312 M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 



(3) On the age since cutting, and npon the species, as whether of ever- 

 green or deciduous kinds, since the radiating and evaporating power are 

 not the same at all seasons ; 



(4) As to whether the rainy season comes in summer, autumn, or win- 

 ter; and 



(5) The proximity of pestilential marshes, &c. 



Whatever the effect of a forest may be, we may assume that it is pro- 

 portioned to its extent ; for a tree or a clump of trees cannot have as 

 much effect as a great mass of woodland. A single tree indicates by 

 its shadow on the ground near it that its presence is injurious to the 

 cultivation of plants within a certain distance, depending upon its 

 height. The higher a forest grows the greater this shadow extends, 

 and thus the effect is felt within a certain limit in its borders and to 

 some distance beyond. 



The height of the trees, (if the forest has a certain density), may afford 

 an obstacle to the winds of greater or less extent, according to the slope 

 of the surface and the relation which this bears to the direction of these 

 winds. It is well understood that the forests do not afford any con- 

 siderable shelter except against the lower winds, and the obliquity of 

 these must be taken into consideration, as we shall hereafter see. Their 

 density, up to a certain point, supplies the place of a solid mass, as we 

 shall further on more fully show. 



The nature of the soil may vary, as follows: 



It may be silicio-argillaceous, silicio-calcareous, or argillo-calcareous, 

 and the subsoil may be permeable or impermeable. The effects are very 

 different in these various conditions. We may classify these various 

 conditions of the soil as follows : 



Pervious soil f ^- Subsoil pervious, 

 ir-ervious sou j 3^ gubsoil impervious. 



TTv,r^«■n^;^,.o o^4i f 1' SubsoU pervlous. 

 Impervious soil J ^^ g^^^^.^ impervious. 



The roots of trees penetrating the soil and the subsoil separate these 

 parts and facilitate the flow of waters that may fall upon the surface, 

 and the older the woods and the greater the number of old reserves the 

 deeper do the roots penetrate into the soil and the easier do the waters 

 pass down. Let us examine the effects of these four divisions of the 

 soil that we have mentioned, upon forest vegetation : 



1. With a pervious soil and subsoil, the waters are never stagnant, 

 whether the ground is wooded or not. 



2. With a pervious soil and an impervious subsoil, there is a stagna- 

 tion of waters if in an open country, as in La Brenne and Sologne. In 

 a wooded country, and the subsoil, has not too much depth, the waters 

 drain off easily by means of the roots that traverse it ; but if not, they 

 remain stagnant. 



3. If the soil is impervious, and the subsoil pervious, it will not agree 

 with certain trees, except the oak. 



4. If both soil and subsoil are impervious, it is least of all suited for 

 forest culture ; nevertheless there are certain kinds of trees that will 

 live and thrive in these conditions. The roots of trees penetrating into 

 the soil have an effect in modifying the distribution of the waters of a 

 country. MM. Gras and Alphonse Surel, from numerous observations 

 made in the Hautes-Alpes, have explained the manner that forests op- 

 erate when planted on the slopes of mountains. When the soil on the 

 slope is overgrown by vegetation, first of low plants and then by trees, 

 the roots interlacing among one another form a net- work, which gives 



