Stress and Disturbance on Forest Ecosystems
A forest is more than its trees. It is a complex
ecosystem - always changing, defined by the interactions
of living organisms and the surrounding environment. For
this reason, management decisions should consider the
potential impacts on the whole forest. Managers have
come to realize that the forest is more than just the
sum of its parts.
An ecosystem can be characterized at any scale, from a
few square feet to thousands of acres or the entire
earth. Whenever plants and animals interact with their
environment and each other, and the ultimate source of
energy is sunlight, the association can be described as
an ecosystem. For example, the interaction of lichen
population on the bark of an oak
tree is as much an ecosystem as the community in which
the oak is found. It is just a different scale. The
study of ecosystems is mostly concerned with the way
different species interact, changes in the community
over time, and the flow of inputs and outputs, such as
energy, nutrients, and water. Even the simplest
ecosystem can be tremendously complex.
Change any part and it influences the rest of the
system.
Forest Equilibrium
Change any part of a forest ecosystem, and it also
influences the rest of the forest. Almost regardless of
the change, whether caused by harvesting, hurricane,
disease, or insects, the forest ecosystem eventually
establishes a new equilibrium. Depending upon the
severity of the disturbance, a new equilibrium may cause
changes in soil organisms, wildlife populations, or
productivity and composition of tree species. The
direction and degree of change is a result of the way
organisms react to one another and to the new conditions
on the site. The new balance can be very desirable or,
as in the case of human-caused deforestation of the
tropical forests, potentially disastrous. Stress is
caused when some important ecological factor changes,
resulting in a strain to reach a new equilibrium. Like a
motor the burns oil, as oil pressure drops, the engine
runs hotter, straining the cooling system. In forests,
stress is the rule rather than the exception, and not
all stresses are bad. But the moment stresses are
compounded (a hotter engine burning more oil) growth
reduction, crown die-back, and mortality are likely to
occur.
Stress: Good & Bad
Some forest managers have used stress to their
advantage. If slight overcrowding is permitted, crown
competition causes stress. The dominant trees will react
to this stress by investing in height, rather than side
branching. However, if the crowding is overdone, the
stress may weaken the trees and reduced air circulation
could lead to an opportunistic pest infestation.
Climate-induced stresses and disturbance, such as
drought, hurricanes, and ice storms, may be expected but
deliver unpredictable results. Other natural
disturbances are those caused by disease and insects.
The presence of some of the most serious pests, however,
is due to human error. Chestnut blight and Dutch elm
disease are examples of seriously destructive pests that
were imported by humans. Each has had a tremendous
impact on the species composition of our forests. Acid
deposition is an example of an additive stress.
[Because researchers have shown that it does not have to
rain to cause deposition of acids, it is no longer
called "acid rain". Weather systems that bring haze and
mist can be quite acidic and also carry other pollutants
that affect trees.] Acid deposition may not kill trees
directly, but it can cause stress. One of the current
theories is that acids from the
atmosphere displace aluminum in the soil which, in turn,
pollutes important nutrient exchange sites on tree
roots. The tree cannot take up important elements such
as calcium, which eventually leads to twig die-back in
the crown. Couple this stress with others and die-back
becomes more severe, in some instances to the point of
death. Acid deposition did not kill the tree, but it may
have been the stress that tipped the scales.
This information has been excerpted from the
"ntroduction to Forest Ecology and Silviculture"
Tom J McCoy, Extension Forester, published by the
University of Vermont in 1995]
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