20 University of California Publications in Botany [VOL. 9 



in the Canadian, the lowest of the boreal life-zones, all months are 

 subject to frost; little significance is to be attached, however, to the 

 rubric "killing frosts," when referring to high mountain stations, 

 for the sufficient reason that though the plants may be frozen they 

 may still survive. 39 



RAINFALL 



Though the Sierra Nevada lies to the south of the majority of the 

 storms entering the continental atmosphere from the North Pacific area, 

 the position of the range, athwart the track of the moisture-bearing 

 winds blowing landwards, ensures to the western side of the mountains 

 sufficient rain not to exclude tree growth away from the water courses, 

 even at the base of the mountains except near the southern end of 

 the region. The Blue Oak (Quercus Douglasii H. & A.), the Interior 

 Live Oak (Q. Wislizenii A. DC.), and the Digger Pine (Pinus Sabim- 

 ana Dougl.) grow on the foothills east of the central valley but little 

 above the valley floor. On the eastern flank no such lowering of the 

 "dry tree-line" exists; west of Honey Lake, Lassen County (3,849 

 feet), this line runs at about 2,500 feet above the lake, rising south- 

 ward to near 8,500 feet west of Owens Valley. The rainfall on both 

 flanks constantly diminishes to the southward: 



Western slope stations: 



La Porte, Plumas County, 5,000 feet 89.2 inches 



Bowmans Dam, Nevada County, 5,500 feet 75.6 " 



Blue Canon, Placer County, 4,695 feet 74.2 " 



Crockers, Tuolumne County, 4,452 feet 55.0 " 



Summerdale, Mariposa County, 5,270 feet 55.1 " 



Tehachapi, Kern County 10.62 " 



Eastern slope stations: 



Truekee, Nevada County, 5,819 feet 27.1 " 



Taboose, Inyo County, 6,200 feet 14.0 " 



Bairs, Inyo County, 6,100 feet 8.7 " *o 



This difference in the mean annual precipitation between the wind- 

 ward and leeward sides of the range is even more clearly seen by com- 

 paring nearby stations : 



West side: Bowmans Dam, 5,500 feet 75.6 inches 



East side: Boca, 5,535 feet 20.14 " 



The altitude of greatest rainfall in the Sierra is between 5,000 and 

 6,500 feet on the western flank. The line for a time rises to the south- 

 ward, the higher mountains of the southern half of the region appear- 

 ing to cause an increased rainfall at a constant level. The rainfall 



