If a tree is allowed to grow in the open, it has a tendency to branch,
and makes a low and spreading crown. In order to lengthen its shaft and
to reduce the number of branches it is necessary to narrow its growing
space, to shade its sides so that the lower branches and their foliage
do not receive light enough to perform their functions. When the side
shade is dense enough, these branches die and finally break off under
the influence of winds and fungous growth; wood then forms over the
scars and we get a clean shaft which carries a crown high up beyond the
reach of shade from neighbors.
[Illustration: Fig. 10.--Scheme to Illustrate the arrangement of annual
growth. 1, 2, 3, etc., represent the parts of the stem grown during the
first, second, third, etc., twenty years of the life of the tree, _k_,
knots; the shaded part of each is the "dead knot" of lumber.]
The branches being prevented from spreading out, the shaft is forced
to grow upward, and hence, when crowded by others, trees become taller
and more cylindrical in form, while in the open, where they can spread,
they remain lower and more conical in form (figs. 11, 12).
There are, to be sure, different natural types of development, some,
like the walnuts, oaks, beeches, and the broad-leaved trees generally,
having greater tendency to spread than others, like spruces, firs,
and conifers in general, which lengthen their shaft in preference
to spreading, even in the open. This tendency to spreading is also
influenced by soil conditions and climate, as well as by the age of the
tree. When the trees cease to grow in height, their crowns broaden, and
this takes place sooner in shallow soils than in deep, moist ones; but
the tendency can be checked and all can be made to develop the shaft at
the expense of the branches by proper shading from the sides.
It follows that the forest planter, who desires to produce long and
clean shafts and best working quality of timber, must secure and
maintain side shade by a close stand, while the landscape gardener,
who desires characteristic form, must maintain an open stand and full
enjoyment of light for his trees.
Now, as we have seen, different species afford different amounts of
shade, and in proportion to the shade which they afford can they
endure shade. The beech or sugar maple or spruce, which maintain a
large amount of foliage under the dense shade of their own crown, show
that their leaves can live and functionate with a small amount of
light. They are shade-enduring trees. On the other hand, the black
walnut, the locust, the catalpa, the poplars, and the larch show by the
manner in which their crowns thin out, the foliage being confined to
the ends of the branches, that their leaves require more light--they
are light-needing trees; so that the scale which arranges the trees
according to the amount of shade they exert serves also to measure
their shade endurance.
In making, therefore, mixed plantations, the different kinds must be so
grouped and managed that the shady trees will not outgrow and overtop
the light-needing; the latter must either have the start of the former
or must be quicker growers.
[Illustration: Fig. 11.--Oak tree grown in the open.]
[Illustration: Fig. 12.--Maple tree grown in the forest.]