CLASSIFICATION OF PLANT COMMUNITIES 309 



the Spartina glabra association are essentially similar through- 

 out; and so on. Habitats which thus are equivalent to one 

 another may be referred to a common habiiat-type. . 



In any given region, owing largely to the existence of these 

 parallel series of habitats, there have been developed corre- 

 spondingly numerous parallel series of associations. Different 

 individual associations which are correlated with the same type 

 of habitat and which as a result agree with one another in their 

 ecological aspect, i.e., which are ecologically equivalent, even 

 though the}^ may differ in their floristic composition, may be 

 considered as belonging to a common association-type. Thus, 

 the -Nymphaea association in one pond is obviously the ecologi- 

 cal homologue of the Nymphaea association in a neighboring 

 pond, while the Scirpus association in the one may similarly 

 correspond to the Typha association in the other. In the same 

 way, an oak forest association in one locality may represent 

 the equivalent, from an ecological standpoint, of a hickory 

 forest association in another locality; and so on. It will be 

 seen that, unlike the association, the association-type is a more 

 or less abstract conception. It may be simply defined as: a 

 type of plant association which is correlated with a given type 

 of habitat. The term association-type has already been used 

 by Schroter (20) in a sense analogous to that here proposed. 



The exact delimitation of association-types of course presents 

 many difficulties, but in a general way it is possible to group the 

 innumerable individual associations of a region into a compara- 

 tively'^ small number of association- types. To a limited degree 

 it is possible to refer to these association-types in terms of the 

 habitat (or of the edaphic unit area) thus : rock face association- 

 type, — middle beach association-type. But such a method 

 of nomenclature, while desirable in theory, has its limitations 

 in practice, owing in part to the difficulty of finding expres- 

 sions which even in a general way are descriptive of the habitats 

 concerned (especially when biotic factors are taken into account) 

 in part to the deficiency of our knowledge concerning the habi- 

 tat factors. It is therefore usually necessary, just as in nam- 

 ing the associations,. to resort to the vegetation for titles; thus: 



