Discovering the Forests of Alaska , Oregon, and Washington

Ecosystems and Change

Change is a normal part of all ecosystems, including forests. Small changes go on all the time--a tree is blown down in a windstorm, or an animal is born, or a small fire burns a patch of forest. Big changes such as large fires, timber harvest, or major insect outbreaks also occur. Certain kinds of changes are more common in some forest types than others. For example, fires may be common in some forest types and infrequent in others.

Forests are changing all the time--they are dynamic. Forests get started through changes, and they change as they develop from one stage to another. The complexity of forests is a result of how forests respond to change.

Three qualities are important when an ecosystem responds to changes--composition, forest structures, and processes.

-Composition: The complexity of forests creates many habitats. These habitats support hundreds or even thousands of plant, animal, fungi, and bacteria species. The more species a forest has, the higher its level of biodiversity. Many species are available to carry out each ecological process. With many species, forests are more resilient when changes occur, as explained under "processes" below.

-Forest structures: A forest has live and dead structures. The structures left after a change or a disturbance are called biological legacies. After a fire or windstorm, the dead trees become snags or fallen trees on the ground. These dead trees shelter many plants and animals, protect the soil, and enrich the soil as they decay. Biological legacies ensure that many species survive a fire or other disturbance, and the legacies help rebuild the ecosystem.

-Processes: Ecological processes include nutrient cycling, regeneration, and symbiosis. Forests that are rich in biodiversity and biological legacies are resilient. Resilient forests continue their ecological processes even through major disturbances. If the abundance of some species decreases for a period of time, other species can carry on the ecological processes.