THE MIXED PRAIRIE. 135 



THE MIXED PRAIRIE. 

 STIPA-BOUTELOUA ASSOCIATION. 



Nature. — Since the first recognition of a prairie and a plains formation 

 (Pound and Clements, 1898 : 244; 1900 : 347) it has been assumed that the 

 one passed into the other through a broad transition region. In the summer of 

 1914 it was found that Stipa, Agropyrum, and Koeleria did not begin to yield 

 to the short-grasses in the central Dakotas and Nebraska and then give way 

 to the plains formation, as had been generally assumed. On the contrary, 

 the three prairie dominants continued across the plains and into the foothills 

 of the mountains of Montana, Wyoming, and Colorado (Clements, 1916 : 180). 

 It was also found that, while Bouteloua, Bulbilis, and the two species of Carex 

 became increasingly abundant, it was as an under-story in the tall-grasses, 

 especially Stipa or Agropyrum. Moreover, where Bouteloua occurred as a 

 pure consociation, or with Bulbilis, this was discovered to be the usual result 

 of overgrazing. This has forced the recognition of a mixed association com- 

 posed of the dominants of both prairies and plains, but essentially prairie in 

 its tall-grasses, numerous societies, and successional relations (plate 24) . 



In order to test this assumption fully, the region has been crossed from east 

 to west during 1915, 1916, and 1917, and in 1918 it was traversed from Col- 

 orado to North Dakota on the west and from North Dakota to Kansas on the 

 east. Especial attention was paid to the community relations of the dominants 

 and the climatic and topographic correlations, particularly where the associa- 

 tion touched the prairies and the short-grass plains. As a consequence, the 

 conclusion has become unavoidable that these northwestern prairies represent 

 a distinct association. They are not a transition community in structure, as 

 they exhibit seven dominants in various combinations throughout the area. 

 Nor are they transitional in position, since the short-grass plains lie south of 

 them, while their major western contact is with the sagebrush formation. 

 They are primarily prairie in character, since the tall-grasses are codominant 

 throughout, the root systems are relatively deep-seated, and the numerous 

 societies are identical or similar in floristic and character to those of the true 

 prairies. The most significant difference is the practically universal presence 

 of one or more of the short-grasses or sedges as a lower layer. 



The constant association of Stipa or Agropyrum with Bouteloua or Bulbilis 

 throughout the community is shown by the following summary: During 

 1914 the climax grassland was studied in 88 localities east of the Rocky 

 Mountains, and tall-grasses and short-grasses were associated as dominants 

 in 83 of these. In 136 local stations the same grouping was found in all but 12. 

 During 1915, of 76 localities visited, 73 showed both types. In 1916, of 65 

 localities, 64 showed Stipa or Agropyrum with Bouteloua or Bulbilis. In 1917 

 the number was 61 out of 64, and in 1918 tall-grasses and short-grasses were 

 associated in 97 out of 100 localities. During the six years, without allowing 

 for duplicate localities, at least one tall-grass and one short-grass were found 

 together as dominants in all but 15 of the 393 localities studied. 



Effect of grazing and climatic cycles. — The study of grazed and protected 

 areas in 1914 disclosed the fact that Stipa and Agropyrum were much more 

 readily affected by grazing than the short-grasses, and that Stipa in particular 

 could be completely eliminated by overgrazing. During the succeeding years 



