FORESTS AND CLIMATE
331
after a thorough study of the different phases of this many-sided problem. It is easy to see why different observers, under different conditions, have reached such divergent results.
Whatever influence forests may exert upon precipitation, run-off and erosion, it is evidently greatest in the mountainous regions where the rainfall is heaviest, slopes steepest and run-off most rapid. Here also the land is less useful for other purposes. The extent of the influence of forests upon these three factors varies greatly, according to circumstances involved in each case. Under one set of conditions, forests may benefit stream flow and mitigate floods, while under other conditions they may have the opposite effect. In no case can they be relied upon to prevent either floods or low-water conditions. There is substantial agreement on this point. Nor is their influence extensive enough to warrant their use as the only means of securing the uniformity of stream flow which is desirable for navigation or the development of water power. For this purpose storage reservoirs would be much more effective. The prevention of erosion undoubtedly outweighs all other benefits of forestation and constitutes one of the most necessary phases of conservation. The commission favors the prevention of deforestation of mountain slopes wherever the land is unsuitable for agricultural purposes, and urges the reforestation of those tracts which have already been denuded, not only when located at the headwaters of navigable streams, but wherever this would be the most valuable use of the land. The increasing pressure of population upon subsistence will make it necessary to use for agricultural purposes all land suitable for cultivation. The influence of forests upon stream flow and erosion is not sufficient to warrant their retention except where the land is unsuited for other purposes. Furthermore, it is possible, if correct methods of agriculture are employed, to retain for cultivation areas located on steep hillsides. This has been successfully accomplished in other countries by terracing and by other means. It must be remembered, however, that reforestation alone can accomplish little toward preventing erosion. The prevention of forest fires, the regulation of hillside farming and the prohibition of complete denudation of mountain tracts, where the soil cover is thin and the land unsuited for agricultural purposes, are also necessary. Forests retard the melting of snow in the spring, and, by allowing the water from this source to be absorbed, exercise a beneficial influence upon stream flow, but should heavy spring rains fall upon the snow thus preserved and cause it to melt within a few hours, the effect of the forest is in such a case to aggravate rather than ameliorate flood conditions. It thus appears that under one set of conditions forests may exercise a beneficial influence upon stream flow and floods, while under another their influence will be harmful.
But these problems do not directly concern the climatologist. He is satisfied if he can make clear, as he sees it, the influence of the forest as a control of climate. If his statements are often disappointingly broad and generalized, it is because he has not the needed scientific basis for making them otherwise.
