THE MIXED PRAIRIE. 137 



and are known to have covered much of northern New Mexico before the 

 period of intensive overgrazing. On the east, the association is found in more 

 or less typical form at Medicine Hat in Saskatchewan, Minot and Mandan 

 in North Dakota, Winner in South Dakota, and Long Pine and McCook in 

 Nebraska, Along the west, it occurs from near Calgary, Alberta, southward 

 to Lewiston and Billings, Montana, Douglas and Laramie, Wyoming, and 

 Colorado Springs and Trinidad, Colorado. Beyond the eastern limit, Bou- 

 teloua and Bulbilis merely persist as alternes in xerophytic situations in the 

 midst of the prairie. 



CONSOCIATIONS. 

 Stipa comata. Stipa viridula. Carex filifolia. 



Agropyrum glaucum. Bouteloua gracilis. Carex stenophylla. 



koeleria cristata. bulbilis dactyloides. 



The distinctive feature of the association is the intimate mixing of the tall- 

 grasses and short-grasses. This is the direct consequence of their relative 

 heights, the short -grasses regularly occurring as a layer beneath the tall ones. 

 The persistence of this relation is explained by the fact that the roots of both 

 types work at much the same level, and there is little opportunity for one to 

 get more water than the other. Moreover, while the tall-grasses shade the 

 others more or less, this is offset by their greater handicap from grazing. The 

 constant mixture is conclusive testimony to the sufficiency of the rainfall and 

 to the close equivalence of the two types of dominants. If the water relations 

 and root penetration were such as Shantz (1911 : 32) has found in the short- 

 grass plains at Akron, the tall-grasses would soon give way to short-grasses, 

 especially during the dry phase. This has nowhere been found to be the case, 

 and the vast area over which they live together not only speaks eloquently 

 of their associational equivalence under the particular subclimate, but is also 

 a compelling argument for the unity of the formation. 



Grouping. — At the edge of the association, the dominants tend to become 

 pure and hence to alternate instead of mingling in layers. This is to be 

 expected in the southwest, where it passes into the short-grass association, 

 and on the west, where there is a broad transition to the sagebrush formation, 

 since both of these mark drier climates in which the competition for water is 

 necessarily keener. Any one of the dominants may appear as a pure con- 

 sociation over limited areas in such regions. Bouteloua and Bulbilis show 

 this tendency chiefly on the southwest, and Stipa, Agropyrum, and Carex 

 filifolia on the west. Nearly every possible combination of dominants occurs 

 within the association, but certain ones are the rule. In eastern Wyoming, 

 in Montana and western North Dakota, the ruling group is Stipa-Agropyrum- 

 Bouteloua or Stipa-Bouteloua. In the moister region south and east of the 

 Black Hills and through South Dakota to the Missouri River, Agropyrum- 

 Bulbilis-Stipa-Bouteloua is the typical mixture. The essential basis of this 

 is formed by Agropyrum and Bulbilis, and hence either of the other two may 

 be lacking. Stipa in particular is much more general than appears to be the 

 case in summer and autumn after it has been grazed down. Carex filifolia 

 or C. stenophylla appears commonly in nearly all the groups but usually in 

 reduced abundance. This is also true of Koeleria in less degree. Other fre- 

 quent groups are Stipa-Koeleria-Bouteloua, Stipa-Carex-Bouteloua, Stipa- 

 Bulbilis-Bouteloua, and Agropyrum-Bulbilis-Bouteloua. Combinations of four 



