April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 173 



important point here is simply this : that in comparing the climax vegetation 

 of different climatic regions, the thing that concerns ns chiefly is the type of 

 climax which most fully expresses the influence of climate as a differentiating 

 factor of the environment. For any particular climatic region this condition 

 is best fulfilled in habitats where localized climatic conditions are absent and 

 where vegetative development is in no way interfered with by unfavorable 

 conditions of soil or topography. Generally speaking, therefore, it seems 

 logical that, for any particular climatic region, the most mesophytic or ad- 

 vanced type of climax which is capable of development on " ordinary uplands," 

 under favorable conditions of soil and topography, should be taken as the 

 climatic criterion. In other words, physiographic climaces of this description 

 represent the climatic climax. 59 



The important part played by biotic factors, especially humus and shade, 

 in compensating the effect of unfavorable physiographic factors, has been 

 emphasized, in particular, by Cowles ('01, '11). In consequence of this influ- 

 ence, during the course of a developmental succession the effect of physi- 

 ographic factors on the vegetation tends to become less and less pronounced, 

 with the result that while the more primitive associations in a series may be 

 closely restricted by physiographic conditions, those which are more advanced 

 tend to overstep and even to ignore physiographic boundaries. Hence, in 

 general, associations which are developmentally mature tend to resemble one 

 another much more closely than associations which are immature ; and vice 

 versa. " The general trend of vegetation is from diversity toward uniform- 

 ity " (Cowles, '11). Notwithstanding this influence of biotic factors, how- 

 ever, it is only in regions of comparatively uniform physiography, or where 

 the influence of physiography is more or less completely offset by that of 

 climate, 60 that it is impossible to distinguish physiographic climaces. 61 



69 In this connection it should be mentioned that the climatic climax does not neces- 

 sarily represent either the most mesophytic or the most advanced type of vegetation that 

 is capable of development in a climatic region. In places such as ravines where the 

 regional climate is ameliorated as a result of topographic conditions, or in localized situa- 

 tions where unusual soil conditions compensate the influence of unfavorable climate (as 

 along streams in desert regions), a degree of mesophytism (or advance) may be attained 

 which surpasses that exhibited by the regional climax (the "post-climax" of Qements 

 '16). Neither is the climatic climax necessarily represented by the most widespread 

 type of natural vegetation in a region, although this is commonly the case. On the 

 Atlantic Coastal Plain, for example, the yellow pine forests, which constitute the 

 prevailing type of vegetation over large tracts of country from North Carolina south- 

 ward, do not represent the type of climax favored by climate on the better soils, but 

 rather a climax . . . due primarily to unfavorable soil conditions. 



00 Extreme humidity in tropical regions may more or less completely offset the influ- 

 ence of physiographic factors ; while extreme aridity in some desert regions may accom- 

 plish a somewhat similar result. 



61 For further discussion of the influence of physiographic factors on vegetation, 

 see pp. 163-167 of this paper, and - also Nichols '17, pp. 313-317. 



