REGIONAL DISTRIBUTION 513 



460. Influence of rainfall on forest distribution. Plant forma- 

 tions in general, and forest ones among the rest, have their 

 boundaries determined largely by the amount and seasonal 

 distribution of the annual rainfall of the region. A careful 

 comparison of the forest map of the United States (Fig. 380) 

 with the rainfall map (Fig. 381) shows that the arid and semi- 

 arid regions are treeless. By far the greater portion of the 

 immense area lying to the west of an irregular line extending 

 from the Red River of the North to the junction of the Pecos 

 with the Rio Grande, has an annual rainfall ranging from 

 twenty inches down to almost nothing. Over this great ter- 

 ritory there are few extensive forests except those covered 

 with xerophytic conifers, and these occur mostly in the moun- 

 tains where the rainfall is somewhat greater. Our true deserts 

 are treeless except for scattered individuals of such extreme 

 xerophytes as the tree yucca (Fig. 379). The Pacific slope, 

 in its northern portion, has an annual rainfall of thirty-seven 

 inches, and is covered with such luxuriant forests as are shown 

 in Fig. 262. The southern portion has an average annual rain- 

 fall of ten inches, and (except on the mountains) has no con- 

 siderable forests but only a scanty growth of trees and 

 xerophytic shrubs (chaparral). 



