There are many approaches to the rational classification of 

 vegetation units, and since these units themselves are more or 

 less arbitrarily delimited, the classifications are likewise arbi- 

 trary.2°^'^3 Various characteristics have been employed as the 

 chief criteria, the choice depending largely upon the purpose 

 and viewpoint of the classifier, widely used ones being species 

 composition, physiognomy and life-form, and the successional 

 status. In empirical classification one of the most widely used 

 criteria, partly because of its convenience, is the nature of the 

 habitat. The choice of criteria is influenced greatly by the geo- 

 graphic extent of the vegetation, by one's knowledge of the plants, 

 and the time and facilities that are available for field work. Re- 

 connaissance of a 100-sq-mile area, especially in a little-known 

 region, requires the use of different criteria from those employed 

 in the analysis and mapping of types on only a few square miles 

 or less. 



When extensive areas are being investigated, differentiation, 

 as a rule, is first made of large physiognomic units such as conif- 

 erous forest, deciduous forest, shrubland, and grassland (Figures 

 1-1 and 2-6). Each of these units may then be subdivided on the 

 basis of life-form, dominance, successional status, or habitat. 

 Some workers, as Riibel, Brockman-Jerosch, and Daniker, have 

 stated that it is impossible to synthesize large units from the an- 

 alysis and description of small communities,^^ but others, includ- 

 ing Braun-Blanquet^^-^*' and Tuxen,^^^ maintain the opposite 

 viewpoint, and have produced systems of vegetation by this in- 

 ductive method. In a limited area such as a few square miles, 

 small units, recognized chiefly on the basis of homogeneity in 

 species composition, may well serve as the starting point. 



Five major criteria, listed below, have been widely used in the 

 classification of plant communities and will be discussed and ex- 

 amples of their use given. 



( 1 ) Floristic or species composition 



(2) Ecological relations or habitat 



(3) Successional status 



(4) Physiognomy 



(5) Geographical characteristics 



Bases and Vnits • 177 



