FORESTS AND CLIMATE 331 



after a thorough study of the different phases of this many-sided prob- 

 lem. 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 reser- 

 voirs would be much more effective. The prevention of erosion undoubtedly out- 

 weighs 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 popula- 

 tion 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 hill- 

 sides. 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 agri- 

 cultural 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. 



V. 



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