CHAPTER X 



THE DISTRIBUTION OF CLIMAX COMMUNITIES 

 PRESENT DISTRIBUTION OF CLIMAXES 



In the early nineteenth century, Humboldt drew attention to 

 the importance of climate in determining the distribution and 

 range of species, and Grisebach showed the possibilities of using 

 communities, instead of species, as units of study. These were the 

 beginnings of modern descriptive plant geography, which deals 

 with the extent and distribution of vegetation types, particularly 

 climaxes, and the reasons they occur where they do. The complex 

 nature of climate necessitated from the first separate consideration 

 of its components, and this led to oversimplified explanations of 

 plant distribution based upon single factors. Even Warming, 266 to 

 whom we are indebted for shaping the foundations of much of 

 our modern ecological philosophy, was confident that communi- 

 ties and their responses are primarily controlled by water. Among 

 the early geographers, Schimper 213 deserves special mention be- 

 cause he emphasized what is now generally recognized, namely, 

 that a complex of interacting factors determines vegetation. There 

 is still no simple means of expressing the effectiveness of the com- 

 plex. 



Merriam's 173 attempt to correlate all vegetational distribution 

 with temperature is illustrative of the search for a single factor 

 whose quantitative value would express climatic conditions. He 

 showed that zones with similar summer temperature character- 

 istics frequently have similar vegetation, but, unfortunately, he 

 assumed that because there was a correlation there must also be a 

 cause and effect relationship. His generalizations are, therefore, not 

 acceptable, and too many exceptions remain unexplained. 



A persistent search was made by Livingston and his associates 159 

 for a single quantitative value of physiological significance, which, 

 when plotted to indicate isoclimatic lines, would closely match the 

 distributions of major vegetation types. Summer evaporation rates, 

 temperature coefficients, and temperature indices based upon 



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