April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 163 



immediately that the climatic formation represents an extremely complex vege- 

 tation-unit. But this great heterogeneity which impresses one at first thought 

 is counterbalanced by the fact that throughout any area of uniform climate 

 there is an unmistakable tendency, through the phenomenon of succession, for 

 vegetation everywhere to progress toward a condition of ecological uni- 

 formity — toward what may be termed the climatic climax association-type. 41 

 From the standpoint of the vegetation it is this climax by which the climatic 

 formation is primarily characterized and from which it is usually named. 

 Thus the climatic climax in the eastern United States is mesophytic deciduous 

 forest ; in the Great Plains region it is xerophytic grassland, etc. 42 



C. Physiographic Unit-areas and Physiographic Plant Formations 



Physiographic Unit-areas. — In regions having a uniform climate, differ- 

 ences in vegetation are associated primarily with differences in topography 

 and soil. These latter are not distributed in a wholly haphazard manner. 

 They have arisen in a more or less orderly fashion during the course of 

 physiographic development, and in consequence the various topographically 

 and edaphically determined habitats are arranged more or less definitely in 

 relation to various of the larger physiographic features of the region. It is 

 these larger and more outstanding features to which the term physiographic 

 unit is here applied ; generally speaking, such features as ravines, valleys and 

 flood plains, rocky uplands and sand plains, lakes, bogs, coastal swamps, and 

 the like. 



Like its climatic counterpart, a physiographic unit-area tends to exhibit 

 more or less pronounced environmental uniformity of a sort; there are certain 

 habitat conditions which prevail throughout the entire area. Thus a ravine 

 is characterized by its relatively high atmospheric humidity and by its protec- 

 tion from sun and wind ; a lake by the presence of water ; a salt marsh by the 

 presence of salt water ; a rocky upland or a sand plain by certain peculiarities 

 of soil or topography, etc. But, on the other hand, the recognition of an area 

 as a physiographic unit does not imply that it possesses absolute uniformity 

 of topography and soil. 43 On the contrary, when examined in detail, there 

 may be very considerable local diversity of habitat ; such, for example, as that 



41 For further discussion of succession and climatic climax, see section VII. 



4J In describing a climatic formation in the abstract, the point to be emphasized is 

 the definite or constant character, in physiognomy and ecological structure, of the climatic 

 climax association-type. In the case of the concrete example of the formation, as already 

 mentioned, it is the homogeneity or uniformity in this respect which is most important. 

 The formation concrete, it may be added, is further characterized by a more or less dis- 

 tinct floristic individuality, but in this respect there may be and commonly is little or no 

 resemblance between the different concrete representatives of the same abstract forma- 

 tion. 



43 Any more than a climatic region is characterized throughout by absolute uniformity 

 of climate. 



