PLANT SOCIETIES 



473 



evident. Long-continued droughts during the growing season 

 which occur now and then, and the great accompanying forest 

 fires, show the inability of the forest to produce rainfall per se. 



644. Importance of the forest in the disposal of rainfall. 

 The importance of the forest in disposing cf the rainfall is very 

 great. The great accumulation of humus on the forest floor 

 holds back the water both by absorption and by checking its 

 flow so that it does not immediately flow quickly off the slopes 

 into the drainage system of the valley. It percolates into the 



Fig. 419. 



Mountain spring in forest, fed by the water held back by the abundant humus and dense 

 undergrowth. From Bureau of Forestry. 



soil. Much of it is held in the humus and soil. What is not 

 retained thus filters slowly through the soil and is doled out more 

 gradually into the valley streams and mountain tributaries, so 

 that the flood period is extended, and its injury lessened or en- 

 tirely prevented, because the body of water moving at any one 

 time is not dangerously high. The winter snow is shaded and 

 in the spring melts slowly, and the spring freshets are thus 

 lessened. The action of the leaves and humus in retarding the 

 flow of the water prevents the washing away of the soil; the 

 roots of trees bind the soil also and assist in holding it. 



