It is now necessary, by careful observation of the surroundings of
each of the "final harvest" crop trees, or "superiors," as we may call
them, to determine what trees of the "subordinate" crop trees, or
"inferiors," must be removed. All nurse trees that threaten to overtop
the superiors must either be cut out or cut back and topped, if that
is practicable, so that the crown of the superiors can develop freely.
Those that are only narrowing in the superiors from the side, without
preventing their free top development, need not be interfered with,
especially while they are still useful in preventing the formation and
spreading of side branches on the superiors. As soon as the latter
have fully cleared their shafts, these crowding inferiors must be
removed. Care must be taken, however, not to remove too many at a time,
thus opening the crown cover too severely and thereby exposing the
soil to the drying influence of the sun. Gradually, as the crowns of
inferiors standing farther away begin to interfere with those of the
superiors, the inferiors are removed, and thus the full effect of the
light is secured in the accretion of the main harvest crop; at the same
time the branch growth has been prevented and the soil has been kept
shaded. Meanwhile thinnings may also be made in the subordinate crop,
in order to secure also the most material from this part of the crop.
This is done by cutting out all trees that threaten to be killed by
their neighbors. In this way many a useful stick is saved and the dead
material, only good for firewood, lessened. It is evident that trees
which in the struggle for existence have fallen behind, so as to be
overtopped by their neighbors, can not, either by their presence or by
their removal, influence the remaining growth. They are removed only in
order to utilize their wood before it decays.
It may be well to remark again that an undergrowth of woody plants
interferes in no way with the development of the main crop, but, on
the contrary, aids by its shade in preserving favorable moisture
conditions. Its existence, however, shows in most cases that the crown
cover is not as dense as it should be, and hence that thinning is not
required. Grass and weed growth, on the other hand, is emphatically
disadvantageous and shows that the crown cover is dangerously open.
The answer to the three questions, When to begin the thinnings, How
severely to thin, and How often to repeat the operation, must always
depend upon the varying appearance of the growth and the necessities
in each case. The first necessity for interference may arise with
light-needing species as early as the twelfth or fifteenth year; with
shade-enduring, not before the twentieth or twenty-fifth year. The
necessary severity of the thinning and the repetition are somewhat
interdependent. It is better to thin carefully and repeat the operation
oftener than to open up so severely at once as to jeopardize the soil
conditions. Especially in younger growths and on poorer soil, it is
best never to open a continuous crown cover so that it could not close
up again within 3 to 5 years; rather repeat the operation oftener.
Later, when the trees have attained heights of 50 to 60 feet and clear
boles (which may be in 40 to 50 years, according to soil and kind) the
thinning may be more severe, so as to require repetition only every 6
to 10 years.
The condition of the crown cover, then, is the criterion which directs
the ax. As soon as the crowns again touch or interlace, the time has
arrived to thin again. In mixed growths it must not be overlooked that
light-needing species must be specially protected against shadier
neighbors. Shade-enduring trees, such as the spruces, beech, sugar
maple, and hickories, bear overtopping for a time and will then grow
vigorously when more light is given, while light-needing species, like
the pines, larch, oaks, and ash, when once suppressed, may never be
able to recover.
Particular attention is called to the necessity of leaving a rather
denser "wind mantle" all around small groves. In this part of the
grove the thinning must be less severe, unless coniferous trees on the
outside can be encouraged by severe thinning to hold their branches low
down, thus increasing their value as wind-breaks.
The thinnings, then, while giving to the "final harvest" crop all the
advantage of light for promoting its rapid development into serviceable
timber size, furnish also better material from the subordinate crop.
At 60 to 70 years of age the latter may have been entirely removed and
only the originally selected "superiors" remain on the ground, or as
many of them as have not died and been removed; 250 to 400 of these
per acre will make a perfect stand of most valuable form and size,
ready for the final harvest, which should be made as indicated in the
preceding chapter.