CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 287 



assume dominance under local special conditions. Several species 

 of Ephedra are characteristic. 



Desert Scrub Formation.- Mojave Desert— This, the smallest of 

 the desert units, lies almost entirely in California below the south- 

 ern end of the Sierra Nevada. Physiographic conditions are similar 



r 



FlG. 152. Joshua tree (Yucca brevifolia), characteristic of the northern 

 Sonoran desert, particularly in the transition from creosote bush dominance 

 to shadscale of the sagebrush formation.— Courtesy Univ. of Nevada Agri- 

 cultural Extension Service. 



to the Great Basin but elevations are generally lower (1,000-4,000 

 feet). The irregular precipitation of less than five inches is dis- 

 tributed over fall, winter, and spring. Summers are very hot and 

 dry. The area includes Death Valley, with a minimum elevation of 

 480 feet below sea level. Its infrequent maximum rainfall is two 



