126 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Bouteloua association). This seems to be related to three interacting causes. 

 Perhaps the most important is the small amount and the character of the 

 summer rainfall, and the rapidity of evaporation. Coupled with this are the 

 dense sod and the fine mass of shallow roots which limit penetration largely 

 to the upper foot or two. A third factor is the extensive grazing which has 

 supplemented the first two by decreasing the competitive vigor of the sub- 

 dominants or by actually destroying them. In the desert plains (Aristida- 

 Bouteloua association), somewhat similar conditions prevail, and societies are 

 relatively few. This essential water relation between the consociations and 

 the societies is well shown in regions where the rainfall or water-content is 

 locally increased. The number, extent, and dominance of societies are greatly 

 augmented in the case of mixed prairies along the Pine Ridge escarpment in 

 Northern Nebraska, the east edge of the Black Hills, and the Front Range of 

 the Rocky Mountains in Colorado. This is likewise true in the sandhill 

 region of central Nebraska, where the chresard or available soil-water is 

 exceptionally high. 



Relation to consociation. — While there is no necessary connection between 

 consociation and society, there is a more or less evident correlation based 

 upon water requirements and floristics. It must be clearly recognized, how- 

 ever, that consociations are not divided into societies as associations are into 

 consociations. The entire area of the association is occupied by its consocia- 

 tions, in pure alternes or in mixtures, except where succession is in process. 

 The same area will likewise show societies, but they will mix and alternate, or 

 replace each other without any clear relation to the consociations. This is 

 partly due to their large number and partly to the fact that subdominants 

 are naturally susceptible to variations in the composition, density, and vigor 

 of the grass communities as well as to local differences in the habitat. For 

 example, Psoralea tenuiflora is one of the most important of grassland societies. 

 It is essentially a formational subdominant in that it occurs in all of the asso- 

 ciations with the probable exception of the bunch-grass prairie. Yet its height 

 and density, and hence its degree of dominance, differ in practically all of them. 

 It reaches its best expression in the Stipa spartea consociation, but is usually 

 replaced in the related Agropyrum and Andropogon consociations by its com- 

 plementary subdominant, Psoralea argophylla. Of the hundred or more 

 societies, the majority occur in at least three associations, and usually three 

 contiguous ones. So closely related are the associations in conditions and 

 floristics that hardly a single society is known to be restricted to one associa- 

 tion. The societies of the most limited extent are those which have been 

 recently derived from other formations. They naturally occur in the sub- 

 climax, the desert plains, or in the bunch-grass prairie, since these are the 

 marginal associations of the grassland. 



Origin. — This fact corresponds with a general grouping of societies with 

 reference to the flora from which they come. The grassland has approximately 

 100 societies, of which more than 40 are derived from the southwest, and about 

 30 from the east or southeast. Some of the latter were doubtless from the 

 south or southeast originally. A few are apparently indigenous and a small 

 number are Pacific, and hence probably southern also. The large number of 

 southern elements agrees with the southern derivation of most of the grass- 

 land dominants. The fact that Bouteloua, Aristida, and Bulbilis have pushed 



