32 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



Jasper Ridge, calculated from those of San 

 Jose and Ben Lomond Weather Bureau Sta- 

 tions, is about 80 cm. 



It seems probable that Jasper Ridge has a 

 smaller total of fog than either the valley or the 

 mountains. The true ocean fogs rarely reach 

 it and the "bay" or "tule fogs," which are 

 frequent during the forenoon in the lowland, 

 do not ordinarily rise so high. The average 

 midday temperature is probably higher than 

 in the valley because of the proximity of the 

 bay to the latter, and also higher than in the 

 mountains because of altitude and fog. Tem- 

 perature and fog affect vegetation through their 

 influence upon the evaporating power of the 

 air, and it is quite certain that the evapora- 

 tion-rate is much higher upon Jasper Ridge 

 than in the mountains, and almost as certain 

 that it is higher than in the valley, because of 

 distance from the bay, absence of fog, and prob- 

 ably higher mid-day temperature. We there- 

 fore conclude that Jasper Ridge is far more 

 xerophytic in climate than the mountains and 

 perhaps somewhat less so than the valley. It 

 may be, however, that the greater rainfall is 

 neutralized by the higher evaporation-rate. 

 The evidence of the vegetation is that there is 

 little climatic difference between the ridge and 

 the valley. 



THE VEGETATION. 



The vegetation of Jasper Ridge was orig- 

 inally mainly chaparral. Large portions have 

 been cleared of the bushy growth, quite cer- 

 tainly within a century, and the cleared areas 

 are now expanses of wild oats (Avena fatua 

 and A . barbata) , with scattered oaks, especially 

 Qtiercus douglasii, which is peculiarly charac- 

 teristic of such secondary areas. Quercus agri- 

 folia and Q. kelloggii are also frequent, and 

 there is a considerable scattering of shrubs of 

 the species that frequent such cleared areas: 

 Ceanothus cuneatus,Heteromeles arbutifolia,Bac- 

 charis pilularis, and especially Rhus diversiloba. 



Via. 1. — Profile of a part of Jasper Ridge, drawn approximately 

 to scale, showing topography along trail, distribution 

 of plant communities, and location of stations. 



8. 



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