68 



A 1'RIMKK OF FORESTRY. 



effect on rain water after it has fallen. When rain 

 falls over a dense forest from less than one-tenth to 

 about one-fourth of it i> caught by the trees. A small 

 part of this water may reach the ground by riinnino- 

 clown the trunks, but the greater part of it is evapo- 

 rated and so increases the humidity of the air. That 

 which passes through the crowns falls upon the forest 

 tloor. which sometimes has an absorbing power so o-reat 

 that it can hold for a while a rainfall of ."> inches. Vet 



this water does not 

 remain in the porous 

 floor, but in the end 

 runs otl into the 

 streams, or is evapo- 

 rated, or sinks into 

 the "-round. That 

 which o-ets into the 

 o-roiind is either 



Jffc 



Fn;. -W. The beginning of erosion in soil 

 trampi-d l).-ire by stock. Sierni Nevada 

 Mountains. California. 



taken up by the roots 

 or o-ocs to feed the 

 springs and water- 

 courses. 



Rain which falls 

 over a bare slope acts differently. It is not caught by 

 the crowns nor held by the floor, nor is its flow into 

 the >t reams hindered by the timber and the fallen 

 waste from the trees. It does not sink into the ground 

 more than half a> readily a> in the forest, as experi- 

 ments have shown. The result is that a OTC;II deal of 

 water reaches the streams in a short time, which is 

 the reason why floods occur. It is therefore true that 

 fmv-N tend to prevent flood-. Hut thiso-ood influence 

 is important only when the forest covers a larre part 



