252 



DISTRIBUTION OF PLANT COMMUNITIES 



is characterized by two rainy seasons, one in winter and one in 

 summer, and by high temperatures and high evaporation rates. 

 This type of desert is found in southern Nevada, southern CaUfornia, 

 southern and western Arizona, southern New IMexico, and western 

 Texas, and extends southward into ]Mexico. This is the most xeric 

 type of vegetation in the United States. For the most part the 

 average annual rainfall is between 3 and 6 inches but sometimes an 

 entire year may pass with no rain at all and sometimes the rain for 

 the entire year may all come in a single downpour. 



The most characteristic dominant of this desert is the creosote 

 bush but there is great variation from place to place. In parts of 

 northwestern Arizona and southern California the Joshua tree 



Fig. 114.— Creosote bush and cactus desert, Southern Arizona. 



{Yucca brevifolia) is by far the most conspicuous plant while in 

 southern Arizona, especially in the Organ Pipe Cactus National 

 Monument, there are extensive areas where the aspect is dominated 

 by large cacti such as the saguaro (Carnegia gigantea), the Organ 

 Pipe Cactus {Lemaireocereus thurheri), senita (Lophocereus shottii), 

 and cholla (Opuntia). Along with these cacti may be found the 

 unique ocotillo (Fouquieria splendens), the palo verde (Parkitisonia) , 

 and many others. If the winter rains come on schedule and bring 

 all of these plants into bloom in early spring along with innumerable 

 winter annuals it makes a floral display that beggars description. 



149. Alpine Communities.— Alpine vegetation is found near the 

 tops of the higher mountains all over the world. In going up the 



