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Page:Popular Science Monthly Volume 82.djvu/317

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THE

POPULAR SCIENCE

MONTHLY

 

APRIL, 1913




THE INFLUENCE OF FORESTS UPON CLIMATE
By Professor ROBERT DeC. 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 influence 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 forest 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 rainfall, 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 concluded 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