INFLUENCE OF FOREST ON HUMIDITY OF SOIL. 203 
Influence of the Forest on the Humidity of the Soil. 
I have hitherto confined myself to the influence of the forest 
on meteorological conditions, a subject, as has been seen, full of 
difficulty and uncertainty. Its comparative effects on the tem- 
perature, the humidity, the texture and consistence, the con- 
figuration and distribution of the mould or arable soil, and, very 
often, of the mineral strata below, and on the permanence and 
regularity of springs and greater superficial water-courses, are 
much less disputable as well as more easily estimated and more 
important, than its possible value as a cause of strictly climatic 
equilibrium or disturbance. 
The action of the forest on the earth is chiefly mechanical, 
but the organic process of absorption of moisture by its roots 
affects the quantity of water contained in the vegetable mould 
and in the mineral strata near the surface, and, consequently, 
the consistency of the soil. In treating of the effects of trees 
on the moisture of the atmosphere, I have said that the forest, 
by interposing a canopy between the sky and the ground, and 
by covering the surface with a thick mantle of fallen leaves, at 
once obstructed insulation and prevented the radiation of heat 
from the earth. These influences go far to balance each other ; 
but familiar observation shows that, in summer, the forest-soil 
is not raised to so high a temperature as open grounds exposed 
to irradiation. For this reason, and in consequence of the me- 
chanical resistance opposed by the bed of dead leaves to the 
escape of moisture, we should expect that, except after recent 
rains, the superficial strata of woodland-soil would be more 
6. About twenty-six per cent. of the precipitation is intercepted and pre- 
vented from reaching the ground by the foliage and branches of forest trees, 
p. 194. 
7. In the interior of thick woods, the evaporation from water and from 
earth is much less than the precipitation, p. 210. 
8. The loss of the water of precipitation intercepted by the trees in the 
forest is compensated by the smaller evaporation from the ground, p. 219. 
9. In elevated regions and during the summer half of the year, woods tend 
to increase the precipitation, p. 202. 
