THE CHAPARRAL CLIMAX. 



179 



the typical zone between forest and grassland or desert, chaparral has an 

 intermediate climatic position. It resembles forest in the wide range of rain- 

 fall conditions in which it occurs, and only a general correlation with the 

 latter is possible. In the Rocky Mountains the chaparral lies between 15 and 

 20 inches of rainfall. In southern California it ranges from 10 to 20 inches, 

 and in northern California and Oregon it occurs on dry slopes under 50 to 60 

 inches. It seems clear that chaparral is possible in 10 inches of rainfall only 

 where the proximity of the ocean cuts down evaporation, and at 50 inches only 

 where insolation greatly increases it. Here, even more than in sagebrush and 

 grassland, it will be necessary to determine water-content, evaporation, and 

 especially transpiration relations before adequate correlations can be estab- 

 lished (fig. 7). 



Durango, Colorado 

 10 in. 



San Diego, California 

 11 in. 



ll. — ll 



Fig. 7. — Monthly and total rainfall for representative localities in the associations of 



the chaparral climax. 



Origin and succession.— The chaparral, like the grassland and desert scrub, 

 is largely southwestern in origin. This would be expected from its general 

 climatic relations as well as from its greater development in the south, and 

 its uniform shading-out to the northward along both mountain systems. 

 With the exception of a few genera such as Prunus and Rosa, all of the domi- 

 nants are either southwestern or have reached their chief development in the 

 Southwest. Thus, it seems probable that the chaparral has moved north- 

 ward from an original southern center and has differentiated into two associa- 

 tions as a consequence of finding ecesis most successful along the two great 

 mountain axes. 



