April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 159 



is inevitable that there should always be " splitters " and " lumpers," and this 

 regardless of how we define our terms. 



General Application of the Association-type Idea. — Unlike the concept of 

 the association, the association-type idea is not limited in its application to 

 the vegetation of regions which are floristically continuous. In fact, its use- 

 fulness becomes most apparent when the vegetation of regions which are 

 totally unlike floristically is compared. The southern California and the 

 Mediterranean coast regions, for example, have nothing in common floristi- 

 cally, but because of the similarity in climatic conditions the associations in 

 the one region tend to be paralleled in the other by associations which are 

 similar in their physiognomy and ecological structure — i.e., by associations 

 which can be referred to a common association-type. And even regions which 

 are both floristically and climatically unlike tend to possess certain association- 

 types in common — types such as occur in lakes or on rock cliffs, or in other 

 situations where physiographic factors exercise a controlling influence in 

 determining the ecological character of the vegetation. 



The Association-type as a Basis for Ecological Classification. — In adopt- 

 ing the association-type as a basis for the ecological classification of plant 

 communities, the vegetation itself, as the product of environment, is taken as 

 the criterion. This indirect method of estimating the relation between vege- 

 tation and environment is by no means illogical, any more than it is illogical 

 to deduce the facts of evolution from the study of existent organisms. At 

 the same time it should be emphasized that the individual plants which go to 

 make up any particular piece of vegetation are capable, for the most part, of 

 adapting themselves to a rather wide range of habitat conditions. It follows, 

 on the one hand, that the vegetation can give but an approximate measure of 

 the habitat, and. on the other, that an exact parallelism between associations 

 growing under essentially similar conditions of environment is not to be 

 expected. The agreement, both of vegetation and of habitat, is only approxi- 

 mate. 35 



Physiognomy and ecological structure have been made the primary basis 

 for an ecological classification of plant associations by various European 

 writers, notably Warming ('09, '18), Brockmann & Rubel ('12), and Raun- 



35 From what has been said, it is to be expected that different associations be'onging 

 to the same association-type will be approximately parallel in their habitat relations ; i.e., 

 the nature of the habitat will be approximately identical wherever these associations occur. 

 Identity in regard to the habitat as a whole, however, does not necessarily mean identity 

 in regard to each individual habitat factor. The nature of the habitat as a whole, as 

 already stated, is determined by the combined influence of all the locally effective habitat 

 factors ; but different individual factors are able to offset one another's influence to such 

 a degree that it is possible for different sets of factors, in combination, to produce essen- 

 tially the same result. Thus it is that the same association, growing in two regions which 

 are somewhat dissimilar climatically, may appear to occupy entirely different habitats, 

 the apparent difference in habitat being explained by the difference in climate. 



