BASES AMD UNITS 



BASES OF CLASSIFICATION 



An extensive area of vegetation varies from one part to another 

 because of differences in topography such as north and south 

 slopes, soil conditions, or stages of succession. A large area pre- 

 sents too much complexity in vegetation and environmental 

 conditions to study as a whole, so it must be separated into sub- 

 divisions that are usually recognized by dissimilarities in physiog- 

 nomy, species composition, or often in topography. Each basic 

 unit is a stand, or individual plant community — an aggregation 

 of species with considerable uniformity in structure and composi- 

 tion, and occupying an area of highly uniform environment.^'*'* 

 Each stand has its own individuality, and is marked by the pos- 

 session of certain biotypes, and often ecotypes also, that are not 

 found in other, similar stands; ^'*°''** however, a number of bio- 

 types or ecotypes may be present in common in several stands. 

 Delimitation of a stand is not difficult when it has sharp 

 boundaries (Figures 1-4 and 1-20), which may be caused by 



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