DESERT PLAINS 



281 



tacts in Mexico are little understood, but it once occupied large areas 

 of the central plateau, and a considerable number of its dominants 

 extend into Central and South America. 



The desert plains extend along the southern line of the mixed 

 prairie in the central regions of New Mexico and Arizona, though 

 direct contact is prevented for long distances by mountain ranges in 

 western New IVIexico and the Mogollon Rim in Arizona. Where bar- 

 riers do not intervene, the two grasslands combine in a wide ecotone. 



Fig. 65. — Desert plains dominated by Bouteloua eriupuda; southwestern Texas. 

 (Photo by Edith Clements.) 



and a similar condition obtains where the desert plains swing around 

 the western flank of the mixed prairie at the edge of the Colorado 

 valley. From northern Mexico to northern Arizona it faces the desert 

 climax of Larrea and its associates. 



Climate. High temperatures during the summer and mild to warm 

 winters in the western portion are responsible for the impression of 

 a desert climate, together with the low humidity and the correspond- 

 ing high evaporation. But the annual rainfall confirms the evidence 

 drawn from the climax vegetation to the effect that the climate is 

 that of xeric grassland, unmatched in this respect elsewhere in the 



