The first care should be to improve the crop in its composition.
Instead of culling it of its best material, it should be culled of its
weeds, the poor kinds, which we do not care to reproduce, and which,
like all other weeds, propagate themselves only too readily. This
weeding must not, however, be done all sit once, as it could be in
a field crop, for in a full-grown piece of woodland each tree has a
value, even the weed trees, as soil cover.
The great secret of success in all crop production lies in the
regulating of water supplies; the manuring in part and the cultivating
entirely, as well as drainage and irrigation, are means to this end. In
forestry these means are usually not practicable, and hence other means
are resorted to. The principal of these is to keep the soil as much as
possible under cover, either by the shade which the foliage of the tall
trees furnishes, or by that from the underbrush, or by the litter which
accumulates and in decaying forms a humus cover, a most excellent mulch.
A combination of these three conditions, viz, a dense crown cover,
woody underbrush where the crown cover is interrupted, and a heavy
layer of well-decomposed humus, gives the best result. Under such
conditions, first of all, the rain, being intercepted by the foliage
and litter, reaches the ground only gradually, and therefore does not
compact the soil as it does in the open field, but leaves it granular
and open, so that the water can readily penetrate and move in the soil.
Secondly, the surface evaporation is considerably reduced by the shade
and lack of air circulation in the dense woods, be that more moisture
remains for the use of the trees. When the shade of the crowns overhead
(the so-called "crown cover," or "canopy,") is perfect, but little
undergrowth will be seen; but where the crown cover is interrupted
or imperfect, an undergrowth will appear. If this is composed of
young trees, or even shrubs, it is an advantage, but if of weeds, and
especially grass, it is a misfortune, because these transpire a great
deal more water than the woody plants and allow the soil to deteriorate
in structure and therefore in water capacity.
Some weeds and grasses, to be sure, are capable of existing where but
little light reaches the soil. When they appear it is a sign to the
forester that he must be careful not to thin out the crown cover any
more. When the more light-needing weeds and grasses appear it is a sign
that too much light reaches the ground, and that the soil is already
deteriorated. If this state continues, the heavy drain which the
transpiration of these weeds makes upon the soil moisture, without any
appreciable conservative action by their shade, will injure the soil
still further.
The overhead shade or crown cover may be imperfect because there are
not enough trees on the ground to close up the interspaces with their
crowns, or else because the kinds of trees which make up the forest do
not yield much shade; thus it can easily be observed that a beech, a
sugar maple, a hemlock, is so densely foliaged that but little light
reaches the soil through its crown canopy, while an ash, an oak, a
larch, when full grown, in the forest, allows a good deal of light to
penetrate.
Hence, in our weeding process for the improvement of the wood crop,
we must be careful not to interrupt the crown cover too much, and
thereby deteriorate the soil conditions. And for the same reason, in
the selection of the kinds that are to be left or to be taken out, we
shall not only consider their use value but also their shading value,
trying to bring about such a mixture of shady and less shady kinds as
will insure a continuously satisfactory crown cover, the shade-enduring
kinds to occupy the lower stratum in the crown canopy, and to be more
numerous than the light-needing.
The forester, therefore, watches first the conditions of his soil
cover, and his next care is for the condition of the overhead shade,
the "crown cover;" for a change in the condition of the latter brings
change into his soil conditions, and, inversely, from the changes
in the plant cover of the soil he judges whether he may or may not
change the light conditions. The changes of the soil cover teach him
more often when "to let alone" than when to go on with his operations
of thinning out; that is to say, he can rarely stop short of that
condition which is most favorable. Hence the improvement cuttings must
be made with caution and only very gradually, so that no deterioration
of the soil conditions be invited. We have repeated this injunction
again and again, because all success in the management of future wood
crops depends upon the care bestowed upon the maintenance of favorable
soil conditions.
As the object of this weeding is not only to remove the undesirable
kinds from the present crop, but to prevent as much as possible their
reappearance in subsequent crops, it maybe advisable to cut such kinds
as sprout readily from the stump in summer time--June or July--when the
stumps are, likely to die without sprouting.
It may take several years' cutting to bring the composition of the main
crop into such a condition as to satisfy us.