.<^o®h 



THE INTERPRETATION AND APPLICATION OF CER^ 

 TAIN TERMS AND CONCEPTS IN THE ECOLOGICAL 

 CLASSIFICATION OF PLANT COMMUNITIES. II 



GEORGE E. NICHOLS 



The Sheffield Scientific School of Yale University, New Haven, Connecticut 



THE UNIT OF VEGETATION WITH REFERENCE TO PHYSIOGRAPHY 



The edaphic formation. As has been stated earlier, from an eco- 

 logical point of view the fundamental unit of vegetation is the asso- 

 ciation, the ecological aspect of which is determined by the habitat. 

 Now in any given climatic region the habitats are not distributed 

 indiscriminately. They are grouped, as has just been suggested, 

 into more or less definite complexes, the boundaries of which are 

 determined primarily by the physiographic features of the region. 

 In brief, the various habitat complexes are associated with specific 

 physiographic unit areas. From the standpoint of the physi- 

 ography of the region as a whole, t^ese habitat-complexes deter- 

 mined by physiography represent edaphic divisions of a higher 

 order than the habitat; and likewise, from the standpoint of the 

 physiographic ecology of the region concerned, the association- 

 complexes which occupy these physiographic divisions represent 

 vegetational divisions of a higher order than the association. 

 Just as a ravine, a salt marsh, a rock hill, or a sand-plain may 

 be regarded in its entirety as a physiographic entity, so its vege- 

 tation, taken in its entirety, is to be regarded as an entity from 

 the standpoint of physiographic ecology. In other words, just 

 as the association can be regarded as a unit with reference to 

 a specific physiographic unit area, so the association-complex, 

 where it is determined by physiography, can be regarded as a 

 unit with reference to the physiography of the region This 

 physiographic unit of vegetation may be designated the edaphic 

 formation. The edaphic formation may be defined as: an as- 



341 



THE PLANT WORLD, VOL. 20, NO. 11 

 NOVEMBER, 1917 



