THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 81 



controlled by chaparral or would come to support chaparral if 

 destructive agencies were eliminated; and the principal causes of 

 the destruction of the chaparral or prevention of its establishment 

 have been clearing and repeated fires. 



As to the exact extent of former chaparral dominance we can not 

 set limits with any degree of assurance. Some areas are made certain 

 by historical testimony and others almost equally so by relict evi- 

 dence. We are thus led to admit the extreme probability that the 

 great mass of the Coast Ranges, the foothills of the Sierras, and such 

 minor valleys as the Santa Clara were originally dominated by broad- 

 sclerophylls ; that areas now inhabited by grasses or xerophytic 

 trees, occurring as isolated patches surrounded by chaparral or 

 forming a more or less continuous zone upon the foothills bordering 

 the major valleys, are such by reason of clearing or repeated fires. 

 It may be necessary to except from this category the ridges of Coast 

 Range and Sierras that immediately surround the southern end of the 

 San Joaquin Valley, which, with the inclosed plains, are desertlike. 



There is a possibility, further, that the northern end of the Great 

 Valley itself may once have been dominated by chaparral. The 

 evidence for this is not absolutely conclusive, but is of sufficient 

 weight to justify presentation. The remnant near Hershey, in 

 the Sacramento Valley, shows that the chaparral species are able to 

 grow to their maximum size and to form a solid cover under typical 

 valley conditions. The practical absence of typical chaparral along 

 the northeastern border of the valley (p. 77) — the only break of 

 importance in the whole circle of surrounding foothills — suggests 

 the probability of a former connection across the valley floor. The 

 climax chaparral has apparently been pinched out between the 

 invading mass of the fire-favored grassland and the relatively resistant 

 barrier of the conifer forest. 



So far as available data show, there are no constant efficient 

 climatic differences between the areas dominated to-day by chaparral 

 and the Sacramento Valley. Referring to table 1 (p. 19) we find 

 that 44 stations in the southern Coast Ranges, southern Sierra 

 foothills, and Cuyamaca Mountains, which are the regions most 

 certainly controlled by chaparral, give an average of 21.67 inches of 

 precipitation, while 23 stations in the Sacramento Valley average 

 21.85 inches. Neither is the seasonal distribution notably different, 

 the proportion of total precipitation occurring in the months May 

 to October being, in the case of the chaparral regions, 11.6 per cent, 

 and in the Sacramento Valley 13.9 per cent. Available temperature 

 data are unsatisfactory, as there are no records of mean summer 

 maxima for the Sacramento Valley. 



Clements (22, p. 150) has advanced the theory that the Great 

 Valley of California, "from Bakersfield to Mount Shasta and from 

 the foothills of the Sierra Nevada and Cascade Mountains, through 



