FORESTS AND THEIR CLIMATIC INFLUENCE. 395 



sabject from a diiferent point of vievr, or embraced but a part of the 

 question. Let us adduce the proofs : 



The action of forests on the climate of a country is very complex, for 

 it further depends, first, on their extent, their elevation, the nature of 

 the soil, and of the subsoil ; second, on their orientation or direction 

 with regard to the winds, whether warm or cold, dry or humid ; third, 

 on the age at which they are cut, on their species, that is to say, whether 

 the leaves be^caducous or persistent, seeing that the radiating and ex- 

 halant lu'operties are.not the same at all seasons; fourth, on the season 

 of rain, whether in summer, autumn, or winter; fifth, on the proximity 

 of pestilential marshes, &c. 



Whatever be the action exerted by a forest, it will of course bear a 

 relation to its extent, for a tree or group of trees does not act like a 

 large mass ; a single tree indicates, by the shade which it throws on the 

 surrounding soil, that its presence is injurious to the culture of plants 

 to a distance which depends on its height ; the loftier the forests, the 

 greater the extent of the shade ; the shade depends only on the skirt of 

 the forest, and to a certain degree on the density of that skirt. 



The height of the trees, if the forest has a certain density, may be 

 an obstacle to the wind, according to their position in regard to the di- 

 rection of the latter. It is well understood that forests principally act 

 as a shelter only in relation to the lower winds ; the obliquity of these 

 is to be taken into consideration, as will be seen hereafter; the depth 

 of the forest supplies to a certain extent the compactness in which it 

 may be deficient. This action will be developed further on. The nature 

 of the soil claims consideration' according to the proportions of clay, 

 lime, and silex which enter into its conqiosition. In the case of dif- 

 ferent combinations of these constituents the effects are quite different, 

 much depending also on the circumstance whether the subsoil be per- 

 vious or impervious. All soils, as is well known, may be reduced to the 

 four following divisions : 



Pervious soil Jo Pervious subsoil ; 



( "2. Impervious subsoil. 



T...,,„.„.^r.r.o, ^^n S •^- Pervious subsoil ; 



Imuervious sou < , t • i i 



^ ^4. Impervious subsoil. 



The roots of trees, by penetrating into the soil and subsoil, separate 

 the particles and thus tacilitate the escape of the surface waters ; the 

 older the trees and the more ancient the preserves, the more deeply do 

 their roots penetrate, and the greater the facility consequently with 

 which the waters traverse the subsoil. 



Let us examine the effects of the four kinds of soil above enumerated 

 on the vegetation of forest trees. First case. With a pervious soil and 

 pervious subsoil the waters never stagnate, be the soil wooded or not. 

 Second case. With a pervious soil and impervious subsoil, stagnation of 

 the waters takes place when the soil is not wooded — of this Brenne and 

 Sologne are examples ; if it be wooded and the subsoil have not too 

 great a depth, the waters readily percolate by help of the roots which 

 traverse it ; in the contrary case they remain stagnant. Third case. The 

 soil impervious, the subsoil pervious : this soil suits only certain species 

 other than the oak. Fourth case. Soil and subsoil impervious: with this 

 soil forest cnltuie agrees least of all ; yet are there certain species which 

 can live and develop themselves therein. The roots of trees, therefore, 

 by making their way into the soil, fulfill an important part in the distri- 

 bution of the waters of a country. MM. Gras and Alphonse Surel, 

 'from numerous observations made in the higher Alps, explain as fol- 

 lows the effect produced by forests situated on the sides of mountains. 



