406 CARNEGIE INSTITUTION OF WASHINGTON. 



between the short-grass plains and the mixed prairie, and the detailed 

 investigation of the effects of grazing disclosed the widespread conver- 

 sion of mixed prairie into short-grass. The survey of the mixed prairie 

 was completed during the summer, and the universal presence of tail- 

 grasses under the slightest protection confirmed the hypothesis that the 

 short-grass plains are everywhere an artificial community that has 

 arisen from mixed prairie as a result of grazing. 



The desert plains resemble the bunch-grass prairie in that the domi- 

 nants are largely bunch-grasses, which are replaced by scrub in con- 

 sequence of grazing. The similarity in both these respects is doubtless 

 due to a certain correspondence in water relations. The desert plains 

 still occupy large areas in a condition but little modified, but the evi- 

 dence is fairly conclusive that they once covered extensive valleys 

 now characterized by desert scrub consisting of Larrea, Prosopis, and 

 their associates. As with other associations, relict grass areas have 

 been found in manj^ places, while on certain Indian reservations, where 

 the use has been slight, the original grassland persists over many 

 square miles. With adequate protection good grassland has been 

 found under a rainfall of 6 inches. Perhaps the most conclusive proof 

 of former conditions has been furnished b}^ seasons of exceptional 

 rainfall, such as the summers of 1919 and 1921, when the perennial 

 grasses reappeared in abundance in the desert valleys at Tucson and 

 elsewhere. 



The subclimax and true prairies have been modified comparatively 

 little by grazing, perhaps chiefly because this was not a general pro- 

 cess in a region given primarily to cultivation. When grazing was too 

 closely restricted, as in many pastures, the tall-grasses yielded to 

 others less affected, or were finally replaced by annual weeds. \^Tiere 

 buffalo-grass was present, it usually became controlling, bu^ in general 

 pastures were sooner or later covered with a blue-grass sod. This 

 grass has also been a successful invader of native prairies that were lit- 

 tle disturbed, and it gives promise of completely replacing Stipa and 

 Kceleria in regions with a rainfall of more than 27 inches. It makes 

 headway more slowly against Andropogon, but may also replace it in 

 time. While there is some question that Poa can invade prairie that is 

 wholly undisturbed, this has no practical bearing, as no areas are known 

 that are not at least mowed for hay. In these its early development and 

 maturing and the abundance of sod-leaves gives it an advantage over 

 the tall-grasses that mature later, and especially over the bunch-grasses, 

 such as Stipa and Kceleria. The question of the westward movement of 

 Andropogon is not yet completely settled, but this seems to be largely 

 a matter of its reappearance after the period of intensive grazing, and 

 perhaps also of its movement into areas left by Stipa and Kceleria as 

 they were grazed out. 



