CLASSIFICATION OF PLANT COMMUNITIES 313 



thesis which the writer followed in his earlier field-studies; but 

 observations continued over a number of years have made it 

 seem increasingly evident that such an assumption is unten- 

 able from the standpoint of contemporaneous dynamic plant 

 geography. This hypothesis fails either to accord with theoreti- 

 cal considerations or to harmonize with observed facts. 



Edaphic influences as limiting factors in succession. Now of 

 course it is not disputed that the trend of succession is almost 

 universally mesotropic; that in the vast majority of areas where 

 the phenomenon of succession is taking place there is a constant 

 tendency for the vegetation to approach in its ecological aspect 

 that which characterizes the regional climax association-type. 

 However, it is certainly true that the rate of succession is in- 

 fluenced to a marked degree by the edaphic factors; and, in the 

 opinion of the writer, the effect of these factors goes even fur- 

 ther: not only do they influence the rate of succession; they also 

 determine the extent to which the succession can proceed. In 

 other words, they place a limit on the degree of mesophytism 

 which is capable of attainment. The point which it is desired 

 to emphasize is simply this: that while in edaphically favorable 

 situations the regional climax association-type is capable of 

 attainment, in edaphically unfavorable situations the succes- 

 sion may become arrested at a point far short of this climax; and 

 that in this way there may arise associations which, while they 

 are less mesophytic than the regional climax association-type, 

 nevertheless are permanent with reference to the coeval climatic 

 conditions and must therefore be regarded as climax-types. 



This point is capable of logical demonstration, somewhat 

 as follows. In the first place, it may be stated that the ecolog- 

 ical aspect of any plant association is the combined function^ of 

 those factors which determine the environment; in other words, 

 the aspect of the vegetation is conditioned by the combined 

 influence of the climatic and the habitat factors. Assuming, 



' It would be perhaps more accurate to state that the ecological aspect of 

 a plant association is a function of eich of the integral factors which in combina- 

 tion determine the nature of the environment. It is thus a function of tempera- 

 ture, a function of light, etc. 





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