This section is from the book "The Gardener's Monthly And Horticulturist V24", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Dr. J. M. Anders, of the Philadelphia Academy of Natural Sciences, who has already contributed some valuable papers on the transpiration of plants, gives some more valuable facts in the January number of the American Naturalist. He finds that one square foot of naked soil will evaporate six times as much moisture as a square foot of leaf surface; but a forest has twelve times more leaf surface than the earth surface on which the trees grow, and, hence, the evaporation from a square foot of forest-land is just double that of a naked surface. He further finds that, say in the vicinity of Philadelphia, twelve inches of annual rainfall is given off during the leafy season of six months, which is about half the annual rainfall. It would thus seem that trees have very little to do with the feeding of springs. It is sometimes supposed that the trees retain snow, which melts more slowly under the trees than in the sunshine, and that it, therefore, runs slowly into the streams instead of melting rapidly in the sun where the trees are absent ; but Dr. Anders shows that the earth under trees is in a very absorptive condition, and that the slowly-melting snow is taken up gradually by the earth under the trees, in order to form the great reservoir of moisture which is to supply the enormous summer demand from the leaves.
Another interesting conclusion, bearing on the literature of forests and climate, may be drawn from these observations of Dr. Anders - namely, the trees have to receive this moisture before they can give it out again, and we may, therefore, say trees are rather the result than the cause of a moist climate. We have to look to the evaporating power of the sun on immense tracts of water and the condensing power of polar currents for our chief sources of rainfall, leaving to trees the playing of a very small part in the operation. - Independent.