THE CLIMAX AS A BASIS. 179 



In studying the forest vegetation of eastern America by plotting the ranges 

 of dominant trees, Transeau (1905 : 886) confirms the results of earlier observers 

 as to the existence of four distinct forest centers, namely : 



" (1) The Northeastern conifer forest centering in the St. Lawrence basin, 

 (2) the deciduous forest, centering in the lower Ohio basin and Piedmont 

 plateau; (3) the Southeastern conifer forest, centering in the south Atlantic 

 and Gulf Coastal plain; and (4) the insular tropical forest of the southern part 

 of the Florida peninsula, centering in the West Indies. The term center, as 

 here used, implies the idea of distribution about a region where the plants 

 attain their best development. Such vegetation divisions are not fixed, but 

 move and increase or decrease in extent depending upon continental evolution 

 and climatic change. 



"It has been found that if the ratios, produced by dividing the amount of 

 rainfall by the depth of evaporation for the same station, be plotted on a map, 

 they exhibit climatic factors which correspond in general with the centers of 

 plant distribution. Further, the distribution of grassland, prairie, open forest, 

 and dense forest regions is clearly indicated. This is explained by the fact 

 that such ratios involve four climatic factors, which are of the greatest impor- 

 tance to plant life, viz, temperature, relative humidity, wind velocity, and 

 rainfall." 



Recently, Livingston (1913 : 257) has integrated the temperature and 

 moistiure relations of the climatic areas, and has developed a general method of 

 determining the climatic control of climax formations. We are still far from 

 the final method for delimiting climaxes and their climates. It seems clear, 

 however, that it must be based primarily upon the range of consociations, and 

 upon the measurement of the growth and reproduction of their dominants in 

 relation to the water and temperature conditions of both the growing and 

 resting periods (plate 50, a, b). 



Climaxes of North American vegetation. — Clements (1902 : 15; 1904 : 160) 

 has made an analysis of North American vegetation upon the basis of tempera- 

 ture and water zonation, in an endeavor to determine the great vegetation 

 centers. The major continental zones were thought to be due to temperature 

 and water, and their interruption to the decreasing rainfall and increasing 

 evaporation toward the interior, as well as to the disturbing effect of mountain 

 ranges. The 17 provinces were supposed to indicate as many vegetation 

 centers, but they were determined floristically, by the superposition of the 

 ranges of dominants, and not developmentaUy. Hence, while most of them 

 correspond to climax formations, some obviously do not. With the recog- 

 nition of the formation as the major unit of vegetation, the question of zones, 

 regions, provinces, etc., becomes of minor importance. These are geograph- 

 ical distinctions based upon floristic, while the developmental method 

 demands vegetation distinctions based upon climaxes and the course of suc- 

 cession. 



The division of North American vegetation into the following climaxes or 

 formations is based upon development in so far as our present knowledge 

 permits. It is necessarily tentative in the highest degree, and perhaps its one 

 value Hes in suggesting probable climaxes for critical investigation. Until these 

 areas have been monographed developmentaUy by means of exact quadrat 

 and instrumental methods, any determination and limitation of chmaxes must 

 remain purely provisional. The arrangement of the chmaxes is based upon 



