point under existing climatic, biotic, and abiotic conditions 

 would probably not result in establishment of potential 

 natural vegetation as previously described. Frequently, 

 human-induced natural plant communities become re- 

 markably stable and appear to be self-regenerating. These 

 relatively stable communities, although different from a 

 perceived potential natural community, represent the 

 highest stage of predictable succession, and should be 

 considered a quasi-potential natural community for the 

 area since the future would be predicted with a high de- 

 gree of uncertainty. For example, portions of the large 

 expanse of existing coniferous forest communities in the 

 Southeastern United States have been perceived to be a 

 substage of a deciduous forest region (Dice 1943, Weaver 

 and Clements 1938). Intensive agriculture such as cotton 

 farming, harvesting of deciduous tree species, and inten- 

 sive forest management for coniferous species has led to 

 remarkably stable communities. Even if humans were 

 removed from the scene, the return of deciduous trees 

 would likely be minor, if at all, depending on the time- 

 span. These communities must be considered the highest 

 stage of natural succession and represent the potential of 

 the area. 



Climax communities were described by early investi- 

 gators (Weaver and Clements 1938) according to zonal- 

 ity; communities developed under "normal" conditions 

 of prevailing and generalized climates. Other communi- 

 ties, developed under restricted conditions within a zone, 

 such as steep slopes or restricted drainage, were regarded 

 as nonclimax. It is now recognized that many climaxes 

 representing potential natural vegetation exist under con- 

 ditions differing from zonal situations. These conditions 

 are recognized by using edaphic, topographic, fire, and 

 zootic adjectives. These modifiers indicate the nature of 

 influences that determine the resulting stable communi- 

 ties which differ materially from the "normal" situation 

 (Daubenmire 1968). 



A description of the hierarchy and a discussion of 

 the linkage between potential natural vegetation and suc- 

 cessional communities follows. Successional communities 

 as used here refers to those communities which develop 

 after disturbances such as logging or grazing and the out- 

 come of the successions are predictable potential natural 

 communities. 



Description of the Hierarchy 



The vegetation element is a six-level hierarchy (see 

 above). The levels are class, subclass, group, formation, 

 series, and association. The upper four levels are pat- 

 terned after a world vegetation classification system 



(UNESCO 1973). These levels are based primarily on 

 growth form and outward appearance, modified at some 

 levels by moisture and temperature regimes. Some crite- 

 ria of the upper levels have been modified to facilitate 

 consistent identification of potential natural communities 

 in the United States. These levels (class, subclass, group, 

 and formation) are shown in appendix A. The two lower 

 levels, series and association, are based on more detailed 

 analyses of the plant communities including plant domi- 

 nance and community structure. Therefore, the highest 

 level of the classification is based on general classes of 

 vegetation; the lowest level of the classification is based 

 on discrete plant communities. 



Descriptions of the six categories of the potential 

 natural vegetation element follow. 



Classes — There are five mutually exclusive classes 

 based on dominant growth form and cover. 



I. Forest: Communities formed by trees 5 with a 

 canopy cover of 61 percent or more at maturity, 

 tree crowns usually interlocked. This class in- 

 cludes forested wetlands. 



II. Woodland: Communities composed of trees with 

 a canopy cover of 26 to 60 percent at maturity. A 

 herbaceous and/or shrub understory is usually 

 present. This class includes woodland wetlands 

 and the "open-forest" described by some authors. 



III. Shrubland: Communities composed of woody 

 perennial shrubs, individual plants generally with 

 multiple stems, 1.6 to 16 ft (0.5 to 5 m) tall at 

 maturity, with 26 percent or more canopy cover. 

 This class includes shrub wetlands. A tree canopy 

 cover 25 percent or less may be present. 



IV. Dwarf-shrubland: Communities with 26 percent 

 or more canopy cover of perennial woody plants 

 rarely exceeding 1.6 ft (0.5 m) in height at matu- 

 rity. This class is sometimes called heaths or 

 heathlike formations and includes dwarf shrub 

 wetlands. Tree or shrub canopy cover 25 percent 

 or less may be present. 



V. Herbaceous vegetation: Communities dominated 

 by grass, grasslike, or forb vegetation with or 



'For the purposes of this classification, a tree is a woody perennial, 

 usually single-stemmed plant that has a definite crown shape and char- 

 acteristically reaches a mature height of at least 16 ft (5 m); this size 

 requirement may be altered where ecological research or experience pro- 

 vides a basis for adjustment. In high altitude, high latitude, or xeric situ- 

 ations, the tree height limit may be only 10 ft (3 m). In tropical situations 

 tree height may be from 27 to 33 feet (8 to 10 m). Some species of oak 

 (Quercus), juniper (Juniperus), willow (SalLx) and other plants may 

 grow either as trees or shrubs. 



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