TBEES, SHRUBS, AND FOEESTS 367 



The great value of trees in this forest domain is enhanced 

 by the fact that a large part of it is in the mountains and in 

 regions like the pine barrens of the Southern states, where 

 the land is not of value for agriculture on account of the un- 

 productiveness of the soil in those regions. The trees thus 

 render an otherwise unfruitful region productive, and serve 

 at the same time as a protection against floods, erosion, and 

 drought by their control of rainfall and other climatic factors. 



FIG. 222. The ornamental function of trees and shrubs 

 Photograph furnished, by the United States Forest Service 



Climate and water supply. The factors of climate which are 

 controlled in any measure by forests are concerned largely with 

 temperature, air movements, and water control. The effect on tem- 

 perature is one which is felt only in the immediate vicinity 

 of the forests themselves, and not over the country at large. 

 It is a well-known fact that the leaves of trees in a forest 

 absorb a large part of the heat which falls upon them, and 

 that they utilize this heat in warming the leaves, in making 

 sugar and starch, and in the evaporation of water vapor. The 

 rich covering of humus on the forest floor also absorbs heat 

 and protects the soil beneath from absorbing and radiating it 

 as the soil in. naked exposed regions would. As a consequence 



