KINDS OF GROUPINGS 



A specific grouping of plants, including all the layers, is a 

 stand (Figures 1-11 and 2-3). It may be defined as a particular 

 aggregation of plants having a high degree of uniformity in com- 

 position and structure and occupying an area of essentially uni- 

 form environment. The terms comniunity, concrete community, 

 individual community, and phytocenose also refer to a specific 

 grouping of plants, but "community" may also be used for any 

 aggregation of plants. Within a more or less limited geographical 

 area similar groupings of species occur in similar habitats, whereas 

 different groupings are found under various combinations of en- 

 vironmental factors. Groupings are not identical, nor genetically 

 related, even within a limited area, but some do resemble one an- 

 other sufficiently, and differ from other groupings sufficiently, so 

 they can be classified together as one community type, abstract 

 community, or association. A community type or abstract com- 

 munity may be defined as a group of stands that are similar in 

 species composition and structure and occupy similar habitats. 

 An association, as defined by the Third International Botanical 

 Congress in Brussels, is ". . . the fundamental unit of phyto- 

 sociology, being a plant community of certain floristic composi- 

 tion, of uniform habitat conditions and of uniform physiog- 

 nomy." 20^ The association in the Clements sense is a subdivision 

 of the formation, occupying a large area, and recognized or de- 

 limited by its floristics, physiognomy, and organic relation to other 

 units in the formation.'*' Obviously, there is no correspondence 

 between the two definitions. 



The grouping of similar stands into one community type does 

 not require that the composition, abundance, and structure of the 

 stands be identical, nor that the habitats be identical; but it does 

 require a certain degree of resemblance, more than with members 

 of other community types (Figures 1-1 and 1-4). The similarity 

 in habitats lies not necessarily in the homology of the soils, but 

 in the similarity of conditions for plant growth, for a well-devel- 

 oped humus carbonate soil may support the same kind of 

 vegetation as an iron podsol soil; the soils are analogous, not 



60 * Species az&cl Popi»latioims 



