Forest Management Is Controlling Disturbances
Logging, though one of the most important silvicultural
tools available, causes stress in many ways. Three are
worth highlighting: (1) by opening the canopy, changing
light, and temperature levels; (2) by disturbing and
compacting the forest soils when heavy equipment is used
for extraction; and the most
serious, (3) by causing wounds in the remaining trees,
disrupting important physiological processes, and
leaving them susceptible to decay and disease-causing
organisms.
Other potentially negative effects from
practices such as high-grading (take the best and leave
the rest) reflect poor silviculture but often do not
cause any more stress than logging to improve the stand.
Stands with closed canopies (highly shaded conditions)
that are opened by logging may experience a phenomenon
known as thinning shock. Although it is not exactly
clear why this happens, it may be that trees divert
energy into crown growth to take advantage of openings
at the expense of providing adequate nutrition to other
parts of the tree.
Increased temperatures, light,
and wind speeds are
factors as well. Thinning shock by itself is not bad.
Usually a stand recovers after a growing season or two.
However, couple thinning shock with other stresses, and
die-back and mortality will occur. Closed stands that
have been defoliated recently, or are suffering from
drought, should be opened only very gradually or not at
all. The way to avoid thinning shock is to thin lightly,
but often. Doing so, however, leads to the potential of
increasing stress from other factors, namely soil
compaction and wounds. By eliminating air spaces in the
soil, and decreasing the rate of water percolation, soil
compaction is a very serious side effect of harvesting.
Planning Can Reduce Stress Usually, 10 to 16 percent of
a harvest area is devoted to roads, landings, and skid
trails. Experts say that, with proper planning and good
felling techniques, these areas could be reduced by 40
percent. Not only does good planning lessen the
ecological impacts of logging, it makes timber
extraction considerably easier and cheaper. For example,
the best way to prevent soil compaction and
root disturbances is to schedule harvesting during
frozen-ground conditions. In any harvesting prescription
- short of clear-cutting - wounding of the remaining
trees is inevitable.
Even the most careful of felling
techniques will cause some crown breakage, and stem
wounding on remaining trees used as bumpers is difficult
to avoid. Of greatest concern, however, is wounding of
feeder roots. As indicated earlier, up to 80 percent of
the total length of a tree's root system is made up of
feeder (or fine) roots. They are mostly near the soil
surface and are extremely susceptible to damage,
especially during the growing season.
Careful with that Equipment!
Root and stem wounding has increased dramatically with
the use of skidders and other mechanized harvesting
equipment. Unfortunately, this equipment
can compensate for a lack of user skill with power and
there is always a tendency to oversize equipment at the
expense of maneuverability. Any time the outside
protective layer of a tree is broken, it creates an
opening called an infection court. In response to
wounding, trees have evolved a method of walling-off or
isolating the injury. Badly scarred trees with full,
healthy crowns standing next to a skid trail are not an
uncommon site. These trees, however, will never recover
the grade and wood loss due to the injury. Timber
management is a process of weighing the benefits of a
prescription against the negative
effects of harvesting. In the epilogue to Principles of Silviculture (Daniels et al., 1979), the authors cite a
German forester, Henrich Cotta, who wrote in
1816 his Advice on Silviculture:
"The good physician lets people die; the poor one
kills them. With the same right one can say the good
forester [landowner, logger, or others who make
decisions about forest use] allows the most perfect
forest to become less so; the poor one spoils them."
Logging is a double-edged sword. Regardless of the
objective - be it for wildlife habitat, recreation,
timber production, or a combination of uses - logging
is the principal means of affecting and controlling
change in the forest. It also has the potential to do
great damage, some of which may be irreversible.
Fortunately, most of the practices that lessen the
negative impacts of logging are common sense.
[This information has been excerpted from the
Introduction to Forest Ecology and Silviculture
Thom J. McEvoy, Extension Forester, published by the
University of Vermont in 1995]
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