DESERT PLAINS 283 



Subdominants. The vast majority of the long list of subdominants 

 are southwestern in origin and distribution; relatively few have been 

 derived from the mixed prairie or from the Gulf region. Owing to the 

 desertlike climate, the western and southern portions arc signalized by 

 a large number of dwarf and half shrubs, which are to be reckoned 

 among the subdominants as a rule. Equally characteristic is the 

 presence of two major aspects, summer and winter, the latter, in par- 

 ticular, marked by a host of annuals. A large number of these are 

 to be found in the desert, where they probably originated, and many of 

 them occur also in the prairie of southern California. These may be 

 regarded as the forb dominants of a subscre of bare soil, as they are 

 in the desert, but it is simpler at present to treat them as annual 

 subdominants of the desert plains or California prairie, as the case 

 may be. 



Proclimaxes. The most striking feature of the desert plains is the 

 general presence of shrubs, giving it the character of savanna. Chief 

 among these are Larrea, Yucca, Flourensia, and Ephedra on the gen- 

 eral levels, with Prosopis or Mimosa at somewhat higher ones, while 

 Prosopis, Acacia, Parkinsonia, and Olneya occur in the washes. These 

 have undoubtedly been present since some distant dry-phase migra- 

 tion of subtropical shrubs and trees to the northward, but they have 

 also multiplied greatly in consequence of overgrazing. This disturb- 

 ance has likewise favored the replacement of perennial grasses by 

 annual ones, such as Bouteloua aristidoides, B. parryi, and Aristida 

 adscensionis, especially in the drier areas, giving rise to a widespread 

 disclimax savanna composed of these "six-weeks grasses" with Larrea 

 and other shrubs. 



Two other shrubby proclimaxes, which appear to be postclimax in 

 nature, are found in the immature soil of the rocky slopes of foot- 

 hills and escarpments. The one is composed of Agave, Dasylirion, and 

 Nolina in particular, and is found at its best in the "sotol" region of 

 western Texas; the other is marked chiefly by Carnegiea, Fouquiera, 

 and Parkinsonia and stretches from eastern Arizona to the Colorado 

 Desert. 



Influents. The bison was never present in this grass type in any 

 abundance, except in west Texas. The pronghorn antelope originally 

 occurred in goodly numbers; the subspecies differs somewhat from the 

 northern form, but chiefly in spending the summer in the higher coun- 

 try and winter in the lower grass-covered plains. The distinction 

 between this community and the Great Plains is reflected in the addi- 

 tion of two species of kangaroo rats. Dipodomys spcctabilis specta- 

 bilis Mer, (Figs. 66-67) is essentially a grassland species, feeding 



