142 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Agropyrum. In central Kansas they play a similar part, but usually occur 

 with Andropogon. Toward the southern limits of the area, Bulhilis mixes with 

 Hilaria cenchroides, H. muiici, and Aristida purpurea in Texas, and Bouteloua 

 gracilis with B. eriopoda and A. purpurea in New Mexico and Arizona. 



When Bouteloua and Bulhilis meet the tall-grasses, both or either may 

 become dominant as a result of overgrazing. There is increasing if not con- 

 clusive proof that overgrazing is the cause of the pure areas of short-grass 

 found in the mixed prairie from Saskatchewan to Kansas, and sometimes 

 covering many square miles. The study of this problem has also led to the 

 plausible assumption that the widespread but erroneous belief in the dis- 

 appearance of the buffalo-grass ia likewise due to the changed conditions 

 following settlement. The disappearance of the enormous herds of buffalo 

 gave the tall-grasses a chance to reappear and to conceal the short-grasses 

 beneath them. At least, it is undeniable that buffalo-grass and grama still 

 grow abundantly in many places where they were said to have disappeared 

 in the late sixties and early seventies. This also explains the frequent state- 

 ment that the bluestems and other tall-grasses entered the Middle West, or 

 at least became much more abundant, after settlement began. A detailed 

 discussion of this point may be found in Chapter VI. 



Factor relations. ^While the factor relations of the dominants have received 

 almost no attention quantitatively, the habitat of the association has been 

 studied by both Shantz and Weaver. The former (1911 : 32; 1906 : 28) 

 found the average chresard from June 7 to September 27 at a depth of to 6 

 inches to be 4.8 per cent, at 6 to 12 inches 2.7 per cent, at 12 to 18 inches 

 1.25 per cent, at 18 to 24 inches 0.56 per cent, and at 24 to 30 inches 0.23 per 

 cent. He also dealt with the distribution of the rainfall, and the relation of 

 runoff, penetration, and evaporation to water-content. Weaver (1919) has 

 measured the water relations on the plains at Colorado Springs, but it is 

 probable that this area belongs to the mixed prairie rather than to the short- 

 grass community. Briggs and Belz (1911) have made a thorough digest of 

 rainfall and evaporation records for the West, which has much significance for 

 the climatic relations of these contiguous associations. A study of their figures 

 makes it clear why grassland goes little beyond the isohyete of 20 inches at 

 the Canadian boundary, but extends to that of 25 inches in central Texas 

 (fig. 3). The change from mixed prairie to short-grass plains, moreover, is 

 in accord with the evaporation values for the respective regions. Over the 

 northern Great Plains, these values are 30 to 39 inches, and over the southern 

 they are 52 to 62 inches. 



Sequence of dominants. The evidence drawn from both the habitat and 

 from succession indicates that Bouteloua is the most and Bulhilis the least 

 xerophytic of the dominants. This agrees essentially with their general dis- 

 tribution, in that Bulhilis becomes more controlling to the east with increasing 

 rainfall, or to the north with decreasing evaporation. As already noted, 

 Bouteloua forms the matrix over most of the south-central Great Plains, and 

 Bulhilis makes dense mats in depressions of all sorts, abandoned roadways, 

 dry pools, playas, etc. The topographic evidence is fully confirmed by the 

 successional, as Shantz has shown in the case of old roadways (1917 : 19), 

 and is especially well exhibited in the playa subsere. While the playa is a 



