CHAPTER III. 
FOREST INFLUENCES. 
Upen 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 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. 
