TREES, SHKUBS, AND FORESTS 



369 



reckoned among the most beneficent and important effects of 

 the forest cover. When rain falls over a dense forest, from one 

 tenth to one fourth of it 

 is caught by the crowns 

 of the trees, while the 

 forest floor of humus soil 

 and roots holds the re- 

 mainder. It is estimated 

 that a forest floor " can 

 hold for a while a rain- 

 fall of five inches." This 

 water is then gradually 

 evaporated or is slowly 

 drained off into streams, 

 lakes, and the sources 

 of springs. Mountain 

 streams, which irrigate 

 fertile valleys, are thus 

 fed and sustained at their 

 source. In like manner 

 the supply of water for 

 the great irrigation sys- 

 tems of the West, and 

 for the water supplies of 

 large cities like Denver, 

 Los Angeles, and San 

 Francisco, comes from 

 mountain springs, lakes, 

 and streams, which are 



protected at their source 



FIG. 224. Pacific-coast forest of Douglas fir 

 and western red cedar, Tacoma, Washington 



Photograph furnished hy the United States 

 Forest Service 



by forests. 



Rain which falls upon 

 unf orested soil has a very 

 different effect from that 



outlined above, especially in mountainous and hilly regions, 

 such as those occupied by most of our national forests. In a 

 region denuded of forests the rain falls directly upon the soil, 



