TREES. 87 



The water evaporated by leaves keeps the air 

 saturated. This moisture aids in the precipitation 

 of rain. When, however, there are few trees to 

 supply atmospheric moisture since rainfall is de- 

 pendent upon additional moisture carried by winds 

 from the seas, long dry periods are likely to be 

 experienced in certain sections during summer 

 when rain is most needed. 



A forest gives out much moisture to the air 

 by leaf evaporation, and holds back the water on 

 the forest floor, so that it is given off gradually. 

 This is an ideal condition. It is not the great 

 amount of water falling upon the ground at one 

 time that does most good, but the constant supply. 

 A forest holds back the water in two ways: by 

 shading the ground so that little evaporation takes 

 place, and by retaining it among fallen dead and 

 decaying leaves and branches. The latter not 

 only has a tendency to produce frequent summer 

 rains, but also prevents the rapid rushing away of 

 heavy rains to cause floods. 



Trees are great barriers against scorching, hot 

 winds in summer, and against strong, destructive 

 winds coming at any time of the year. In the west- 

 ern prairie states many groves have been planted 

 purely as wind-breaks for farm buildings. They 

 have proved to be very efficient in that respect. 

 Besides, the people there are now able to use many 

 of the trees for fuel and timber because of the 

 necessary thinning as the trees grow in size. 



