22 PRINCIPLES OF AMERICAN FORESTRY. 



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 which 

 prevents the drying out of the soil. They check the flow 

 of water over the land, thus preventing the washing away 

 or compacting of the soil by heavy rains, and giving the 

 water a better chance to soak into the ground. 



5. A tree protects from the destructive force of severe 

 winds. A single tree or group of trees may seem to have 

 little effect on tornadoes, but large groups of trees may 

 possibly prevent their formation or greatly lessen their 

 violence. Protection from severe winds may greatly affect 

 the growing of plants, since, on account of the winds, many 

 plants that may be successfully grown when protected by 

 shelter-belts cannot be grown on the open prairie. This pro- 

 tection, when present, serves to lessen the fuel necessary to 

 warm dwelling-houses and also lessens the food eaten by 

 animals. It also keeps the surface soil in fields from being- 

 blown away. 



In these five principal ways a single tree affects the con- 

 ditions of climate and soil in its immediate vicinity. To be 

 sure, some of them are not so very evident where a single 

 tree grows in an open field, but where trees are growing in 

 groups or on large tracts of land all of these factors are 

 important in modifying climate and soil conditions, and 

 will be referred to at greater length. 



INFLUENCE OF FORESTS ON WATER SUPPLIES. 



It is very evident that the proper disposition of water 

 upon the land is the most important factor in the growing of 

 crops, and it is equally evident that nature's changeful and 

 wasteful ways of supplying water to crops are not the best 



