classes. However, they are subordinate characteristics; their 

 presence modifies the dominant soil-forming processes. 



Families — There are currently 6,111 families defined 

 by physical and chemical characteristics that affect soil 

 use and response to management. Particle size, mineral- 

 ogy, temperature regime, and depth of root penetration 

 are examples. 



Series — There are about 14,800 series recognized by 

 the kind, thickness, number, and arrangement of hori- 

 zons. The range of properties, such as particle size, used 

 to define series may be more restrictive than those used at 

 the family level. 



Phases of classes within any category are sometimes 

 developed to serve specific purposes in individual soil sur- 

 veys. They are not part of the soil taxonomic hierarchy. 

 Any property or combination of properties that does not 

 duplicate limits for a taxonomic class can be used to dif- 

 ferentiate phases. The choices of properties and limits are 

 determined by the purpose of the particular soil survey 

 and by how consistently the phase criteria can be applied. 

 Some of the features commonly used in defining phases 

 in soil surveys are: texture of the surface layer; presence 

 of rock fragments, such as gravel or stones; slope; physi- 

 ographic position; salinity; kind of substratum; degree of 

 past erosion; and flooding potential. 



Vegetation Element 



The vegetation element embodies the modern con- 

 cepts of climax plant communities that distinguish between 

 original (pristine climax) and potential natural vegeta- 

 tion. The purpose for dealing with climax vegetation or 

 its facsimile is to establish natural vegetation potential. 

 Knowledge of potential natural vegetation provides a 

 basis for determining the natural biological potential of 

 an area. Natural plant communities with their individual 

 species evolve over long periods of time to adapt to specific 

 habitat conditions. The adaptation takes place through 

 adjustments of the individual plants for light, heat, space, 

 moisture, nutrients, and wild animals. These adjustments 

 eventually evolve to a plant community structure in 

 which individual species are regenerating, competition 

 for basic elements is quite stabilized, and a future differ- 

 ent community is unpredictable except when disturbances 

 such as logging, excessive grazing, and fire significantly 

 change the community structure. The recognition of 

 these phenomena by trained plant ecologists establishes 

 some guidelines for land management decisions. For 

 example, reforestation of a species within a specific area 

 where it does not naturally occur, should usually be 

 avoided, although exceptions may sometimes be justified. 



The rest of this section establishes the differences 

 among climax, original, natural, and potential natural 

 vegetation; it is a brief synthesis based on work by Tuxen 

 (1956, 1975) as translated by Kuchler (1967) and implied 

 and applied by Mueller-Dombois and Ellenberg (1974) 

 and Daubenmire (1978). 



Climax (pristine) plant communities, defined by 

 early writers, were communities that developed over 

 thousands of years to establish stabilized groupings of 

 plants (Weaver and Clements 1938). These groupings 

 constitute original vegetation that exists in the landscape 

 unaffected by recent human activities and that is in bal- 

 ance with the regional biotic and abiotic forces of its site. 

 Because much of the earth's surface has been affected by 

 recent human activity, the original vegetation is often 

 chiefly of historic and geologic interest. 



The natural vegetation of today, as compared to 

 original vegetation, exists in landscapes that have not 

 been converted to agricultural or urban uses. Most areas 

 of natural vegetation have been impacted to a greater or 

 lesser degree by human activities. However, in many 

 parts of the world, including the United States, natural 

 vegetation exists where human activity has been minimal, 

 and the vegetation is in balance with the biotic and abiotic 

 forces of the site. A future plant community is unpredict- 

 able in the absence of major disturbances. Thus, poten- 

 tial natural vegetation is related to "climax" vegetation, 

 provided "climax" is used in this modern context. Two 

 assumptions are necessary to obtain potential natural 

 vegetation: first, that human disturbances that cause 

 basic vegetation change are removed from the scene, and 

 second, that the resulting succession of plant commu- 

 nities is telescoped into a single moment to exclude effects 

 of major climatic changes and earth-shaping events, such 

 as vulcanism or diastrophism. Removing humans from 

 the scene does not exclude their marks in the successional 

 pattern toward potential natural vegetation. For example, 

 cheatgrass (Bromus tectorum) was introduced from 

 Eurasia about 1850, spread profusely throughout the arid 

 and semiarid Western United States, and became firmly 

 established under a wide variety of environmental condi- 

 tions. The species is an exotic in the area, but has natural- 

 ized because of its adaptive capabilities. Therefore, 

 cheatgrass should be regarded as part of the potential 

 natural community (Driscoll 1964b). 



Another kind of situation to establish potential natu- 

 ral vegetation of an area must be considered where human 

 influence has been extensive and dramatic. The reduction 

 or removal of human activity from the scene and the pro- 

 jection of vegetation succession to a relatively stable end 



