Page:A primer of forestry, with illustrations of the principal forest trees of Western Australia.djvu/29

23 CHAPTER V.

FORESTS AND CLIMATE.

Reference has already been made to certain aspects of the indirect value of forests. But beyond these ways in which forests are indirectly of great moment, there is one whose importance must be specially stressed. In every part of the world where forests are great, there is a constant annual rainfall, sometimes spread with fair evenness over the whole year, and sometimes occurring as seasonal rains. Again, wherever forests are absent or tree-life sparse, comparatively little rain falls. The presence of forests implies a sufficient rainfall; their absence often means little or no rain and desert country. The world furnishes many examples of this. There are no forests in Arabia, and the rainfall is of the scantiest. The same may be said of North-Central Africa, where there stretches over vast arid distances the Sahara Desert, Central Australia has no great forests, and such trees as there are are few in number and stunted in size, and the rainfall there is very small. History tells us quite clearly that the destruction of forests alters the climate of a country by reducing the rainfall to a point that makes the growing of food-crops impossible. Time was when Mesopotamia was a well-wooded country and when grains of various kinds were produced in abundance. Many centuries ago its forests were almost wholly destroyed. The rainfall diminished to such a degree that agriculture could not be carried on except in the immediate neighbourhood of the two great rivers—the Euphrates and the Tigris—and such crops as were grown owed their existence to irrigation. Sicily, at one time, was one of the granaries of ancient Rome—that is, it grew and sent to Rome large quantities of wheat every year—but its forests were almost wholly cut down and it ceased to be a great wheat-growing country. The island of Cyprus is another instance in proof of the fact that the wholesale destruction of forests has a disastrous effect upon a country's productive power.

But forests are in other ways of inestimable value so far as moisture is concerned. When rain, especially heavy rain, falls upon a hard, bare country, much the greater portion of it immediately runs off into the streams and rivers and on into lakes or into the sea. When the country, however, is forested, the case is different. Much of the surplus water runs off of course, but a great deal is retained by the forest. In every forest there is upon the ground an accumulation of what is known as "humus," made up of the leaves and twigs that have fallen from the trees. This humus has to become thoroughly soaked before any water begins to run off. Further, the ground within a forest is more porous than ground that has no forest cover. The reason is obvious, for beneath the surface of the forested ground are vast masses of roots, and these break up the sub-soil, permitting the absorption of water. Experiments in various parts of the world have proved beyond question that forests have a beneficial effect on streams and springs. Rivers that are fed from forested watersheds have a more uniform discharge and carry less debris than streams coming through an unforested watershed. It has also been proved conclusively that extensive damage from floods occurs less frequently in streams coming from forested watersheds than in streams rising in poorly forested or treeless watersheds

In forested country, too, streams run throughout the year, because the forest gradually gives off the water it holds, while in unforested regions, the water passes quickly to the streams, and, in the absence of further rain, these dry up. Springs also are favourably influenced by the presence of forests, as their flow is regulated and made continuous by the water conserved by the forest and given off gradually. Ringbarking large areas of tree-covered land has much the same effect on streams and springs as does cutting down the trees, and more particularly is this the case if fire sweeps through the ringbarked country and destroys the humus. When the Mundaring Weir was formed, a considerable part of the water catchment area