!74 GEORGE E. NICHOLS Vol. IV, No. 2 



VIII. Summary and Conclusions 



The essential points which it is aimed to emphasize in the present paper 

 are as follows : 



It is proposed that the term plant association be recognized as applicable 

 both to the abstract vegetation concept and to the concrete individual pieces 

 of vegetation upon which this concept is based. This two-fold usage tends to 

 harmonize the diverse senses in which the term has been applied by different 

 ecologists, and the idea has been indorsed by a large majority of American 

 and Canadian ecologists. 



Viewed in the abstract, an association may be described as a vegetation- 

 unit characterized by its essentially constant physiognomy and ecological 

 structure and by its essentially constant floristic composition, at least with 

 regard to dominant species. Viewed in the concrete, it may be described as a 

 piece of vegetation characterized by its essentially homogeneous physiognomy 

 and ecological structure and by its essentially homogeneous floristic composi- 

 tion, at least with regard to dominant species. 



The idea of the association as an organic entity is best fulfilled by the 

 association concrete. 



The proposed interpretation of the term does not preclude its further 

 application to a series of similar plant communities of associational rank, 

 considered as a concrete aggregate rather than an abstract vegetation unit. 



As subordinate communities within the association, societies of two sorts 

 are distinguished : layer societies and group societies. 



Taken in its entirety, an association is characterized throughout by essen- 

 tial homogeneity or constancy of habitat, but subordinate variations of habitat 

 within the association make it necessary to distinguish between the general 

 habitat relations of the association as a whole and the specific habitat relations 

 of its constituent elements. 



In defining the association, habitat uniformity is not specified as a criterion 

 for the reason that the association, as a vegetation-unit, is naturally delimited 

 by its inherent characteristics of vegetative form and structure. 



The nature of any habitat is determined by the combined influence of all 

 the locally effective habitat factors. These latter fall into five categories, vis., 

 climatic (regional climatic factors, local climatic factors, rhythmical changes 

 in climate, progressive changes in climate, etc.), physiographic (topographic 

 factors, edaphic factors, rhythmical changes in physiography, progressive 

 changes in physiography, etc.), biotic (shade, root competition, humus, soil 

 micro-organisms, plant or animal invasion, etc.), anthropeic (factors associ- 

 ated with the influence of human agencies), and pyric (factors associated 

 with the influence of fire). 



The ecological classification of plant associations consists in the arrange- 

 ment into common groups of different associations which are related to one 



