CHAPTER rX 

 CHAPPARAL 



General Conditions. — The plant formation in which occur 

 sclerophyllons, dwarf, evergreen shrubs and trees is confined to 

 southern California and southern Arizona. . Various species of 

 shrubs are found but the type takes its name of " chapparal " 

 from the dwarf oaks which occur in it. Approximately 10,000,000 

 acres are included. 



The rainfall according to the Weather Bureau records varies 

 from 9.54 inches at San Diego to 21.42 inches at San Luis Obispo. 

 The average is, however, less than 15 inches. 60° F. is the mean 

 annual temperature with a range from 22 degrees to 109 degrees 

 and hence the evaporation is high. As a consequence there is 

 a very small amoun,t of moisture available for plant growth 

 more especially since the rainfall is concentrated during the 

 winter months and runs off rapidly. 



The length of the growing season is not determined so much 

 by the number of months during which the thermometer is above 

 freezing but by the period during which there is enough moisture. 



Chapparal is confined to the slopes of the mountain ranges 

 which are high enough to be above the arid plains country. 

 This is arid because the mountains intercept the rain bearing 

 clouds from the Pacific Ocean. Even the drought resisting vege- 

 tation of the chapparal cannot creep down into these areas of 

 scant rainfall. The type is therefore confined to the lower 

 slopes of the mountains of California and Arizona between 

 elevations of o to 8000 feet above sea level. The lower limit is 

 only reached near the Pacific Ocean where the rainfall is relatively 

 abundant. Inland the chapparal has to ascend the mountains 

 to obtain enough soil moisture. As a consequence the topogra- 

 phy of the type is characteristically steep and much cut up while 



the soil is thin. 



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