

THE 



POPULAR SCIENCE 



MONTHLY. 



APRIL, 1913 



THE INFLUENCE OF FOEESTS UPON CLIMATE 



By Professor ROBERT DbC. WARD 



HARVARD UNIVERSITY 



Introduction: Popular Belief in Forest Influences, and its 



Possible Origin 



FAR and wide, the world over, we find a popular belief in an influ- 

 ence of forests upon climate, especially upon rainfall. This is 

 not difficult to explain. Take our own experience, for example. 

 On a summer day we leave the hot, sunny road and walk along a 

 narrow forest path. The trees give shade; the glare and heat of the 

 road are replaced by the soft, dark carpet of leaves and moss ; the air 

 seems cool and damp. It is all a great relief, and the impression 

 is inevitable that a forest climate is different from that of the open. 

 Again, on a spring day, when the snow has disappeared from the 

 fields, but when a chilly, wintry wind is blowing, we leave the open 

 meadow and cross a patch of woodland. There is snow still lying 

 deep under the trees; there is welcome protection from the biting 

 wind; it seems pleasantly warm. Has not, we naturally say, the for- 

 est a climate all of its own? Once more. We observe, the world 

 over, that where there are extended forests there is heavy rainfall, and 

 we see deserts and treeless areas where the rainfall is light. We infer 

 that the forests have something to do with producing the heavier rain- 

 fall, and some of us may even go a step farther and think that the great 

 treeless areas were once forested, and that deforestation has made them 

 dry. Or, to give one more case, we may have noticed the increasing tree- 

 growth with increasing elevation on our mountains, and may have con- 

 cluded that the denser forest is the cause of the heavier precipitation 

 which is generally observable as we ascend our mountain slopes. 



Thus it may come about, naturally enough, that people believe in 

 forest influences upon climate. Yet, if we ourselves happen to have 



VOL. LXXXII. — 22. 



