CLASSIFICATION OF PLANT COMMUNITIES 317 



which it occupies has been uninterruptedly out of water 

 and presumably covered with vegetation since upper Miocene 

 times; yet in spite of this fact, today it is still occupied by a 

 series of associations which, while they represent ''an old and 

 climax condition, ancestrally infinitely more ancient than any- 

 thing in the surrounding area" (23, p. 242), nevertheless, with 

 reference to the climax association-type of the region, must be 

 classed as primitive. It is significant that this particular series 

 of edaphic climax associations has apparently maintained it- 

 self in spite of changes in climate A stronger argument in 

 favor of the potency of edaphic factors in limiting succession 

 could hardly be conceived. 



A similar noteworthy example of a widespread edaphic climax 

 is seen in the natural prairie of western Long Island (see Harper, 

 13). Although, like the pine-barrens, situated in a region of 

 deciduous forests, this area, some 50 square miles in extent, at 

 least during the present chmatic era apparently has never be- 

 come forested, and there is no reason to believe that under the 

 present climatic conditions, even if left undisturbed, it would 

 undergo anj' appreciable changes in the future. 



The association complex. Sometimes the nature of the habitat 

 is essentially uniform throughout a given area, but more often, 

 within an area which from a physiographic or some other stand- 

 point it is desirable to treat as a unit, several different habitats 

 are represented. Thus, for example, in the case of a pond, very 

 largely as the result of differences in the depth of the water, 

 there are numerous distinct types of habitat. Similarly, a 

 salt marsh, a flood plain, a ra\'ine, a rock hill, a sand plain, or 

 a burned area may include a series of habitats which it is de- 

 sirable to treat collectively. Any such series of habitats may 

 be designated collectively as a habitat-complex. The term may 

 be defined as: a group or series of habitats which occupy a 

 unit area and are alike with reference to one or more habitat 

 factors. 



The conception of the habitat-complex, like that of the habi- 

 tat-type is capable of algebraic expression. In the case of a 

 pond, for example, let it be roughly assumed that P = pond, 



