Open main menu

Page:Popular Science Monthly Volume 19.djvu/189

This page has been validated.
177
THE VALUE OF OUR FORESTS.

the amount of precipitation has for some time engaged the attention of naturalists. Such an influence has been asserted, partly from theoretic considerations and partly on account of the entire change presented by the climatic relations of the countries in which the forests have disappeared.... It is probable that such influence exists; but while on the one hand its consequences may be over-estimated, on the other hand there is want of direct proof, inasmuch as the rain measurements have been continued for too short a time, both at stations situated within the woods and outside of them in the open fields....

"The commission consequently concluded that an influence of the woods upon the amount of rain deposited, and especially upon the yearly contribution, is probable, although direct observation does not give sufficient evidence to determine its extent, or positively its existence."

Dr. Rogers, of Mauritius, gives this testimony: "So late as 1864 the island was resorted to by invalids from India, as the 'pearl' of the Indian Ocean—it being then one mass of verdure. But, when the forests were cleared to gain space for sugar-cultivation, the rainfall diminished, the rivers dwindled down to muddy streams, the water became stagnant in cracks, crevices, and natural hollows, while the equable temperature of the island entirely changed, drought was experienced in the midst of the ocean, and thunder-showers were rarely any longer witnessed.... The hills were subsequently planted with trees, and the rivers and streams resumed their former dimensions."

The Island of Ascension was formerly almost a barren rock. The supply of water was very scanty, derived solely from a few springs, and water was often brought from the Cape of Good Hope, and even from England, for the needs of the garrison. About twenty-five years ago the planting of trees and shrubs and the cultivation of the soil were undertaken vigorously. The water-supply has increased with the progress of this work, until now it is excellent, and the garrison and ships visiting the island are supplied with abundance of water and vegetables of various kinds.

Observations in France by M. Fautrat, reported to the Academy of Sciences, showed that, in a dense wood of five hundred hectares, a rain-gauge fixed on a tall poplar received much more water than one of similar height three hundred metres beyond the borders of the woods. Experiments continued during two years confirmed the first results, and an instrument placed over a forest of Pinus sylvestris, at twelve metres' elevation, received ten per cent. more water than one at the same height in the open fields.

But, however the case may be as to the effect of forests upon the amount of rainfall, there can be no doubt that they secure a more equable distribution of the rains than is usual in the open country. They are also great storehouses of moisture. By their myriad leaves they intercept the moisture of the passing clouds or the damp winds, and convey it to the ground, or hold it within their embrace ready to