11 



This table shows that the balance of water retained by the soil increase ^ 

 rapidly with altitude, and that the evaporation in mountain forests may be 

 reduced to about 10 per cent of the rainfall. If it be remembered that the 

 moisture is most effectually preserved in forests, it will easily be understood why 

 the mountain forests have from time immemorial been looked upon as the 

 preservers of moisture and feeders of springs. No doubt, a certain portion of the 

 water is again taken out of the ground by the roots of the trees and evaporated 

 through the leaves. The quantity thus consumed is not known at present, but it 

 cannot be more than 12 inches, the total quantity available in plain forests, 

 and probably it becomes less with elevation, so that a considerable balance 

 remains available in hill forests for the feeding of springs. 



MECHANICAL EFFECT OF FORESTS. 



The mechanical effect of forests makes itself chiefly felt in regard to the 

 distribution of the rain-water, the preservation of the soil on sloping ground, the 

 binding of moving sand, the prevention of avalanches, and the moderation of air 

 currents. 



(a) Feeding of springs and rivers. 



Most of the rain-water falling on a bare slope rushes down into the nearest 

 water course in a comparatively short time, thus causing a rapid rise in the level 

 of the stream. Only a comparatively small portion sinks into the ground, so as 

 to become available for the feeding of springs. Of the rain falling over a forest, 

 close on one-fourth is intercepted by the crowns of the trees, and the other three- 

 fourths fall upon a layer of humus, which possesses a great capacity to absorb 

 water and to retain it for a time. It has been shown, for instance, that mosses 

 of the species Hypnum, which grow under the shade of conifers, can absorb up to 

 five times their own weight of water, and peat mosses of the genus Sphagnum up 

 to seven times, while the leaf -mould to be found in a middle-aged well-preserved 

 beech wood can absorb and retain for a time a rainfall of five inches. Part of the 

 water thus absorbed penetrates into the ground and becomes available for the 

 feeding of springs, while the rest gradually finds its way into the nearest stream. 

 In this manner well-preserved forests must have a decided effect upon the 

 sustained feeding of springs, and the moderation of sudden floods in rivers. 

 When, however, the humus has been saturated with water and rain continues, the 

 effect of forests as regards inundations must cease, because the additional water 

 follows the laws of gravity, and finds its way into the valleys. 



(6) Protection of the soil. 



Water rushing down a bare slope possesses a great mechanical power, by 

 means of which it loosens the soil, and carries it down hill. In this way land- 

 slips are often caused, ravines are formed, and fertile land situated at the foot of 

 the ravine may be covered with silt and rendered valueless. Frequently the 

 debris collects in rivers and forms obstructions, which are followed by a diversion 

 of the bed and erosion of fertile lands. The rate at which this process proceeds 

 depends on the geological origin and the formation of the surface; the less binding 

 the soil and the looser the formation the greater will be the damage. If, on the 

 other hand, such a slope is covered with a well-preserved forest the roots of the 



