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West Cascade Range Crest, Oregon and Washington

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Fire is a key process in changing forests.Arrow.Many pathways can lead to old-growth forests. One trend is consistent among all of them—increasing structural complexity as a forest develops.


Arrow.Some of today's old-growth forests started hundreds of years ago after some kind of catastrophic change—perhaps a large forest fire or windstorm—destroyed the previous forest. Click on this link to learn more about the pathways that lead to old-growth forest.


Arrow.But change is a normal part of all ecosystems, including old-growth forests. Small changes go on all the time. A snag falls over, or an animal is born. Bigger changes such as fires, windstorms, or insects kill some trees and create openings in the forest where new trees can grow, adding to the complexity of the forest.

Toadstools grow on decaying logs.

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

  • Composition—The complexity of the old-growth forest creates many habitats. These habitats support thousands of species, including soil arthropods, spiders, insects, mites, millipedes, lichen, fungi, mosses, small mammals, bats, birds, and large mammals. This high level of biodiversity means that many species carry out each ecological process.
  • Forest structures—A forest has live and dead structures. The structures left after a disturbance are called biological legacies. After a fire or windstorm, the dead trees become snags of 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—Forests are changing all the time; they are dynamic. Forests continue their ecological processes through all the changes, a quality known as resilience. Click on the heading '"processes" to learn more about them.


USDA Forest Service - Pacific Northwest Research Station
Last Modified: Monday, 16 December 2013 at 14:18:41 CST

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