Whether forests increase the amount of precipitation within or near
their limits is still an open question, although there are indications
that under certain conditions large, dense forest areas may have such
an effect. At any rate, the water transpired by the foliage is certain,
in some degree, to increase the relative humidity near the forest,
and thereby increase directly or indirectly the water supplies in its
neighborhood. This much we can assert, also, that while extended plains
and fields, heated by the sun, and hence giving rise to warm currents
of air, have the tendency to prevent condensation of the passing
moisture-bearing currents, forest areas, with their cooler, moister air
strata, do not have such a tendency, and local showers may therefore
become more frequent in their neighborhood. But, though no increase in
the amount of rainfall may be secured by forest areas, the availability
of whatever falls is increased for the locality by a well-kept and
properly located forest growth. The foliage, twigs, and branches break
the fall of the raindrops, and so does the litter of the forest floor,
hence the soil under this cover is not compacted as in the open field,
but kept loose and granular, so that the water can readily penetrate
and percolate; the water thus reaches the ground more slowly, dripping
gradually from the leaves, branches, and trunks, and allowing more time
for it to sink into the soil. This percolation is also made easier by
the channels along the many roots. Similarly, on account of the open
structure of the soil and the slower melting of the snow under a forest
cover in spring, where it lies a fortnight to a month longer than in
exposed positions and melts with less waste from evaporation, the snow
waters more fully penetrate the ground. Again, more snow is caught and
preserved under the forest cover than on the wind-swept fields and
prairies.
All these conditions operate together, with the result that larger
amounts of the water sink into the forest soil and to greater depths
than in open fields. This moisture is conserved because of the reduced
evaporation in the cool and still forest air, being protected from
the two great moisture-dissipating agents, sun and wind. By these
conditions alone the water supplies available in the soil are increased
from 50 to 60 per cent over those available on the open field.
Owing to those two causes, then--increased percolation and decreased
evaporation--larger amounts of moisture become available to feed the
springs and subsoil waters, and these become finally available to the
farm, if the forest is located at a higher elevation than the field.
The great importance of the subsoil water especially and the influence
of forest areas upon it has so far received too little attention and
appreciation. It is the subsoil water that is capable of supplying the
needed moisture in times of drought.