January, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 19 



habitat relations are essentially the same throughout the area occupied by the 

 association. It therefore follows, with regard to the various layer societies 

 within the association, that for each of these the habitat relations are essen- 

 tially homogeneous throughout, but that for different layer societies the habitat 

 relations may be quite dissimilar. Group societies may or may not be associ- 

 ated with local peculiarities of habitat. 



In this connection, it may be pointed out also that, in the case of the 

 dominant species of an association, the habitat relations differ at different 

 stages of development. In this respect immature individuals of these species 

 rank with the subordinate life-forms which they most nearly approximate. 

 Thus, immature trees in a forest association, at various stages of their develop- 

 ment, are subject successively to the same environmental influences (above the 

 surface of the ground, at any rate) as low shrubs, as tall shrubs, and as small 

 trees. This observation is an important one in its bearing on the successional 

 relations of plant associations, to be considered later. 15 



B. The Habitat Factors 



The character of the environment in an area occupied by a plant associ- 

 ation is determined by the combined influence of all the locally effective habitat 

 factors; in the words of Gleason ('17), it "consists of the resultant of all the 

 external factors." 16 The nature of these factors, which are responsible not 

 only for the ecological characteristics of the vegetation as it exists today, but 

 for the successional changes in vegetation which ensue in the course of time, 

 will now be briefly outlined. For convenience, they are divided into five 

 categories, namely, climatic, physiographic, biotic, anthropeic, and pyric. 17 



15 Attention should be called here to a recent contribution by Yapp ('22) on "The 

 concept of habitat," which was received after the present paper had gone to press. Yapp 

 describes the habitat as " the place of abode of a plant, a plant community, or in some 

 cases even a group or a succession of related plant communities, together with all factors 

 operative within the abode, but external to the plants themselves." He distinguishes 

 four main classes of habitat, viz., the successional habitat ("the changing habitat occu- 

 pied by an allied group of plant associations which, as a rule, comprise the stages of a 

 normal succession or sere") ; the communal habitat ("the general habitat of any recog- 

 nizable plant community, such as an association or a society"); the individual habitat 

 ("the habitat of an individual plant, whether solitary or forming part of a plant com- 

 munity") ; and the partial habitat (" the habitat of an individual plant during any given 

 period or stage of existence"). My own views, it need hardly be stated, are essentially 

 in agreement with those expressed by Yapp. 



16 Under the term habitat factor is included any condition which influences the nature 

 of the habitat. In the present paper the term factor is interpreted in a very general 

 sense and no effort is made to restrict its use with the exact precision which might perhaps 

 be deemed theoretically advisable. 



17 Anthropeic from the Greek avdpwireios, meaning human; of or belonging to man; 

 of which man is capable. Pyric from Greek ir^p, meaning fire. 



