65 



the Stipa-Agropyron prairie, the BulhiUs-Bouteloua plains, and 

 the Aristida-Bouteloiia desert plains. 



"The consociation is the unit of the association. It is char- 

 acterized by a single dominant. The association is actually a 

 grouping, the consociation is pure dominance." In the usual 

 treatment most consociations appear as associations. In the 

 Aristida-Bouteloua association, for example, the dominance of 

 Aristida gives rise to an Aristida consociation. 



Turning now to the serai units, "the associes is the develop- 

 mental equivalent of the association." "It is composed of two 

 or more consocies, i. e., developmental consociations, just as the 

 association consists of two or more consociations. Like the 

 association it is based upon life-form, floristic composition, and 

 habitat, but differs from it in as much as all of these are under- 

 going constant or recurrent developmental changes." While the 

 association is permanent, in so far as development is concerned, 

 the associes is transient, although it may persist for many years. 

 Thus Pinus-Pseudotsuga and Larix-Abies form associes in Idaho 

 which are developing towards the Thuja consociation of the 

 Pacific coast forest formation. Now where Pseiidotsuga domi- 

 nates over an area we have a Pseudotsuga consocies, for a 

 "consocies is a serai community marked by the striking or com- 

 plete dominance of one species, belonging of course to the life- 

 form typical of that stage of development." It differs from the 

 consociation only in that it is a developmental or serai, while 

 the latter is a climax community. 



Whether the investigator is dealing with climax or develop- 

 mental units can be determined only by the application of such 

 exact quantitative methods as are outlined under "the investiga- 

 tion of succession" (Chapter XV). An application of such 

 methods will go a long way towards solving the problems of 

 vegetational development of a region — a phase of ecology which 

 American investigators have recently shown to have a funda- 

 mental bearing upon forestry, grazing and agriculture. 



The concept of succession as a series of invasions, a sequence 

 of plant communities marked by the change from lower to higher 

 life-forms is analyzed at length. "The essence of succession lies 



