THE FOREST AND SPRINGS 373 



it is there that nearly all springs are concentrated. Forests existing on mountains, 

 notably on those whose aspect is perpendicular to those of moist winds, cause the pre- 

 cipitation of the greatest quantity of aqueous vapor which they contain. It is enough 

 to cast a glance at a hydrographic map to be convinced of this fact. Bare, denuded 

 mountains have only a very feeble action in this respect; the countries bordering on the 

 Adriatic as well as on a part of the Mediterranean, which are renowned for their dry- 

 ness, show this in a very striking manner. Deprived of forests, these mountains lack 

 the means of coohng the air and drawing to themselves in consequence the vaporous 

 precipitations it contains. The denuded soil, which the sun penetrates with intense 

 heat on those parts exposed to the west and the southwest, does not certainly possess 

 this property." 



"A second distinction consists in the enormous diminution in the proportion of 

 surface flow on wooded mountains, compared with the same slopes when they are 

 denuded. . . . The water, instead of precipitating itself into the thalweg and caus- 

 ing thereby sudden and dangerous inundations, penetrates slowly through the covering 

 and into the soil which it soaks to a great depth. Therefore it is unquestionable, and 

 we believe an uncontested fact, that mountain forests are favorable generally to the 

 production of springs." 



There is a stronger reason stiU for this being the case when mountain forests grow 

 in a hot climate where physical evaporation is considerable. 



The Influence of Forests on the Infiltration of Waters in the Plains. — The 

 influence of the surface flow is complete in the case of forests in the plains. There 

 the feeding of the subterranean sheet will depend only on physiological evaporation 

 and on the permeability of the soil. Let us first examine the action of forests on this 

 permeability of the soil. 



Forest soil in good condition is naturally light. The roots of trees penetrate deeply 

 into it, sometimes to a depth of 10 and 13 feet and more; in swelling out they produce 

 the effect of wedges which divide the soil mechanically. When the trees have been 

 felled the roots decompose and their place is taken by a network of channels filled with 

 hygroscopic matter, which directly conducts the water to considerable depth. Along 

 the roots of trees under foot, especially near the stem between the soil and the bark, 

 there exist empty spaces which are caused by the swaying of the tree when shaken by 

 the wind; rain water, which has run along the stem, arrives directly, one might say 

 instantaneously, by means of these at the soU. Lastly, forest vegetation is favorable 

 to the division of the soil through the action of earthworms. 



In winter, the temperature of the forest soil is appreciably higher than that of the 

 open ground. It often results from this that during the cold season the rainfall or 

 melting snow acts upon a frozen surface, which has become impenetrable at the surface, in 

 such a manner that all the water disappears in surface flow. In the forest, the soil which 

 is less cold need not necessarily be frozen, and can therefore absorb the fallen water. 



According to all the evidence collected, physical evaporation of the water of the soil 

 is less under trees than on an agricultural soil. The forest covers the ground with a 

 double protective screen; first the covering of dead leaves, an eminently hygroscopic 

 substance, and in consequence always cool, which, superimposed immediately on the 

 ground, opposes evaporation with great energy. Higher up, the crown, often very dense, 

 offers its maximum density in summer, at a period when evaporation is greatest. The 

 temperature of the air is also lower under the trees than outside, especially in summer. 

 This forms a powerful impediment to evaporation. The lowest temperature of the 

 forest soil in summer acts again in the same way. 



Finally, evaporation is much favored in a flat country by the wind, which is con- 

 tinually renewing the strata of the air, saturated by direct contact with the soil. 



