Not only do different species grow more or less rapidly in height and
girth, but there is in each species a difference in the rate of growth
during different periods of life, and a difference in the persistence
of growth.
It stands to reason that trees grow differently in different soils and
situations, and hence we can not compare different species with respect
to their rate of growth except as they grow under the same conditions.
Thus the black walnut may grow as fast as or faster than the ash on
a rich, deep, moist, warm soil, but will soon fall to the rear in a
wetter, colder, and shallower soil.
Given the same conditions, some species will start on a rapid upward
growth at once, like the poplars, aspen, locust, and silver maple,
making rapid progress (the most rapid from their tenth to their
fifteenth year), but decreasing soon in rate and reaching their maximum
height early. Others, like the spruce, beech, and sugar maple, will
begin slowly, often occupying several, sometimes as many as 10 to
15, years before they appear to grow at all, their energy all going
into root growth. Then comes a period of more and more accelerated
growth, which reaches its maximum rate at 25 or 30 years; and when the
cottonwood or aspen has reached the end of its growth in height the
spruce or pine is still at its best rate, and continues to grow for a
long time at that rate; in later life the rate decreases, yet height
growth sometimes does not cease altogether for centuries. As a rule,
the light-needing species are the ones which show the rapid height
growth at the start, while the shade-enduring are slow at the start,
but persistent growers.
This fact is important in explaining the alternations of forest
growth in nature; the persistent shade-enduring species crowd out the
light-needing, and the latter rapidly take possession of any openings
that fire or storm has made. It is also important with reference to
the management of wood crops and starting of mixed plantations; the
light-needing species must be mixed only with such shade-enduring
species as are slower growers than themselves.
The diameter growth shows also periodic changes in its rate, and is,
of course, influenced in the same way by soil, climate, and light
conditions, as the height growth.
In the juvenile or brush stage, lasting 6 to 10 years in light-needing
and 20 to 40 years in shade-enduring species, the diameter grows
comparatively little, all energy being directed to height growth and
root growth. When the crown has been definitely formed, more food
material is available for wood formation, and the increase in foliage
is accompanied by a more rapid increase of trunk diameter; in favorable
situations, the highest rate occurs between the fortieth and sixtieth
years; in the poorer situations, between the fiftieth and eightieth
years, which rate continues for some time. Then comes a period of
slower rate, which finally in old age dwindles down almost to zero.
But neither the diameter growth nor the width of the annual rings alone
tells us directly what amount of wood is forming. The outer rings,
being laid over a larger circumference, although thinner than the
preceding rings, may yet have greater cubic contents. The statements
of diameter growth are, therefore, misleading if we are interested in
knowing how much wood is forming.
Accordingly the growth in volume must be considered separately, as
determined by the enlargement of the cross-section area and the height.
The growth in volume or mass accretion is quite small in young trees,
so that when wood is cut young the smallest amount of crop per year
is harvested, while, if it is allowed to grow, an increase more than
proportionate to the number of years may be obtained.
Only when the tree has a fully developed crown does it begin to make
much wood. Its volume growth progresses then at a uniform rate, and
continues to do so for decades, and sometimes for a century or more.
On poorer sites the rate is slower, but remains longer on the increase,
while on good sites the maximum rate is soon reached.
Of course, in a forest, where light conditions are not most favorable,
because form development and soil conditions require shade, the total
wood formation is less than in an isolated tree, favorably placed.
Just so the dominant trees in a forest--i. e., those which have their
crowns above all others--show, of course, the advantage they have
over the inferior trees which are suffering from the shade of their
neighbors.
Finally, if we would take into consideration an entire forest growth,
and determine, for instance, how much wood an acre of such forest
produces at different periods, we must not overlook the fact that the
number of trees per acre changes as the trees grow older. Some of them
are overshaded and crowded out by the others, so that a young growth of
spruce might start with 100,000 little seedlings to the acre, of which
in the twentieth year only 10,000 would be alive, while in the fortieth
year the number would be reduced to 1,200, and in the hundredth year to
280. Hence the rate of growth of any single tree gives no idea of what
the acre of forest will do.
Tims, while a single good white pine might grow the fastest in volume
when about one hundred years old, then making wood at the rate of,
say, 1.5 cubic feet per year, an acre of pine on good soil, containing
about 1,600 trees, may make the most wood in the thirtieth year, then
growing at the rate of 170 cubic feet per acre, while in the hundredth
year the rate would not exceed 70 cubic feet; and an acre of pine in a
poorer location, with about 1,400 trees, may make the most wood in the
fortieth year, at the rate of 100 cubic feet per acre.
From the consideration of the relation of light conditions to soil
conditions, to form development, and to rate of growth, we may make the
following deductions of interest to the forest planter:
In order to secure the best results in wood production, in quantity
and quality, at the same time preserving favorable soil conditions,
the forest should be composed of various species, a mixture of
light-needing and shade-enduring kinds. The light-needing ones should
be of quicker growth; the shady ones, in larger numbers, should
be slower growers. For the first fifteen to twenty-five years the
plantation should be kept as dense as possible, to secure clear shafts
and good growth in height; then it should be thinned, to increase crown
development and diameter growth; the thinning, however, is not to be so
severe that the crowns can not close up again in two or three years;
the thinning is to be repeated again and again, always favoring the
best developed trees.