312 GEORGE E. NICHOLS 



Permanent and temporary associations. Postulating thus the 

 continuance of a relatively uniform climate, from the standpoint 

 of dynamic plant geography two secular classes of plant associa- 

 tions are distinguishable: permanent and temporary, A per- 

 manent association is one which has reached a condition of 

 equilibrium with respect to climatic factors, on the one hand, 

 and habitat factors, on the other. It has attained the highest 

 degree of mesophytism which the nature of its environment will 

 permit. It represents the culminating member of a specific 

 successional series. A temporary association, on the other hand, 

 is one which has not reached such a condition of equilibrium. 

 Through the influence of various habitat factors it is destined 

 to become superseded sooner or later by a (usually) more meso- 

 phytic type of vegetation. It therefore represents merely a 

 transient stage in a given successional series. 



The concept of a uniform regional climax association-typ3. Un- 

 der favorable conditions, as a consequence of the progressive 

 reaction of the habitat factors and the concomitant succession 

 of plant associations, the vegetation of a given area may attain 

 the big'. est degree of mesophytism which t^ e climate of t'le 

 region permits. This type of association, which represents the 

 climax for the region, may be designated the regional climax 

 association-type.^ Now it is commonly stated or implied in 

 ecological literature that in every region, as the logical consum- 

 mation of progressive successional changes, the vegetation of 

 all soils and all types of topography is destined eventually to 

 acquire the same degree of mesophytism that characterizes the 

 regional climax association-type; that, while in unfavorable 

 situations the influence of certain habitat factors may diminish 

 the rapidity of the succession, it -does not alter the final out- 

 come; that ultimately, although the time may be indefinitely 

 postponed, the regional climax is destined to be attained in 

 all areas. If this idea is correct, then of course no associations 

 can be regarded as permanent except those which belong to the 

 regional climax association- type. This is the working-liypo- 



2 Clements (4, p. 128) would restrict the use of the term association to these 

 climax associations. 



