Forest Succession and Tolerance
In the absence of a disturbance, changes in the species
composition of a forest are slow but continuous. The
process of a continual change is called succession. The
fact that the direction of forest succession is both
predictable and controllable is the foundation of
forestry practice. Succession can be sped up, or slowed
down, or stopped altogether simply by altering the
composition and density of trees in a stand. A fire,
wind storm, or high-grading
harvest can have the same effect, but it is purely
chance without careful planning.
Primary succession is
the progression of plant and animal communities from the
colonizers of bare mineral soil to a relatively stable,
self-sustaining community many years later. Secondary
succession takes place after disturbance in an existing
plant community. It is an interruption in a primary
cycle. Forest management, then, deals principally with
secondary succession.
Of the 8 or 10 stages of primary succession recognized
by ecologists, only 4 or 5 apply to forest management.
They are herbaceous, shrub, intolerant (or pioneer)
trees, mid-tolerant (or sub-climax) trees, and tolerant
(or climax) trees. As a site proceeds from pioneer to
climax, the complexity of the ecosystem usually
increases and its stability, or resistance to change in
the absence of disturbance, increases as well.
Pioneering Tolerance
Pioneer species, such as aspen or white birch, grow fast
but are short-lived. Their size at maturity is much
smaller than climax species such as sugar maple.
However, the most distinctive difference between pioneer
and climax species is that pioneer trees are incapable
of establishing themselves in a forest
understory; they are said to be intolerant. Though
tolerance is really the degree to which a species can
share resources on a site and still be successful, it is
most often thought of as shade tolerance. Pioneer
species have virtually no tolerance of shaded conditions
while climax species do. As a result, forest succession
usually proceeds toward more tolerant species. Since
tolerant species can establish themselves in shade, they
remain in a forest stand in the absence of a
disturbance. This is known as a climax forest.
Disturbance
An understanding of species tolerance is fundamental to
forest management. Pioneer stands that are thinned will
move toward a climax association more quickly than if
left alone. By the same token, climax stands that are
thinned will continue to be climax stands. Pioneer
stands that are clear-cut usually result in early
successional stands. The more drastic the disturbance,
the further back succession is set. For example, if the
forest management prescription
calls for regenerating pioneer or early-successional
species such as aspen or white birch, a logger might be
encouraged to churn up the forest floor and disturb the
site as much as possible.
The small wind-borne seeds of
pioneer species require a bare, mineral soil to
germinate well. Also, the root systems of some pioneer
species such as aspen will sprout new stems called
suckers when the stand is clear-cut and the root systems
are injured. This is how aspen got its reputation as
"the tree that loves to be hated". In some instances,
where a given species falls in the forest succession
depends upon the site. White pine
is a good example. Usually intermediate in tolerance, on
good hardwood sites it acts like a pioneer species, but
on drier "pine" sites it acts more like a climax
species. So tolerance, and the speed and direction of
succession, is often tied to site characteristics.
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
Introduction to Forest Ecology and Silviculture
Tom J. McVoy, Extension Forester, published by the
University of Vermont in 1995] |