January, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 1 7 



it is going but one step further to definitely consider all the pitch pine com- 

 munities in existence as collectively comprising one great association. This, 

 of course, is one sense in which the association has actually been interpreted 

 by some ecologists, including Tansley ; 9 it is a sense in which some will doubt- 

 less continue to apply the term. 



Plant Association Defined. Comparing the idea of the plant association 

 as a concrete piece of vegetation with that of the association as an abstract 

 vegetation-unit, the point to be emphasized in the first case is the homogeneity 

 or uniformity in the structure of the vegetation ; in the second it is the con- 

 stancy or definiteness of this structure. Viewed in the concrete, a plant asso- 

 ciation 10 may be defined as a plant community characterized by its essentially 

 homogeneous physiognomy and ecological structure and by its essentially 

 homogeneous floristic composition, at least with regard to dominant species. 

 Viewed in the abstract, the association may be defined as a vegetation-unit 

 characterized by an essentially constant physiognomy and ecological structure 

 and by an essentially constant floristic composition, at least with regard to 

 dominant species. 11 



C. The Naming of Plant Associations 

 In a general way, associations can be designated in terms of their domi- 

 nant species: for example, pitch pine association; oak-hickory association; 

 Scirpus-Typha association. More accurately they should be named in terms 

 of both dominant and subdominant species : for example, pitch pine-scrub oak 

 association ; oak-hickory-laurel association. 



III. The Plant Association in Its Relation to the Habitat 



A. The General Situation 

 Habitat Defined. The term habitat may be taken to include " everything 

 relating to the factors operative in a geographically definite locality, so far as 

 these factors influence plants " (Flahault & Schr6ter, '10). " It is the exact 

 equivalent of the term environment, though the latter is commonly used in a 

 more general sense" (Clements, '05, p. 18). Habitat, to be sure, somehow 

 suggests more the idea- of ground relations — the "kind of situation" a plant 



9 Tansley has expressed himself as completely in agreement with the proposals set 

 forth in my circular letter, but he also strongly advocates this aggregate interpretation. 

 "It is this larger concrete entity," he writes (25 Oct., 1921), "which I should regard 

 as the ' quasi-organism.' " To me the aggregate view of the association seems altogether 

 too intangible to be of practical value ; at any rate I am extremely reluctant to regard 

 the association in this sense as being in the nature of an organism. The organism or 

 quasi-organism concept is one that I shall not discuss further in the present paper. 



10 The " Assoziationsindividuum " of Braun-Blanquet ('21). 



11 These definitions, with minor differences in wording, were recommended in my 

 circular letter of October, 1921, and met with the approval of a large majority of the 

 ecologists to whom they were submitted. 



