January, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 1 3 



is meant its general outward appearance — its external morphology, so to 

 speak. When we refer to a piece of vegetation as deciduous forest or ever- 

 green forest or thicket or meadow, for example, we are speaking of it in terms 

 of physiognomy. The physiognomy of a plant community, in turn, is deter- 

 mined by the nature of the more prominent plants which enter into its com- 

 position; more precisely, it is determined by the character and abundance of 

 the dominant life-forms. The term life-form is here used in its broadest 

 sense to include all features of form and structure by which a plant is adapted 

 to cope with various conditions of environment — peculiarities, for example, in 

 size and habit of growth, in the duration of the plant as a whole and of the 

 stem above ground, in the duration and structure of the leaves, in the position 

 and structure of the resting buds, in the extent and structure of the roots, 

 and in the methods of vegetative reproduction. 4 Briefly stated, life-form is 

 the visible expression of those features in a plant which determine its adapta- 

 tion to environment. 



Under the head of ecological structure are included all peculiarities in 

 vegetation which are of ecological significance ; in fact, physiognomy repre- 

 sents but one phase of ecological structure. The idea of ecological structure 

 is much more comprehensive than that of physiognomy, however, since it 

 takes into consideration various peculiarities of the vegetation which do not 

 express themselves visibly through life-form, as well as those that do — such, 

 for example, as the behavior of the constituent plants in relation to various 

 conditions of light, temperature, and atmospheric humidity, or to various 

 water and other conditions in the soil. Thus, when we speak of plants as 

 being tolerant or intolerant of shade, mesophytic or xerophytic, we are con- 

 sidering features which have an important bearing on the ecological structure 

 of the vegetation, though not necessarily expressed through life-form. Eco- 

 logical structure also goes further than life-form in taking into consideration 

 the character and abundance of the various subordinate life-forms within the 

 community. Physiognomy, for example, would take no account whatever of 

 the various shrubby and herbaceous elements within a forest association, but 

 these would be included under ecological structure. In short, ecological 

 structure is to the plant community what morphological and physiological 

 structure are to the plant. 



As regards the floristic composition of a plant community, it is the rela- 

 tively common or conspicuous species which are of greatest importance — 

 species which are constantly preponderant either numerically or in mass effect : 

 primarily it is the species which typify the dominant life-form of the com- 

 munity (i.e., the dominant species) ; but species belonging to life-forms of 

 subordinate rank (i.e., subdominant species) have also to be considered. In 

 a forest, for example, both the dominant species among the trees which form 



4 For discussion of life-form and related concepts, see Pound & Clements ('98), 

 Raunkiaer ('05, '07), Warming ('09), Drude ('13), Gams ('18), and Clements ('20). 



