To insure the largest amount of growth, full enjoyment of sunlight is
needed. But as light is almost always accompanied by heat and relative
dryness of air, which demands water from the plant, and may increase
transpiration from the leaves inordinately, making them pump too hard,
as it were, young seedlings of tree species whose foliage is not built
for such strains require partial shading for the first year or two. The
conifers belong to this class.
In later life the light conditions exert a threefold influence on the
development of the tree, namely, with reference to soil conditions,
with reference to form development, and with reference to amount of
growth.
The art of the forester consists in regulating the light conditions so
as to secure the full benefit of the stimulating effect of light on
growth, without its deteriorating influences on the soil and on form
development.
As we have seen, shade is desirable in order to preserve soil moisture.
Now, while young trees of all kinds, during the "brush" stage of
development, have a rather dense foliage, as they grow older they
vary in habit, especially when growing in the forest. Some, like the
beech, the sugar maple, the hemlock, and the spruce, keep up a dense
crown; others, like the chestnut, the oaks, the walnut, the tulip
tree, and the white pine, thin out more and more, and when fully
grown have a much less dense foliage; Anally, there are some which do
not keep up a dense shade for any length of time, like the black and
honey locust, with their small, thin leaves; the catalpa, with its
large but few leaves at the end of the branchlets only, and the larch,
with its short, scattered bunches of needles. So we can establish a
comparative scale of trees with reference to the amount of shade which
they can give continuously, as densely foliaged and thinly foliaged,
in various gradations. If we planted all beech or sugar maple, the
desirable shading of the soil would never be lacking, while if we
planted all locust or catalpa the sun would soon reach the soil and
dry it out, or permit a growth of grass or weeds, which is worse,
because those transpire still larger quantities of water than the bare
ground evaporates or an undergrowth of woody plants would transpire.
Of course, a densely foliaged tree has many more leaves to shed than a
thinly foliaged one, and therefore makes more litter, which increases
the favorable mulch cover of the soil. Another reason for keeping the
ground well shaded is that the litter then decomposes slowly, but into
a desirable humus, which acts favorably upon the soil, while if the
litter is exposed to light, an undesirable, partly decomposed "raw"
humus is apt to be formed.
Favorable soil conditions, then, require shade, while wood growth is
increased by full enjoyment of light; to satisfy both requirements,
mixed planting, with proper selection of shade-enduring and
light-needing species, is resorted to.
As the different species afford shade in different degrees, so they
require for their development different degrees of light. The dense
foliage of the beech, with a large number of leaves in the interior
of the crown, proves that the leaves can exist and perform their work
with a small amount of light; the beech is a shade-enduring tree. The
scanty foliage of the birches, poplars, or pines shows that these are
light-needing trees; hence they are never found under the dense shade
of the former, while the shade-enduring can develop satisfactorily
under the light shade of the thin-foliaged kinds. Very favorable soil
conditions increase the shade endurance of the latter, and climatic
conditions also modify their relative position in the scale.
All trees ultimately thrive best--i. e., grow most vigorously--in the
full enjoyment of light, but their energy then goes into branching.
Crowded together, with the side light cut off, the lower lateral
branches soon die and fall, while the main energy of growth is put into
the shaft and the height growth is stimulated. The denser shade of the
shade-enduring kinds, if placed as neighbors to light-needing ones, is
most effective in producing this result, provided that the light is not
cut off at the top; and thus, in practice, advantage is taken of the
relative requirements for light of the various species.[1]
[1] This relation of the different species to varying light conditions;
their comparative shading value and shade endurance, is one of the most
important facts to be observed and utilized by the forester. European
foresters have done this, but since they had to deal with only a few
species and over a limited territory, they could quite readily classify
their trees with reference to their shade endurance, and take it for
granted that shade endurance and density of foliage or shading value
were more or less identical. With our great wealth of useful species it
will be necessary and profitable to be more exact in the classification.
The forester finds in close planting and in mixed growth a means of
securing tall, clear trunks, free from knots, and he is able, moreover,
by proper regulation of light conditions, to influence the form
development, and also the quality of his crop, since slow growth and
rapid growth produce wood of different character.
There are some species which, although light-foliaged and giving
comparatively little shade, are yet shade-enduring--i. e., can subsist,
although not develop favorably, under shade; the oaks are examples
of this kind. Others, like the black cherry, bear a dense crown for
the first twenty years, perhaps, seemingly indicating great shade
endurance; but the fact that the species named soon clears itself
of its branches and finally has a thin crown, indicates that it is
light-needing, though a good shader for the first period of its life.
Others, again, like the catalpa, which is shady and shade-enduring, as
the difficulty with which it clears itself indicates, leaf out so late
and lose their foliage so early that their shading value is thereby
impaired. Black locust and honey locust, on the other hand, leave
no doubt either as to their light-needing or their inferior shading
quality.
That soil conditions and climatic conditions also modify crown
development and shade endurance has been well recognized abroad, but in
our country this influence is of much more importance on account of the
great variation in those conditions. Thus the box elder, an excellent
shader in certain portions of the West, is a failure as soil cover in
others where it nevertheless will grow.
We see, then, that in determining the shading value as well as the
shade endurance of one species in comparison with another, with
reference to forestry purposes, not only soil and climate but also the
character of foliage and its length of season must be considered.