusion: habitat types appeared to have more limitations in designating units of range. Habitat types are units of vegetation, the descriptions of which can include details of habitat. Range sites are units of range including biotic and abiotic factors. Range sites are units for management of range and not for classification of vegetation; habitat types are units in a vegetation classifiction scheme and have less application for range or forest management practices because they are pre-selected on basis of vegetation alone.
The U.S. Forest Service has defined range site noticeably different from the SCS-SRM concept. A typical example from the California Region is:
"Range sites are broad, ecologic areas within which soils, climate and other environmental factors of strong similarity exist or are potential" (U.S. Forest Service, 1969, 160).
At first flush, this seems consistent with the definitions and concept above, but the emphasis on "broad" is substantially different, in contrast to the more restrictive scale of interpretations by the SCS and, to a somewhat lesser extent, standard Forestry texts.
In conjunction with range site the Forest Service used the units of vegetation type and their subtypes. The latter were defined as: "A subtype is a relatively large portion of a vegetation type which appears significantly different in composition, slope, site, density, trend or condition class (U.S. Forest Service, 1969, 150-1). Vegetation types appeared to correspond to generally accepted views of cover type with examples including pinyon-juniper, annual grass savanna, annual grassland (U.S. Forest Service, 1969, 150- 4 and 150-5). Others are broader or more general (eg. meadow, sagebrush, conifer; 150- 2, 150-3). However, the real difference—going back again to the criterion of "broad"—between Forest Service and the other interpretation of range site is that the Forest Service range site can encompass several vegetation types. For example: "Various vegetation types … may exist on any or all of these range sites" (U.S. Forest Service, 1969, 160).
Clearly, Forest Service range sites are of greater scale than vegetation (= cover) types given that the latter exist inside the former. This is just the opposite of the SCS-SRM perception of range site. Furthermore, and probably because of the larger size of range sites, the Forest Service has made little use of the range site as either a concept or as a unit for analysis and management. This is in contrast to the other two major Federal agencies dealing with range conservation. Historically it has been the range (= vegetation) types that are mapped and serve as the basic units for Forest Service range resource inventories (U.S. Forest Service, 1964) and range analysis of allotments (U.S. Forest Service, 1964, 1970).
The more traditional interpretation of range site (eg. the series of SCS-SRM definitions) is that range site is the smallest unit or kind of range that can be recognized and distinguished as unique. It is the least scale of range that could in theory be managed. Almost all managed parcels of natural pasture (termed variously as ranges, pastures, allotments, traps, paddocks, etc.) will have several to many range sites. Thus these range sites are not managed separately (ranges or pastures not range sites are the management units) but, as with individual forage species, each unique range site contributes to the pasture or range plant community that is managed as a collective whole.
In theory, range site is the smallest distinctive (recognizable or discernable) unit of native grazing land. This unit consist of the range environment, especially soil, and the unique potential vegetation that is the highest ecological state possible for that environment. Range site is the most specific (relatively smallest) unit of potential natural vegetation (usually climax vegetation) for each specific habitat of natural pasture that is capable of being identified and managed.
In the context and vocabulary of taxonomy, range site is the "lowest" (at the bottom of the taxonomic hierarchy), smallest, most precise "taxon". Range site can be visualized as "the species level or unit of range". Range site is roughly analogous to species of organism or series of soil. It is analogous, not homologous, because classification of vegetation is classification and not taxonomy. At least this is the case when the arrangement of units is not necessarily based on relatedness (not phylogenetic; the hierarchy is not consistently based on origin). This seems more likely to be so when classification of land and its vegetation is based on the use to which that land and vegetation are put (eg. classification of land for agricultural purposes vs. taxonomy of soil; classification of range [natural vegetation used for grazing/browsing], vs. a more taxonomic classification of, say, grassland, forest, or desert).
This comparison of range site as the lowest identifiable, manageable unit in a range heirarchy to a lower taxonomic level is readily apparent when compared to soil series. Range site as developed by the young Soil Conservation Service and later adopted by the Bureau of Land Management is based largely on soil series or soil association. Likewise, the SCS-SRM range site is described vegetation-wise by the entire plant community (most of it anyway) and not just the regionally dominant species, as is the case with range vegetation (= cover) type. As usually defined, range cover type is the next higher unit of vegetation. In other words, range site is both more habitat (edaphic) and species-specific.
In the historical development of ecological theory that formed much of the basis of both Range Ecology and range practice, acceptance of the range site is the ultimate victory of the polyclimax theory of Arthur Tansley over the monoclimax theory of F.E. Clements. Those interested in the instructive drama of this debate between two close friends can piece it together from such works as Tansley (1935), Tobey (1981), McIntosh (1985), and Glenn-Lewin et al. (1992).
Many (probably most) of the first generation of grassland, forest, and tundra ecologists in North America were dyed-in-the-wool Clemetsian ecologists, often vocally proud of their allegiance to Clementsian concepts. This was less true for students of desert vegetation some of whom agreed with the conclusions of Forrest Shreve that there was not a desert climax per se and that succession did not occur in desert vegetation (Bowers, 1988, p. 134-136). Other contemporaries fell more in line with the opposing—anti-Clementsian—theory of Henry A. Gleason. Gleason proposed that plant communities as such did not exist except as arbitrary groups of plants envisioned by human imagination (ie. vegetation does not consist of natural units) (Barbour et al. 1999, 23-24). The Gleasonian interpretation came to be known as the individualistic hypothesis or continuum concept of vegetation. It was consistent with the views of Shreve regarding desert vegetation.
In spite of the insightful contributions of Gleason, Shreve, and some others as well, the vast majority of pioneer range and forest ecologists continued to adhere to the sweeping and usually practical Clementsian theory of "dynamic vegetation". "For the most part, Shreve's fellow ecologists were so steeped in orthodoxy that they saw only what Clementsisan doctrine prescribed" (Bowers, 1988, p. 62). In fact, this was the case more often than not even for more theoretical ecologists as well as range and forest practitioners. A case in point is Lucy Braun, the leading authority on the eastern deciduous forest of North America (Braun, 1950, ps. 10-17). "Clements is probably still the one person most responsible for the shape of ecological thought in the United States and, perhaps, throughout the world" (Brewer, 1988, p. 503).
Many of the cornerstones of Clements' model of vegetation-- the "Clementsian paradigm" as it is currently labeled-- are so deeply ingrained in the theory ("dogma" according to detractors) of applied Ecology that contemporary resource professionals like range managers and foresters have accepted this foundation subconsciously or de facto. Brewer (1988, p. 503) again stated it well when he observed that Clements' views and ecological philosophy "have been modified but not lost"; his methods and terms have become "part of the permanent fabric of ecology". For instance, succession, climax (or potential natural vegetation as the less rigid or doctrinaire category), and biome are bedrock Clementsian concepts. Clearly Clements built such concepts on the writings of earlier workers like Henry C. Cowles and Oscar Drude, but it was Clements alone who breathed the life of practical knowledge into them that enabled sweeping, abstract theory to become applied to on-the-ground management by agricultural producers and agency personnel alike.
Given the lasting impact of Clementsian theory and its practical applications to Range Management and Forestry (see especially Plant Indicators [Clements, 1920] and Plant Ecology [Weaver and Clements, 1929, 1938]) it is initially somewhat surprising and ironic that the polyclimax theory of the British Tansley prevailed over the much more widely disseminated "homegrown" monoclimax theory of the invincible, entrenched Clements. On closer examination, however, this outcome was predictable.
To begin with, Clements was "a theorist on a grand scale" (Brewer, 1988, p. 509). Elaborate schemes and meticulously crafted terms notwithstanding, Clements was not a reductionist and he did not concern himself with details of exceptions that deviated from general patterns of vegetation. The temporal and spatial dimensions of his all-encompassing view were millennial and regional. He envisioned cycles of vegetation development within ever-larger and longer cycles of dynamic plant communities (successional paths called seres inside of longer seres inside of still yet longer seres) from the beginning of earliest plant life on Earth until death of the planet. This general pattern was repeated in miniature—though more rapidly—along the path of the "innermost", temporally the most rapid, seres. This was the analogy of ontogeny that was part of Clements overall organismic view of vegetation ("climax as a complex organism" [Clements, 1936, p. 253]). Few ecologists, even co-authors like Victor Shelford, could understand completely this grandiose scheme (see for example Croker [1991, ps. 88-89]). It stands to reason that this overall sweeping theory would not be accepted: it could not be grasped by enough of Clements' audience no matter how loyal and "orthodox".
Secondly, was the issue of Clements' terminology, which proliferated at what must have seemed exponential rates as more and more communities were found that appeared to be climax, not the climax of the region or the climax that was typical of the prevailing climate (ie. there were a lot of exceptions to monoclimax, the climatic climax, and the general regional rule). The response of Clements and his devout followers was to create ever more elaborate hierarchial schemes (Ricklefts, 1979, p. 733). The consummate product of this effort was his paper, "Nature and Structure of the Climax" (Clements, 1936), usually thought to have been at least partly in response to the major paper advocating polyclimax by his friend Tansley (1935). Some of the clarifications in "Nature and Structure" were helpful and terms like consociation and faciation found some use (eg. they were used by Braun [1950] and throughout the encyclopedic The Ecology of North America [Shelford, 1963]), but they did not turn the tide for Clements' monoclimax.
Most importantly was the problem of application of the monoclimax perspective to actual management. It is ironic that the man who wrote for natural resource practitioners in order that they could apply realistic practices to range, forest, and farm fields did so on a spatial scale and in a time frame that it was impossible for them to apply other than in vague, general terms. A climax of regional scale (ie. a formation) that was thus determined by regional climate (climax, climatic climax, regional climax, formation, and, later, biome were all synonyms) was simply too large to be used by farmers, ranchers, lumbermen, or those in the fledging conservation agencies attempting to advise producers in proper use of vegetation.
Clements took as the basic pattern for his model of plant succession the Geographical Cycle Theory of William Morris Davis. In Davis' theory land forms were formed primarily by land wasting due to climate both directly and indirectly (Davis, 1898, chapter XI, Climatic Control of Land Forms). Atmosphere and oceans directed the cycles of building up and wearing down of land and thus indirectly (as well as by direct controls) determined plant and animal life on Earth. As such, Clements interpreted succession as a series of nested seres and cycles within one another corresponding with the cycles of land formation. Clements' time scales were thereby inherently those of geologic time, at the longer span, and of the age (time duration) of a given climate at the shorter span. Clements' spatial scale was that corresponding to the size of climates, usually of a regional scale. As mentioned below in the Biome section below, the latest version of Holism applied at regional scale is the ecological region or macroecosystem devised by R.G. Bailey (1996, 1998) as units in Ecosystem Geography. In Bailey's system of ecosystem classification and hierarchy one hears the echo of Davis, Cowles, and Clements.
Further discussion of the role of Davis' theory of geological cycles was presented in the Origin of Clementsian Organicism portion of the Organicism section below.
By contrast Tansley's polyclimax theory was, in effect, more of local rather than regional size and scale. The polyclimax perspective allowed "closer looks". It made possible ecological management of smaller units of vegetation that were of the realistic size of pastures, allotments, forest blocks or even natural units of obviously different potential plant communities within the management units. These latter were range sites. While most authors in so many words imply that it was Clements vs. Tansley, monoclimax vs. polyclimax, this is a oversimplification. It is in fact, a misrepresentation of two perspectives or two views of vegetation where the difference was one of size: regional vs. local. Instead of being one or the other, it was one inside the other (numerous local polyclimaxes inside of the climatically determined average regional climax, hence the monoclimax). The conflicting "theories" were, simply and precisely stated, an issue of resolution— of spatial and temporal scale— and not, as is often mistakenly stated, a debate over whether climate, soils, fire, aspect, or buffalo determined the climax. In the terminology of what Schneider (1994, p. 19) called quantitative ecology (the use of scaled quantities in understanding ecological patterns and processes), the two views, while not expressed in numbers, hinged on scale-dependent patterns and scale-dependent processes (Schneider (1994, p. 4).
As the polyclimax enabled more local and thus more precise (probably also more accurate) management prescriptions at more workable scales, polyclimax was the overarching theory (or philosophical view) that came into acceptance (more or less) by rangemen, foresters, and wildlifers. At risk of oversimplifying, what seems most likely to have eventually "sifted out" in the United States was that polyclimax cleared the way for major use of edaphic climaxes within (ie. "inside of") the regional scale of the more general factor of climate. Again, several local polyclimaxes (especially edaphic climaxes) within one regional monoclimax. In other words, a co-existence just fell into place so that both viewpoints could be accepted: one inside the other so to speak (Tansley polyclimax units inside a larger, longer-term Clementsian monoclimax).
The thing that was responsible for this development (and relatively painlessly given the battles in the scientific literature) was widespread soil mapping, especially by the Soil Conservation Service. The edaphic climax nudged out climatic climax— the original climax as coined by Clements from the word climate— when edaphic (or perhaps topographic) climaxes were mapped and described as range sites.
It was thus that units of vegetation were rendered amenable for management by going from the largest to the smallest through an alignment of the two major prevailing theoretical and philosophical perspectives on plant communities.
From the earliest days of range work there were publications dealing with plant communities and ecological status in response to grazing (eg. Sampson, 1919). This research was later translated into published guidelines by which to judge range condition and trend or to determine utilization (eg. Pickford and Reid, 1942; Bisbee and Vogt, 1948; Beetle, 1950). These early guides did not include range sites. Instead they were generic for range types or larger geographic areas (ie. they reflected the prevailing monoclimax view and were not site-specific).
Somewhat later in the climax drama there was a brilliant third hypothesis-- like monoclimax and polyclimax, it is usually labeled a "climax theory"-- by Robert H. Whittaker (1953). This hypothesis viewed climax vegetation as a pattern. Unlike most of his contemporaries Whittaker realized that the mono- vs. polyclimax debate was more about scale than causal agents. Prior to his climax pattern paper Whittaker (1951, p. 28) wrote: "Succession does not really lead to a regional climax type. It leads to some kind of climax suited to the specific conditions of the site where succession occurs". Note the word site.
Whittaker's climax pattern theory (or concept= hypothesis) "…recognizes a regional pattern of open climax communities whose composition at any one locality depends on the particular environmental conditions at that point" (Ricklefs, 1979, p. 733). Spurr and Barnes (1980, p. 415) interpreted the climax pattern concept as "a logical extension of the polyclimax approach" where there is a continuum of successional paths and climax communities (the climax pattern) that change gradually or, sometimes, abruptly "…as site factors change along environmental gradients".
The climax pattern concept grew out of Whittaker's use of gradient analysis in forest vegetation. Findings from various studies that used gradient analysis (such as those of Whittaker) supported the Gleasonian individualistic or continuum concept of plant communities (Barbour et al., 1987, ps. 158, 228) so the climax pattern theory is sometimes interpreted as "… an extension of the continuum idea and the approach of gradient analysis to vegetation" (Krebs, 1972, p. 430 where he cited Whittaker [1953]). But this simplistic interpretation is not the exact or entire interpretation of Whittaker. Whittaker (1953) stated that the monoclimax and polyclimax theories assume that community-units exist as associations which are "valid, discrete units of vegetation" that comprise "distinct climaxes". The climax pattern view does not make this "community-unit assumption" but instead permits the relating of "community gradients to environmental gradients" in turn relating "the whole diversity of climax stands in an area to the whole range of environments" (Whittaker, 1953, p. 59). This view did indeed lead to gradient analysis, but climax pattern theory is "… both a synthesis of the other theories … " "… and an advance beyond them" (Whittaker, 1953, p. 60). As such, this "third climax hypothesis" "… is in part intermediate to or synthetic of the monoclimax and polyclimax …" and it regards these two "… as parts of a single, often continuously gradating climax pattern" (Whittaker, 1953, p. 60).
Students of climax and succession literature have frequently remarked that there is not nearly as much difference among these three climax theories as is often believed, and that anyway much of the argument has been over names while differences in perception are more of scale and time frame (eg. Krebs, 1972, p. 430; Spurr and Barnes, 1980, p. 415; Brewer,1988, p. 524). None other than Witttaker (1953, p. 60) volunteered that there was a "semantic aspect" to the climax argument, but in his view the refinement was a "decreasing degree of abstraction " in going from monclimax to polyclimax to climax pattern views. Similarity and common ground notwithstanding, debate over climax allowed interpretations and mental inventions that led to fairly standardized meanings of range site and range and forest cover type, at least within the Ecology based professions.
Relatively close timing of several events was probably important in establishment of range sites. It is probably more than coincidental that Whittakers climax pattern theory followed fairly closely after Watt (1947) reported on the element of "pattern" and the processes associated with patterns of vegetation. Sandwiched in between was the landmark paper by Dyksterhuis (1949) that either somewhat officially ushered in or provided a formalized basis for the Soil Conservation Service method of range condition and trend analysis. This method was based on range site descriptions and site-specific responses of plant species to grazing like those described by Clements (1920). Responses were also similar to those of disturbances like drought such as reported by the Nebraska school of Ecology (Weaver and Albertson, 1936, Weaver and Hansen, 1941; Weaver and Albertson, 1944). The great drought of the 1930s had tremendous impact on ecologists like John Weaver and Frederic Clements (Tobey, 1981, p. 191-221) and it was pivotal to formation of the Soil Erosion (later, Conservation) Service. Perhaps most time-critical of all was death of the immortal Clements in July 1945 (Tansley, 1947).
The concepts and perspectives generated by the climax theory debate has continued to spawn ideas and inventions in areas of Vegetation Science. The climax pattern concept visualized vegetation as a "mosaic of plant communities whose distribution is determined by a corresponding mosaic of habitats" (Whittaker, 1953, ps. 59-60 quoting Arthur Tansley in describing the vegetation of the British Isles). Here is a prelude of the concept of landscape mosaic that became a central concept and perspective in Landscape Ecology (cf. Forman and Godron, 1986) and Ecosystem Geography (cf. Bailey, 1996; Bailey, 1998), the newest subdisciplines based on ecological organization level and size. Landscape mosaic also became an important concept in Forestry as forest practices shifted away from the intensive management that dominated the profession after World War II (cf. Perry, 1994).
This sequence in the refinement of the climax concept moved from larger to smaller scale-- from regional units of vegetation (= climatic climaxes= regional climaxes= formations) to local units (sites, as in range sites). It set the stage for Ecosystem Geography to devise a hierarchial system of ecosystem units of scale spanning from the smallest (microscale) unit designated as site to the largest or broadest scale (macroscale) unit called ecosystem region or ecoregion (Bailey, 1996, ps. 23-26). The ecoregions are delineated by the ecoclimatic zones or regional climates (= macroclimates) of Earth (Bailey, 1996, p. 74-82) and the sites are determined by edaphic-topographic differentiation (Bailey, 1996, ps. 117, 121-127). In hierarchial classifications of ecosystems such as that by Bailey's ecoregion scheme are found pure components of monoclimax, polyclimax and climax pattern theories of vegetation. For example, the ecoregion unit essentially corresponds to the original monoclimax of Clements, the regional (= climatic) climax. After a half century of berating a cornerstone of the Clementsian paradigm, the most recent ecological classification returned full cycle to monoclimax in it's original form. "What goes around, comes around".
In the interim, however, range (and, probably, forest) site became established as the most specific work-a-day unit of management.
Microsite is the smallest possible unit of environment or habitat affecting a plant or small group of plants. It is an important part of the ecophysiology of individual or closely aggregated individual plants and is usually regarded as the microhabitat, the plant's microenvironment. This is usually thought of and expressed as microclimate (Rosemberg et al., 1983), the climate near the ground, climate in a small place, local climate, or position climate (Geiger, 1965).
Microsite and microclimate are certainly critical and constitute much of Autecology. Practical considerations in Range Management include the seedbed microclimate of reseeded range or hydrologic properties of range watersheds as affected by stubble or mulch conditions on the soil surface as resulting from, say, stocking rate.
Yet, as was explained in context of habitat type, microsite is too small to fit into a classification of vegetation or to be a management unit per se (which is not to say that manipulation of the microsite so as to achieve management objectives is impossible or unimportant). From a theoretical perspective the microsite could be interpreted as the most reductionist extension of the polyclimax. Alternatively, given the emphasis on microclimate, it could be seen as climatic climax reduced to its smallest size, as the Clementsian paradigm writ small.
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