48 



While again, the humidity of the climate is maintained from the simple fact that the 

 green, moist foliage of the trees constituting the forests has the -well-known tendency of 

 preventing the increase of the sun's rays by radiation, and thus reducing the chances of 

 evaporation. 



" The Count de Gasparin has found that soUs covered with low vegetation or with 

 woods, and in which the soil is composed of humus, mingled with sand and lime or clay, 

 absorb more water than those which contain no humus, and consequently retain it longer 

 than the latter. These effects vary, according to the proportions of the various elements 

 of which the soils are composed. The infiltrations are greater in wooded lands than in 

 those covered with sod. The roots penetrate deeper, and thus facilitate the passage of 

 waters, which would be only stopped by an impervious stratum. 



" The branches of trees in leaf not only oppose the evaporation of the water in the 

 soil, but the leaves themselves are constantly yielding a vapour from exhalation, and 

 which tends to reduce the evaporation of waters, so far as the moisture exhaled goes to 

 saturate the air, the infiltration at the same time going on into the soil. Herbaceous 

 plants, not in masses, do not produce similar effects ; in fact, whoever has been in places 

 partly wooded and partly sodded must have observed, after a rain and a rest of some 

 duration, that the sodded grounds were dry, while the wooded soil was always damp. 



" We will now speak of the water absorbed by the roots, and that which is exhaled 

 into the atmosphere. 



" The roots of trees, as shown by the experiments of Hales and others, absorb a 

 large amount of water, charged with various elements constituting the sap. The surplus 

 water is evaporated from the leaves, which are constantly surrounded by a humid atmos- 

 phere. The water thus evaporated is drawn, not only from the upper strata, but likewise 

 from the deeper layers of the soil into which the roots penetrate, and which supply little 

 or no water to herbaceous vegetation. These lower strata are fed by subterranean sheets 

 of water that often come from a distance. Furthermore, this water remaining in these 

 lower strata, being thus given to the atmosphere, fall again as fog, dew or rain, and thus 

 increase the quantity of water that the surface of the soil receives from some distance 

 away. 



" The amount of water absorbed by the roots is so great that it is practically difficult 

 to make much of it remain near the trees, several reasons for preventing it occurring. The 

 soil in contact with the roots, and for a little distance away, is in a certain state of desic- 

 cation ; little by little it loses its nutritive properties, the lime, etc., and when these 

 elements are gone, the soil contains little but sand and clay, which then becomes perme- 

 able. It is, therefore, well demonstrated — 



"(1.) That a difference exists between the evaporation from a naked soil and a soil 

 covered with sod. 



• " (2.) That there is a like difference between a soU covered with sod and one that is 

 wooded, with the further advantage of the latter in facilitating the infiltration of water. 



"(3.) That the amount of water absorbed by the roots does not produce drought in 

 the soil, since it is returned after evaporation in the condition of fog, dew or rain. The 

 drought does not take place till the soil is exhausted." 



The thermal influence of forests has been established by^ Humboldt as follows : — 

 " They shelter the ground against the sun's rays, they maintain it in a greater degree of 

 humidity, and facilitate the decomposition of the leaves and litter, which they change into 

 humus ; and they act as a cooling cause by producing active aqueous transpiration from 

 the leaves and by multiplying in the expansion of their branches, the surfaces warmed by 

 the solar heat, and the surfaces cooled by nocturnal radiation. In regard to the action 

 last inentioned, positive experiments show that the layer of atmosphere in contact with a 

 meadow or a field covered with herbage or vegetable leaves, becomes cooled by nocturnal 

 radiation, other things being equal, several degrees below the temperature of the atmos- 

 phere at some meters above, while nothing of this kind takes place over a naked soil, 

 which becomes warm or cool according to the nature of its component parts. We will 

 add, as we have demonstrated, that the leaves as well as the trunk and branches become 

 warmed by solar heat, and retain into the night a portion of this acquired heat. This 

 effect should counterbalance the cooling from nocturnal radiation. We have not thus far 



