The Origin of the Desert Climax and Climate 89 



the Larrea climax is replaced by sagebrush and the latter then 

 by grassland. 



It should be clearly recognized that the limits of the desert or 

 Larrea climax cannot be drawn with exactness at present, and 

 that no more than a fair approximation will ever be possible. 

 Quite aside from the difficulties arising from the slow fluctua- 

 tions caused by cycles of varying length and intensity, are those 

 produced by overgrazing and its consequences in a climate 

 where this process is the most destructive to grassland. Hence, 

 the limits of the desert plains and mixed prairie associations 

 must be determined by scattered relicts, which are sought most 

 diligently and with greatest success in the wet phase of the 

 cycle, when they are most in evidence. Moreover, these are the 

 very regions deficient in weather records, and the isohyets are 

 based upon relatively few and recent observations so that they 

 serve only as a general index. However, if it be borne in mind 

 that all climaxes are characterized by broad ecotones, it will be 

 seen that these criteria serve as a practicable basis for circum- 

 scribing the desert region. 



From the essential nature of climates and climaxes, it follows 

 that deserts usually lie in direct contact with grassland rather 

 than woodland or forest. This is particularly true of the Colo- 

 rado complex and its wide extension in Mexico, though the 

 detailed relations are there much less known. Since the domi- 

 nants are typically bushes or shrubs, they may come into touch 

 with another scrub formation, such as the sagebrush on the 

 north or along the middle slopes of bordering ranges, but this is 

 partly at least a consequence of grazing disturbance. The desert 

 is largely encircled by three associations of the grassland forma- 

 tion, namely, the mixed prairie on the northeast, desert plains 

 along the extended eastern boundary, and bunchgrass prairie to 



