GENERAL RELATIONS. 107 



climax may persist for a long period and appear to be a true climax. In the 

 great majority of cases, however, the successional movement though slow is 

 constant, and there can be no question of the climax, especially when the 

 permanent quadrat is employed to reveal changes. 



Each climax formation falls readily into two or more major subdivisions 

 known as associations. Toward their edges these blend into each other more 

 or less, making a transition area or ecotone. The latter is broad in the case of 

 relatively level regions, and narrow in that of the climax zones of mountain 

 ranges. The associations have one or more dominants in common, or at least 

 belonging to the same genus, and there is a certain degree of similarity as to 

 subdominants also. Each association consists of several dominants as a rule, 

 though there may sometimes be as many as eight or ten or more, as in scrub 

 and chaparral. Each dominant constitutes a consociation. It may occur 

 alone, though as a rule it mixes and alternates with the other dominants of 

 the same association. This is the direct outcome of the similar requirements 

 of the dominants, and hence it is a guiding principle that two or more con- 

 sociations are regularly associated in the larger unit. This is emphatically 

 true of the associations covering a large area and possessing a rich flora, such 

 as prairie, chaparral, and forest. It is less striking in desert associations where 

 the dominants are often few, but even in the case of sagebrush and desert 

 scrub an extensive survey indicates that mixing of dominants is the rule. 

 Since no two consociations are exactly equivalent, there are often large areas 

 in which a single one occurs, such as the yellow pine in Arizona and the 

 Douglas fir in Oregon. Such areas are often due as much to migration and 

 successional factors as to differences in climatic requirements (cf. Zon, 1914 : 

 124). 



As will be seen later, there are more groupings of consociations than are 

 represented by the associations actually named. This is illustrated most 

 clearly by the basic association of the grassland climax, the Stipa-Bouteloua 

 poion. This association is named from the two most characteristic con- 

 sociations, but it contains several dominants, e. g., Stipa comata, Agropyrum 

 glaucum, Koeleria cristata, Bouteloua gracilis, Bulbilis dactyloides, Carex 

 filifolia, and C. stenophylla. It seems clear that a community of Stipa and 

 Bouteloua, or Agropyrum and Bulbilis, differs in nature and in indicator value 

 from one containing most or all of these. When detailed mapping of vegeta- 

 tion is undertaken on a large scale, all of these actual groups will demand 

 recognition as well as definite names. But for the present, it seems sufficient 

 to give names to the association and to each consociation, while recognizing 

 that the former will often be represented by groups containing only two or 

 three of the several dominants. 



Societies. — A subdominant is a species which controls an area within that 

 of a dominant or group of dominants. The actual community formed by a 

 subdominant is called a society. Its exact nature is best seen in forest or 

 prairie, where the control of the dominant vegetation-form, tree or grass, is 

 complete, though at the same time it permits a secondary control by a domi- 

 nating species of a different vegetation-form. Thus the yellow pine consocia- 

 tion of Oregon frequently shows a typical layer or society of Purshia tridentata, 

 the Douglas fir forest of the Rocky Mountains one of Thalictrum fendleri, and 

 the Stipa spartea prairies, mixed societies of Psoralea, Amorpha, and Petalo- 



