HOW THE FORESTS HELP MANKIND 125 



yond doubt that forests make a land more temperate less 

 subject to sudden changes. A forested country is neither so 

 hot in the day, nor so cold at night. During the day the sun's 

 heat falling upon the forest is largely absorbed by the leaves so 

 that less heat is available to raise the temperature of the earth's 

 surface. Much of this heat that the tree absorbs is used for 

 growth and to form new wood. So in a sense, the wood of a 

 tree is stored-up heat and when we burn it we are again releas- 

 ing the imprisoned warmth of long past summer days. This 

 heat, then, the forest stores up and prevents it from raising the 

 temperature of the earth's surface. When night comes and the 

 air cools quickly, the forest leaves slowly return some of this 

 heat into the atmosphere and so the nights tend to remain 

 warmer. This is why a desert country is visited by greater ex- 

 tremes of temperature than a forested region. 



Forests serve another useful end when, as windbreaks, they 

 prevent the hot winds of summer from drying out the soil and 

 killing farm crops. For this reason in treeless regions long belts 

 of lombardy poplar, locust, or evergreens are often planted 

 about farm houses and between cultivated fields. They prevent, 

 too, the great damage sometimes caused by sand dunes, those 

 areas of shifting sands which, blown by the wind, move across 

 the country destroying farm lands and covering highways and 

 buildings. Once trees can get a foothold in such areas as these, 

 they destroy the force of the wind and by holding the soil 

 with their roots, at last make these shifting sands prisoner and 

 keep them motionless for all time. 



Forests have a tremendously important influence on the flow 

 of streams. To understand best just how this influence is ex- 

 erted, one may imagine two tracts of land on two adjacent 



