Another method by which a forest belt becomes a conservator of moisture
lies in its wind-breaking capacity, by which both velocity and
temperature of winds are modified and evaporation from the fields to
the leeward is reduced.
On the prairie, wind-swept every day and every hour, the farmer has
learned to plant a wind-break around his buildings and orchards, often
only a single-row of trees, and finds even that a desirable shelter,
tempering both the hot winds of summer and the cold blasts of winter.
The fields he usually leaves unprotected; yet a wind-break around his
crops to the windward would bring him increased yield, and a timber
belt would act still more effectively. Says a farmer from Illinois:
My experience is that now in cold and stormy winters fields protected
by timber belts yield full crops, while fields not protected yield
only one-third of a crop. Twenty-five or thirty years ago we never
had any wheat killed by winter frost, and every year we had a full
crop of peaches, which is now very rare. At that time we had plenty
of timber around our fields and orchards, now cleared away.
Not only is the temperature of the winds modified by passing over
and through the shaded and cooler spaces of protecting timber bolts
disposed toward the windward and alternating with the fields, but their
velocity is broken and moderated, and since with reduced velocity the
evaporative power of the winds is very greatly reduced, so more water
is left available for crops. Every foot in height of a forest growth
will protect 1 rod in distance, and several bolts in succession would
probably greatly increase the effective distance. By preventing deep
freezing of the soil the winter cold is not so much prolonged, and
the frequent fogs and mists that hover near forest areas prevent many
frosts. That stock will thrive better where it can find protection from
the cold blasts of winter and from the heat of the sun in summer is a
well-established fact.