Stress and Disturbance in the Forest

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.