CHAPTER II. 



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: 



(1.) During the day the ground under the tree is pro- 

 tected 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 con- 

 stantly 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 tempera- 

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

 period of warm weather, it may, on the other hand, increase 

 the temperature 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 evaporation. The large amount of water 

 which is transpired by a tree is largely drawn from the sub- 

 soil 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. 



(4.) The leaves that fall to the ground form a mulch 



