MECHANICAL EFFECT OF FORESTS. 17 



mountain forests have, from time immemorial, been looked upon 

 as the great preservers of moisture. 



3. Mechanical Effect of Forests. 



The mechanical effect of forests makes itself felt chiefly with 

 regard to the distribution of the rain water, the preservation of 

 the soil on sloping ground, the binding of shifting sand, the 

 prevention of avalanches, the moderation of air currents and the 

 aeration of the soil and subsoil. 



(a) Feeding of Springs and Rivers. — Most of the rain-water 

 falUng on a bare slope rushes down into the nearest watercourse 

 in a comparatively short time, thus causing a rapid rise in the 

 level of streams. Only a comparatively small portion sinks into 

 the soil and becomes available for the feeding of springs. Of the 

 rain falhng over a forest, close on one-fourth is intercepted by the 

 crowns of the trees, and the other | fall upon a layer of humus 

 which possesses a great capacity of absorbing water and of 

 retaining it for a time. It has been shown, for instance, that 

 mosses of the genus Hypnum, which grow under the shelter 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 found in a middle-aged well-preserved beech wood 

 can absorb and retain for a time a considerable quantity of water. 

 Some of that water evaporates from the soil covering, but the 

 greater part penetrates into the soil ; some of it is taken up by the 

 roots, and the balance becomes available for the feeding of springs. 

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

 the sustained flow of springs. 



The interesting point is to determine the proportion of the 

 annual rainfall which actually reaches the under-ground water 

 level. The problem is very complicated, as its solution depends 

 on a great variety of factors, such as the amount of rainfall and 

 its distribution over the different seasons of the year. It should 

 also be remembered that about one-fourth of the rain is inter- 

 cepted by the leaf canopy, and in the case of a light shower every 

 drop of it may evaporate and never reach the ground. In this 

 respect the species of tree causes great differences. Of the 

 ordinary European trees the firs retain more water than the pines 

 or the deciduous broad-leaved species. On the whole, the view 



