CHAPTER III. 

 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 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 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 

 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 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 retard- 

 ing 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 subsoil, 



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