282 TRANSFER OF WATER THROUGH THE PLANT. 



air of a forest is scarcely greater than that in air over open 

 ground. But the relative humidity in the former case is about 

 six per cent greater than that in the latter. 



752. It has been held by many that forests have a direct effect 

 in increasing the amount of rain-fall, presumably by bringing, 

 through transpiration, the amount of moisture in the atmosphere 

 of a wooded place nearer the point of precipitation. But the 

 weight of evidence now available is against this view. 1 



753. On account of the shelter which they afford, the trees of 

 a forest play an important part in the storage of a water-supply. 

 Under their branches small plants can thrive, and by their hold 

 upon the ground impart to even very porous soil a good degree 

 of stability. 



Soil covered with mosses and other humble plants which live 

 in the shade not only holds back a large part of any given rain, 

 so that the water drains off more slowly, but it is not likely to 

 be itself washed down to lower levels. Upon a treeless slope, 

 however, the rains which fall sweep down at once. 



754. There is, furthermore, less evaporation from a soil 

 covered by a growth of trees than from open ground. Obser- 

 vations during the summer months recorded by Ebermeyer 2 

 show that the evaporation of water from the soil of a forest, 

 when the surface is not covered b} T grass, is only sixt} r -two per 

 cent of that which takes place from open ground. But if the soil 

 under the shade of a forest is covered with grass, the evaporation 

 is eighty-five per cent of that in the open ground. 



Von Mathieu found that the evaporation from open ground 

 from April to October was about five times as much as from 

 wooded soil ; but he does not state whether the soil in the latter 

 case had grass upon it or not. 



1 "Forests increase the annual relative moisture of the air, but this in- 

 fluence is much more noticeable at high elevations than at low elevations. 

 The precipitation of moisture (dew, cloud, rain, snow) takes place more readily 

 on this account in wooded than in treeless regions, and the frequency and 

 intensity of these precipitations increase with elevation above the surface of 

 the sea. Moisture descends more readily and frequently upon a wooded than 

 upon a treeless mountain of the same height. Forests affect rain-fall only so 

 far as they increase the relative amount of water held in the air, and thus 

 bring the relative amount nearer the point of saturation ; thus with the fall of 

 temperature in the forest, a part of the moisture is easily precipitated. . . . 

 Forests make the climate of a country moister, and especially so in summer " 

 (Ebermeyer : Die physikalischen Einwirkungen des Waldes auf Luft und 

 Boden, 1873, p. 151). 



2 Die physikalischen Einwirkungen des Waldes, p. 175. 



