72 FORESTRY 



The Influences of Forests on Climate. 



The subject about which there is the most dispute is the 

 effect of the forests upon the climate and rainfall. Popu- 

 lar interest is easily aroused on this theme. But a great 

 many statements made for or against the beneficial effects 

 of forests are not proven. A long series of records has 

 been kept at European experiment stations established for 

 the purpose of finding out the facts, and from such conflict- 

 ing data, some points have been proved. The total amount 

 of rainfall over a large area for a number of years is prob- 

 ably not perceptibly increased by forests, though it is al- 

 most impossible to prove this. Nor do forests have much in- 

 fluence on storms of a general character. They may, and 

 probably do, slightly increase the number of summer show- 

 ers. The influence of forests upon the air under the crowns 

 is to moderate extremes of temperature, both hot and cold. 

 They are very effective in checking strong winds, so that 

 not only within a forest is the air almost still on a windy 

 day, but the effect is felt for a long distance on the lea side. 

 Single rows of trees are planted as wind-breaks in prairie 

 countries. Both by checking the wind and cutting off the 

 rays of the sun, forests check evaporation and tend to pre- 

 serve the moisture in the air and in the soil. The effect of 

 moisture in the air is to check undue loss of heat by radia- 

 tion and to dispel the direct heat rays of the sun. This 

 largely accounts for the modifying influence of the forest 

 on extremes of temperature. But taken as a whole, we 

 cannot prove that the presence or absence of forests in- 

 creases the rainfall or modifies the climate to a great de- 

 gree, though all the evidence points for rather than against 

 the theory. Nor is it possible to establish any connection 

 between the destruction of forests in one region and 

 changes of climate in an adjoining region separated by 20 

 to 200 miles in distance. Whatever influence the forest has 



