68 



A PRIMER OF FORESTRY. 



effect on rain Avater after it has fallen. When rain 

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

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

 part of this water may reach the ground by running- 

 down 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 

 floor, which sometimes has an absorbing power so great 

 that it can hold for a while a rainfall of 5 inches. Yet 



this water does not 

 remain in the porous 

 floor, but in the end 

 runs off' into the 

 streams, or is evapo- 

 rated, or sinks into 

 the ground. That 

 which gets into the 

 g round is either 

 taken up by the roots 



t' 



■#^ 



Fig. 43.— The beginning of erosion in soil 

 tramped bare by stock. Sierra Nevada 

 Mountains, California. 



M\ or goes to feed the 



water- 



springs and 

 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 streams hindered by the timber and the fallen 

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

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

 ments have shown. The result is that a great deal of 

 water reaches the streams in a short time, which is 

 the reason why floods occur. It is therefore true that 

 forests tend to prevent floods. But this good influence 

 is important only when the forest covers a large part 



