MECHANICAL EFFECT OP FOKESTS. 19 



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 trees and the layer of humus keep together 

 and protect the soil against the action of water ; besides, the 

 crowns intercept and retain, at any rate for a time, a considerable 

 portion of the water. On the whole, a series of obstacles are 

 opposed to the movement of the water, which reduce its velocity 

 and force it into numerous small channels, thus dividing it. The 

 beneficial effect of tree vegetation in this respect can be observed 

 in most mountain ranges, as in the Alps from France to Austria. 

 Wherever in these parts extensive deforestation has taken place 

 the consequence has been the gradual formation of a series of 

 torrents in all places where the surface did not consist of hard 

 rock ; the debris brought down has covered more and more land 

 at the base of the torrents, and this evil had grown to such an 

 extent that efforts have been made to re-afforest the denuded 

 areas at a great outlay. In such cases, afforestation had to be 

 preceded by the construction of dams, dykes, walls, etc., to steady 

 the surface, to enable the new forest growth to estabhsh itself 

 and once more to lay hold of the surface soil. 



The importance of maintaining a complete cover of vegetation 

 in all such cases was recognised centuries ago, so that already in 

 the Middle Ages so-called " Protection Forests " existed, which 

 the then existing laws protected against devastation. 



Forests protect the soil also in low lands, wherever it consists 

 of shifting or moving sand, along the sea shore as well as inland. 

 The action in this case is due, partly to moderating the force of 

 air currents, and partly by keeping the soil together through the 

 action of the roots, by the formation of humus and the retention 

 of moisture. Along the sea-coast frequently artificial dunes have 

 to be constructed before afforestation can be attempted. In this 

 way the Landes of France have been converted from a dreary 

 waste into extensive forests, fields and vineyards. 



(c) Protection against Avalanches. — Although most avalanches 

 in the higher mountains originate above the upper limits of tree 

 growth, there are many cases where the presence of a well- 

 preserved forest protects towns and villages lying below, by 

 preventing the formation of avalanches, or by stopping their 

 forward movement while they are still small. When they 



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