CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 283 



inal extent of sclerophyll dominance has been much reduced by 

 fire, since, once they are destroyed, the return of the sclerophyll 

 species is long delayed. 



Desert Formations.— The major area of the North American 

 desert extends from southeastern Oregon and southern Idaho 

 southward through the Great Basin, including most of Nevada 

 and Utah except high elevations, continues southward into south- 

 ern California and western Arizona, down most of the peninsula 

 of Lower California and, on the mainland, through Sonora as far 

 south as the Yaqui River. The highlands of eastern Arizona and 

 western New Mexico interrupt the continuity of desert, but from 

 south-central New Mexico, there is almost continuous desert 

 through eastern Chihuahua and most of Coahuila in Mexico. 238 



In spite of the great extent of this area, there are certain environ- 

 mental features characteristic throughout. Precipitation is low and 

 erratic; temperatures of air and soil are extremely high by day and 

 drop abruptly at night; atmospheric humidity is usually low, and 

 bright sunny days are the rule. These factors serve to explain why 

 predominating plants are those that can survive desiccation with- 

 out injury or that store water in their succulent tissues. This is not 

 to imply that desert vegetation is uniformly similar throughout. 

 Climatic differences, associated with latitude and altitude, are ac- 

 companied by differences in species and life forms. Locally, the 

 physical differences in topography, exposure, and soils produce 

 distinct vegetational variations just as in moister climates. Finally, 

 there are numerous undrained depressions into which the water of 

 winter rains flows and, upon evaporation, deposits the silts and 

 clays it has transported as well as salts of various kinds. The re- 

 sulting mud flats (playas) in themselves constitute a special habitat 

 with associated species, but the nature and concentration of salts 

 in the soil is even more effective in controlling the communities 

 there. 



Four desert areas are distinguishable on the basis of regional en- 

 vironments and, likewise, by the nature and importance of the 

 major dominants : 238 namely, the Great Basin, Mojave, Sonoran, 

 and Chihuahua deserts. In each of these areas there are communi- 

 ties that occur with minor variations wherever conditions are not 

 extreme. These may be recognized as climax. Other communities, 



