CLASSIFICATION OF PLANT COMMUNITIES 345 



THE UNIT OF VEGETATION WITH REFERENCE TO CLIMATE 



Tfie climatic formation. Climatic factors determine the larger 

 features of the plant covering of the earth. Owing to widespread 

 differences in climate, there are correspondingly widespread 

 differences in the general ecological aspect of vegetation. The 

 vegetation of any region in which the essential climatic relations 

 are similar or uniform throughout, taken in its entirety, is here 

 regarded as constituting a climatic formation. The climatic 

 formation, then, bears a similar relation to the climate of the 

 earth that the edaphic formation bears to the physiography of a 

 climatic region; a similar one to that w^hich exists betw^een the 

 association and the habitat-complex of a physiographic unit area. 

 In other words, if the association is the ecological unit of veg- 

 etation from the standpoint of the habitat, and the edaphic 

 formation is a unit from the standpoint of physiography, then 

 the climatic formation is a unit from the standpoint of climate. 

 Individual climatic formations are usually designated by com- 

 bining the name of the geographic region concerned with that 

 of the climax association-type of the formation; thus: the de- 

 ciduous forest formation of eastern North America, the Great 

 Plains short-gi'ass formation, the sage-brush desert formation 

 of the Great Basin. 



The developmental concept of the climatic formation. Clements 

 (4, pp. 3, 124-127) has adopted the developmental concept of 

 the formation with reference to the climatic formation (in w^hich 

 sense he uses the woi'd formation). He states that, "The unit 

 of vegetation, the climax formation, is an organic entity. As 

 an organism, the formation arises, growls, matures, and dies. 

 Its response to the habitat is shown in processes or functions and 

 in structures which are the record as w^ell as the result of these 

 functions. Furthermore, each climax formation is able to 

 reproduce itself, repeating with essential fidelity the stages of 

 its development. The life-history of a formation is a complex, 

 but definite process, comparable in its chief features with the 

 life-history of an individual plant. The chmax formation is 

 the adult organism, the fully developed community, of w^hich 



