xviii LIFE ZONES 



fields of the Great Plains region of southern Canada and the 

 adjoining United States. In the west it is the home of the giant 

 sequoias, the coast redwood, and the magnificent yellow and sugar 

 pines. It reaches in the mountains as far south as Guatemala. 



The Upper Sonoran includes the wide expanse of gray sage of 

 the Great Basin and the eastern base of the Rockies. Its humid 

 eastern portion, the Carolinian Zone, designates the corn belt and 

 winter-wheat region of the central United States. 



The Lower Sonoran is the Zone of the deserts, of date palms 

 and cotton of Imperial Valley and the Colorado River basin. In 

 the East it is known as the Austroriparian and is characterized 

 by the sugar-cane and cotton plantations of the Gulf States. 



The regularity of order or succession of Zones is affected locally 

 by a wide variety of influences. A south-facing slope is much 

 warmer than one facing northward; ascending currents of heated 

 air from deserts elevate the Zones of neighboring mountains; 

 extensive bodies of water modify temperatures and humidity of 

 adjacent land areas; the more precipitous the slope of a mountain 

 the higher extend the low Zones. 



Each Zone has its so-called "indicators," or characteristic 

 species. Among mammals, plants, and trees, these are the ones 

 occurring most abundantly in any certain Zone, while among 

 birds the characteristic species are those nesting principally in the 

 given Zone, regardless of whether they may reside throughout the 

 year or be summer visitants only. 



For illustrative purposes, the Zones of western North America 

 will be chiefly considered, more particularly those of the com- 

 paratively limited region covered in this book. 



SOUTHWESTERN UNITED STATES 



The Southwestern United States (see map) is taken to include 

 southern California and Arizona. The Tehachapi Mountains are, 

 with other cross ranges, the natural boundary between the 

 Coast Ranges and Sierra Nevada of the north, and the broken 

 and irregular mountain masses of the south. More than one half 

 of the entire area of southern California lies east of its coastal 

 mountains, the so-called desert divide, and is separated by lesser 

 ranges into the Mohave and Colorado deserts. In the San Gabriel 

 Mountains is San Antonio, rising to 10,080 feet. San Gorgonio, in 



