Silviculture similar to vegetable gardening
By JAMES C. FINLEY
Area Forest Resource Agent
tire Extension Service Penn State University
J PARK • Silviculture is the art and
growing trees. An analogy can easily be drawn
silviculture and a vegetable garden. When a
ilanted, weeded, and thinned, one can expect to
;ood crop, provided the plants receive adequate
1 are protected from disease and insects. But
planted in the wrong site, forced to compete
i, or left in crowded rows will not produce
crops.
like'gardens, need to be weeded and thinned in
iroduce good crops of timber and water. In time,
. do the weeding and thinning, but often we do
the time or patience to wait,
the years foresters have learned how in*
, and other organisms interact to influence
s growth. They have studied tree species and
specific requirements for growth. This study
•equirements is called “silvics.” Silvics has
several needs which influence a plant’s ability
and grow. Light, water, and nutrients are
•gulaton of a species’ competitive ability.
id nutrients are primary regulators of a species’
! ve ability.
observation reveals that certain species are
-ticular locations within a stand. For instance,
mently occurs on north fadng slopes or along
' in coves, but is seldom seen on south facing
temlock grows best on these sites because of
moisture. It does not grow well on dry areas,
has a wide-spreading shallow root system
toisture near the soil surface.
. frequently occurs in nearly pure stands
ir areas that contain essentially nothing but
Hemlock’s tolerance to shading is the reason
characteristic. Hemlock forms a very dense
and allows very little light to reach a forest floor,
ues that can grow under very heavy shade will
and hemlock can survive in heavy shade. Con-
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sequently, hemlock seedlings willbecome established in
the shade of the parent tree. They will survive, grow, and
replace older hemlocks (when the older hemlocks die).
Hemlock is classified as a climax species because once it
fully occupies a site, it will continue to dominate the site
and exclude other species. Species composition of a
climax forest will persist until either a natural distur
bance or man alters the stand.
Another example of a species influenced by moisture is
chestnut oak (rock oak). This species is most often found
on dry ridge sites. It grows on more moist sites, but is
unable to compete successfully for light against species
such as hemlock. As a result, chestnut oak grows where
other species cannot find sufficient moisture.
It soon becomes evident that light and moisture can
limit successful growth of any species. Foresters have
studied the natural occurrence and growth rates of
species and have constructed charts of relative tolerance
to shade. These charts are used to explain why trees occur
where they do within a forest and to understand their
vigor.
A list of relative tolerance to shade follows:
Very Tolerant - Eastern Hemlock, Beech, Sugar Maple,
Basswood, Hop Hornbeam.
Tolerant - Red Maple, Silver Maples, Yellow Birch.
Intermediate - E. White Pine, Elm, Amer. Hornbeam.
Intolerant - Black Cherry, White Ash, Yellow Poplar,
White Oak, Red Oak, Hickory.
Very Intolerant - Paper Birch, Black Locust, Aspen,
Larch.
Tolerance within a species is recognized by a number of
characteristics. Tolerant species such as hemlock or
beech persist in a forest understory. They respond to
release from competition for light and moisture with
accelerated growth and improved vigor, even after being
suppressed for many years.
Under the same competitive conditions, lower branches
of intolerant species usually die and break off many years
earlier than those of tolerant species. This natural process
is called self-pruning. Crowns of shade tolerant species
have a denser appearance, and more leaf layers than do
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intolerant species, because lower branches remain alive
much longer than those of intolerant trees. As a result,
tolerant species frequently are slow to self-prune. They
also tend to have more taper in the sawlog portion of their
boles (trunks) in order to support their crowns.
Within mixed stands, tolerant trees persist and compete
very well with species of equal size. Consequently, they
tend to form stands that are dense, with more stems per
acre than stands of intolerant species.
In even-age situations, such as occur after fires or clear
cuts, tolerant species are quickly overtopped by faster
growing intolerant species. The tolerant trees will persist
until the intolerant species is removed. Hie tolerants will
then dominate the stand.
However, in a situation where both tolerant and in
tolerant trees are being established under an existing
canopy, tolerant trees will dominate from the start.
Because tolerant species prosper under shaded conditions
they eventually become a major stand component,
regardless of circumstances leading to stand
regeneration.
The chart of relative tolerances shows that some of our
more valuable species are found among intolerant species
(black cherry, yellow poplar, white oak). These intolerant
species have growth characteristics that make their wood
valuable. Stands of intolerant species often have widely
spaced stems, thus promoting rapid growth in height and
diameter of each tree.
Crowns on intolerant trees will be small, resulting in
relatively little log taper. Intolerant species in properly
stocked stands will self-prune quickly, yielding high
quality lumber with few knots.
Various species compete for a position within a
developing stand. Left to nature, this competition can
leave a lot to chance. Timber Stand Improvement (TSI)
Practices shorten the time required to develop desirable
forests. One common set of TSI practices is intermediate
harvest cuttings.
TSI practices include weedings (of inferior trees and
tree species), thinning (so that growth within the stand
(Continued on Page 381
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