THE DESERT PLAINS. 145 



in New Mexico and to Prescott in Arizona. It was perhaps much broader 

 at one time, as Bouteloua eriopoda still occurs in some abundance about 

 Albuquerque and from Adamana to Winslow in Arizona. 



Range. ^The desert plains association extends from Snyder and Sweetwater 

 in Texas on the northeast through the southern two-fifths of New Mexico 

 into southeastern and south-central Arizona. In Texas and New Mexico, it 

 is the typical community of the regions indicated, with saUne associes in the 

 lower valleys and the mesquite along the benches and upper levels. From 

 southwestern New Mexico through southern Arizona, it occupies a broad 

 belt several to many miles wide around the major mountain chains, and 

 covers the broad intermountain plateaus. Its general range in altitude is 

 from 3,400 to 5,500 feet. 



The association sweeps southward through Chihuahua, Sonora, and 

 Durango into the high tablelands of central Mexico. It has received no 

 ecological study beyond a few miles south of the boundary, and its nature and 

 extent in Mexico must be inferred from floristic and grazing sources. The 

 inference seems clear that Mexico is the real center of the desert plains grass- 

 land and that it is richer in dominants and more varied in structure there than 

 in the United States. This is confirmed by the fact that the best expression 

 of the community is found in southern Arizona near the border. The extent 

 of this grassland in Mexico is probably much greater than in this country, 

 but nothing definite is known about it. 



The name "desert plains" is thought to indicate the nature and location 

 of the association. As to the kind of grassland and topography, "plains" is 

 clearly the best term to be applied. This conclusion is emphasized by the 

 relationship with the short-grass plains. In addition, this is not only the 

 characteristic grassland of the desert region of the Southwest, but it is also 

 in direct contact with the desert all along its lower edge. A further reason is 

 found in the fact that there exists a broad transition region between the 

 scrub desert and the Aristida-Bouteloua grassland. Indeed, Larrea or Proso- 

 pis is scattered over so much of the latter that it has often been regarded as 

 mesquite rather than grassland. Finally, relict patches of Bouteloua roth- 

 rockii, Aristida divaricata, or Muhlenhergia porteri have been found in various 

 protected places iu the desert, at altitudes as low as 2,400 feet, especially at 

 Tucson. These indicate that the desert grassland once extended well down 

 into the scrub desert, and that it was replaced by scrub as a consequence of 

 overgrazing. The significance of these relict areas is confirmed in some degree 

 by the statements of stockmen to the efifect that the desert was formerly well- 

 grassed. 



CONSOCIATIONS. 



Bouteloua eriopoda. Bouteloua gracius. Aristida caufornica. 



Bouteloua rothrockii. Bouteloua racemosa. Aristida arizonica. 



Bouteloua bromoides. Aristida divaricata. Hilaria cenchroides. 



Bouteloua hirsuta. Aristida purpurea. Muhlenbergia porteri. 



All of these may form pure consociations, but Bouteloua eriopoda and 

 Aristida purpurea are the only ones known to do so for long stretches. Both 

 are dominant over the northern and lower areas, particularly in New Mexico 

 and Texas. In these they mix somewhat, but as a rule either one is much more 

 important than the other wherever they occur together. The others rarely 



