288 THE STUDY OF PLANT COMMUNITIES * Chapter X 



inches, and official records show at least one period when tempera- 

 tures held above 100° E for 538 days. 192 



Conditions are not too different from those of the Great Basin 

 although somewhat more extreme. This is borne out by the vege- 

 tation, which includes many of the same species, their distribution 

 controlled here, too, by soil texture and salt concentration. Certain 

 character species do stand out, however, and this justifies the vege- 

 tational distinction from the Great Basin. At the upper elevations 

 (3,000-4,000 feet) and in the transition from sagebrush, with maxi- 

 mum precipitation (near five inches), Joshua tree (Yucca brevi- 

 folia) is conspicuous. With decreasing altitude and precipitation, 

 creosote bush (Larrea divaricata), with burro weed (Franseria 

 dumosa) in association, becomes the major dominant. This com- 

 munity occupies 70 percent of the total area of the iMojave Desert. 



Sonoran Desert.— The lowlands around the Gulf of California 

 in Mexico and Lower California, which lie chiefly below 2,000 

 feet, constitute the Sonoran Desert. Much of the area is made up 

 of dunes and sand plains. Precipitation is extremely uncertain, not 

 exceeding two to four inches in the vicinity of the Gulf, although 

 increasing some with altitude. Its effectiveness is counteracted by 

 the extremely high temperatures. 237 



Fig. 153. Sahuaro (Carnegiea gigantea), the giant of the columnar cacti 

 that characterize the uplands of the Sonoran Desert.— U. S. Forest Service. 



