Ecological Principles Help Predict Forest Changes

Forests are dynamic systems--they are constantly changing. Plants grow, die, and are replaced by others. Disturbances occur frequently on various scales: fire, insect damage, disease, animal activity, etc. While some changes are obvious, others are subtle or occur too slowly to be readily noticed. Some changes occur in somewhat predictable patterns. These provide the basis for much of forest management.

Succession

Succession is one such pattern. Succession is the change in species composition that occurs in plant communities, in a generally sequential manner. The entire sequence is called a sere and each of the characteristic changes, a seral stage. While the stages occur in a continuum and cannot be precisely predicted, each seral stage has a characteristic species composition, structure, and processes.

We can see succession at work most clearly in a situation such as a newly burned forest area. First, plants that colonize quickly and are tolerant of sun exposure will become established. Grasses and certain annual species are often the first pioneers into a burn. Next come fast-growing shrubs and certain trees. Eventually, if the area remains undisturbed, slower-growing, shade tolerant trees will become re-established and a mature, self-sustaining community will dominate. This is known as the climax stage.

Of course, succession in the real world is much more complex than described here. Which species become established depends on innumerable factors including: what seeds are available in the soil or nearby gene pool, competition, climate, soil characteristics, and chance. Localized disturbance may open up smaller areas to early seral stages while other parts of the forest continue through later stages.

Many forestry techniques depend on the concept of succession. Timber harvest, wildlife habitat improvement, and other activities are based on influencing succession, creating disturbances that keep an area in the earlier seral stages.

Conversely, after a fire, shrubs are often removed (manually or chemically) to allow trees to grow quickly, speeding up the process to encourage later seral stages.

Wildlife

While we normally think of plants when discussing succession, animals are also profoundly affected by the seral stage of the forest. The plant components of an ecosystem provide food, cover, and other habitat requirements that determine what wildlife species can live there. Changes in vegetation are accompanied by corresponding shifts in wildlife.

Major disturbances, whether natural or human-induced, that profoundly affect the community seral stage require animals to adapt quickly--or to move or die. If suitable habitat is nearby, mobile animals may be able to re-establish themselves but are likely to perish anyway if that habitat is already fully occupied. Less mobile species may not be able to adjust to the changes and will not survive.

There are many species that thrive in early successional forests, however, others require late succession ecosystems for survival while others live at the edge between successional stages or may require different stages at different times of their life cycle. Understanding some of the complexities of these ecosystem dynamics allows more informed forest management decisions.

Silviculture

The art and science of tending a stand of trees based on ecological principles is called silviculture. Silvicultural treatments alter the composition and structure of a stand of trees to achieve management objectives.

Thinning, for example, is a silvicultural treatment that is used to reduce competition for the remaining trees, allowing them to grow more quickly--in other words, speeding succession. Thinning can accomplish other objectives, such as improving habitat for certain species of wildlife or decreasing the danger of wildfire in areas of high risk.

Some silvicultural treatments, e.g. clearcutting, mimic to an extent very large disturbances in the ecosystem such as a wildfire. Other treatments, e.g. single tree selection, may mimic smaller disturbances such as the openings that occur when individual trees die.

While there are important differences between natural disturbances and analogous silvicultural practices, incorporating ecological principles into forestry practices helps to minimize damage and maintain the health of these complex ecosystems.