18 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



The particular reasons for its presence differ in different places. 

 In the Sierras and the isolated mountain areas of southern California 

 the dominating cause is high total precipitation, which furnishes a 

 sufficient amount of soil-moisture to tide the relatively mesophytic 

 vegetation over the unfavorable season. In the higher mountains 

 the slowly melting snows are of very great importance. In the 

 northern end of the State, in addition to a high total precipitation, 

 there is a decrease in the duration of the dry period. Along the 

 northwestern coast there is, in addition to the other factors, the 

 abundance of summer fog, which is the particular reason for the 

 redwood forest. 



The area with 10 to 30 inches of rainfall is the region of broad- 

 sclerophyll dominance. The correspondence with the region deter- 

 mined upon through other lines of evidence is quite striking, even 

 the Sacramento Valley being included (see p. 81). Only the narrow 

 strip along the east slope of the Sierras, dominated by the vegetation 

 of the Great Basin, must be left out. In the north Coast Ranges 

 the broad-sclerophylls, both trees and chaparral, are of great impor- 

 tance, as plate 1 indicates. Observation shows, however, that the 

 conifers are competing with them on at least equal terms (see p. 73). 

 The climatic map, indicating conifer forest conditions for this region, 

 is therefore not deceptive. 



Finally, those regions with less than 10 inches of rainfall correspond 

 accurately with the deserts — the western portions of the Colorado 

 and Mojave Deserts and the Owens Valley region — and in addition 

 the southern part of the San Joaquin Valley, which closely approaches 

 desert conditions. 



The same points are brought out in another way by table 1. 

 The three great regions are seen by the summary to be thoroughly 

 distinct in total precipitation, the proportion being approximately: 

 desert 1; broad-sclerophyll 6; conifer forest 15. 



The seasonal distribution in conifer forest and broad-sclerophyll 

 regions is much alike, the summer precipitation averaging well 

 below 20 per cent. In the desert the percentage is distinctly higher, 

 because a number of stations east of the 20 per cent line were included 

 in order to give a true picture of the California desert region as a 

 whole. 



Certain temperature figures are added, though the data are incom- 

 plete and their interpretation unsatisfactory. The mean annual 

 temperature is of little use; the seasonal extremes, i. e., the means 

 of the hottest and coldest months (in nearly every case July and 

 January) and the mean maxima and minima of the same months 

 are more significant. The mean annual temperature of the broad- 

 sclerophyll region is only slightly below that of the desert. The 

 mean of the hottest month, on the contrary, is much lower, being 



