270 THE STUDY OF PLANT COMMUNITIES ■ Chapter X 



slope drops abruptly to the floor of the Great Basin, which lies at 

 about 4,000 feet. At the base of the west slope, there are only ten 

 to fifteen inches of rainfall and a long, unbroken, dry summer sea- 

 son. Upward precipitation increases, temperatures decrease, the 

 dry summer season shortens, and a larger proportion of precipita- 

 tion falls as snow. 



FlG. 139. Interior of the red fir (Abies magnified) forest that occupies 

 most of the subalpine zone of the Sierra Nevada. 



Because the general north-south axis of the range lies across the 

 path of the prevailing westerly winds, climatic conditions for the 

 region as a whole are influenced by them and east slopes are much 

 drier than west slopes. Winter precipitation makes up 80-85 per- 

 cent of the total, and at high elevations, most of the moisture falls 



