368 



GENERAL BOTANY 



the forest has a general cooling effect on the air in its vicinity 

 and so protects 'the soil from drying up. Air movements in the 

 form of winds and storms are also restricted by forests, which 

 therefore serve as effective windbreaks and at the same time 

 affect the temperature of a region in both winter and summer. 

 The early settlers in the West soon learned this advantage of 

 trees as windbreaks and planted cottonwoods and other quick- 

 growing trees on the north and west sides of their holdings. 



1 Eastern Region \. % 



2 Central Treeless Region 



3 Western Region 



TROPICAL 

 FOREST 



FIG. 223. General map of the forest areas of the United States 



In addition to the effect on the relative humidity and tem- 

 perature of a region the forests have an important function in 

 the control of water falling in the form of rain or snow. 



Forest control of rainfall and floods. Rainfall is supposed by 

 some to be increased by the presence of great forests, and the 

 investigations of European foresters would seem to bear out 

 this assumption. Other data, however, gathered with equal 

 care by experienced scientific foresters, yield opposite results, 

 and it is doubtful whether the forests have any marked effect 

 on precipitation. 



Floods and erosion, or the wearing away of soil by water, 

 are so largely controlled by forests that this control is now 



