CHAPTER IIL 



FOREST INFLUENCES. 



Upon careful observation it will be found that a single large 

 spreading tree growing in an open field appreciably affects 

 climatic and soil conditions in the following ways: 



(i.) During the day the ground under a tree is protected from 

 the sun's rays and is therefore cooler than soil not protected. 

 As a result of this protection, the air under the tree is cooler 

 than the air in the open, and, as it is constantly in circulation, 

 tends to cool the air in the immediate vicinity of the tree on 

 sunny days. 



(2.) At night a tree retards the radiation of heat from the 

 ground under it. This tends to equalize the temperature of not 

 only the soil and air under the tree, but that in the near 

 vicinity. Therefore, though a tree may reduce the temperature 

 of the soil and air on sunny days or during a short period of 

 warm weather, it may, on the other hand, increase the temper- 

 ature at night or during a short period of cool weather. For 

 example it may be noticed that vegetables growing near trees 

 are frequently uninjured by autumn frosts which destroy those 

 growing in the open. 



(3.) A tree aids in retaining water in the surface soil to the 

 leeward by breaking the force of the wind, and thus retarding 

 evaporation, for it is known that evaporation increases with the 

 rapidity of the air currents. It retains the water in the surface 

 soil under the tree by shading the soil and thus retarding evap- 

 oration. ^The large amount of water which is transpired by a 

 tree is largely drawn from the subsoil, and this increases the 

 humidity of the surrounding air without drawing on the water 

 of the surface soil. But some kinds of trees take up so much 

 of the water from the soil as to preclude the growing of crops 

 in such places near them. 



