Pacific Northwest Research Station: Old growth--A Unique Ecosystem

An excellent fact sheet on old-growth forests from the Pacific Northwest Research Station.


The Pacific Northwest Old-Growth Forest--A Unique Ecosystem

  • The Pacific Northwest old-growth forest is a conifer forest, dominated by large, old trees. In the Pacific Northwest, the most common type of old-growth ecosystem is forests dominated by Douglas-firs and western hemlocks, generally 350 to 750 years old. The youngest old-growth forests are 200 years old, and the oldest are about 1,000 years old.
    - The Pacific Northwest also has old-growth forests dominated by Sitka spruce and western hemlock, along the Pacific Coast, and at higher elevations in the Cascade Mountains, true fir and hemlock old-growth forests.
  • Among all the forests of the world, the Pacific Northwest old-growth forest is unique because of the size and old age of its trees, the accumulations of biomass (weight and density of living organisms), and the climate, with its wet, mild winters and dry, warm summers.
    - No other forest has an entire group of tree species that equals the trees in the Pacific Northwest old-growth forest for their size and long lives. Some of California's giant redwoods are bigger than the biggest Douglas-fir tree. But several species of big trees grow in the Pacific Northwest old-growth forest, not just one. In other forests, some junipers and bristlecone pines live longer. But several species of trees live for hundreds of years in the Pacific Northwest old-growth forest, not just one.
  • Scientists have done a lot of research on old-growth forests in the last 30 years. Much of this work was done by Pacific Northwest Research Station scientists. Click on the other headings on this page to learn more about Pacific Northwest old-growth forests.


Where the Pacific Northwest Old-Growth Forests Are Found

  • The Pacific Northwest old-growth forests are found only in parts of western North America--the area from southeast Alaska and southwest British Columbia, down through western Washington, western Oregon, and the edge of northern California; and from the Pacific Ocean inland to the crest of the Cascade Mountains.
    - Old-growth forests do not cover all the land in this area. Some of the forests are younger. Other types of ecosystems are found in the valleys and along rivers.

The Old-Growth Forest Is the Result of Change

  • 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.
  • 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.
  • Three qualities are important when an ecosystem responds to changes --biodiversity, biological legacies, and resilience.
    - Biodiversity--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, and bats. This high level of biodiversity means that many species carry out each ecological process.
    - 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.
    - Resilience--Although forests are changing all the time, they are dynamic. Forests continue their ecological processes through all the changes, a quality known as resilience. A forest rich in biodiversity and biological legacies is resilient.

What Is An Ecosystem?

  • An ecosystem includes living organisms and the climate, soil, water, and air of the place where they live, and all the interactions of the living organisms with each other and their environment.
  • Ecosystems can be studied in terms of their structures, composition, and processes.
  • One ecosystem is the Pacific Northwest old-growth forest. The structures, composition, and processes of the old-growth forest are described under other headings on this web page.

The Main Structures of Old-Growth Forests

  • Structures are the physical arrangement of various pieces of an ecosystem, such as the spacing of trees in the forest.
  • Old-growth forests have four main structures.
    - Big trees--The huge trees are the main "factories" of the old-growth forest, because the trees bring energy into the forest through photosynthesis. The trees are also storehouses. Each tree stores many tons of organic material and nutrients, which are eventually recycled back into the ecosystem. The big trees are also the source of the next two structures.
    - Large snags (standing dead trees)--The big trees die from tree diseases such as heart rot and root rot, or are killed by lightning strikes or insect damage, or their tops are broken off in a windstorm or snowstorm. Snags are used by many different kinds of wildlife, including pileated woodpeckers and spotted owls.
    - Large fallen trees on the forest floor--Fallen trees decay on the forest floor. It takes many decades for a fallen tree to decay completely. As the fallen trees decay, they become homes for many living creatures, including carpenter ants, folding-door spiders, centipedes, salamanders, and shrews. Mushrooms and other fungi grow on the rotting trees, and eventually the rotten trees turn into nurse logs, as young trees grow on top of them.
    - Multi-layered or continuous canopy--The big old trees have large branches and deep crowns. Younger, smaller trees spread their branches through spaces between big trees. Shrubs such as rhododendrons create another layer. An old-growth forest has so many layers of branches that the canopy is essentially continuous. Lichens and mosses that live on these branches survive on rainwater and moisture from the air.

Composition: The Trees, Animals, and Other Life in the Old-Growth Forest

  • Composition is defined as the different species of animals, plants, and other living organisms that are found in the ecosystem, and the abundance of each species.
  • The Pacific Northwest old-growth forest has thousands of species. The main tree species and just a few of the animal species are described on this web page.

Five Basic Processes That Go On in an Ecosystem

  • An ecosystem is alive and dynamic. Although the same processes go on in all ecosystems, different species carry them out in each ecosystem. As ecosystems develop, the pathways become increasingly complex.
  • All the activities that go on in ecosystems can be classified into five basic processes:
    Input-- Energy, organic materials, or living organisms that come into the ecosystem fuel all the life and activity. Trees and other plants bring energy into an ecosystem through photosynthesis. Animals that move into an ecosystem, such as deer walking into the forest, are also inputs.
    Production--The production of living organisms goes on constantly in every ecosystem. As trees carry out photosynthesis, they grow and become larger. When animals have babies and plants produce seeds, they produce new life.
    Storage-- Trees "store" wood in their trunks and other organic materials in their needles. The old-growth forest stores huge amounts of organic materials. The soil stores huge amounts of water.
    Recycling--When a fallen tree rots on the forest floor, or an animal dies and its body decays, then the organic materials are recycled in the ecosystem. These organic materials include nutrients and essential materials that become food or energy for the living organisms in the ecosystem.
    Output--Some energy and resources leave the ecosystem. When a wildfire burns trees, much of the wood is consumed in the fire. Animals that leave the ecosystem, such as a spotted owl flying to another forest, are also outputs.


See also the links to PNW General Technical Reports and PNW Science Findings.

The main General Technical Reports (GTRs) used for this web page are listed below. Some information was quoted directly from GTR-118.

Franklin, J.F., et al. 1981. Ecological characteristics of old-growth Douglas-fir forests. U.S. Department of Agriculture, Forest Service, PNW Research Station, GTR-PNW-118.

Maser, C., and J.M. Trappe, editors. 1984. The seen and unseen world of the fallen tree. U.S. Department of Agriculture, Forest Service, PNW Research Station, GTR-PNW-164.

Ruggiero, L.F., et al. 1991. Wildlife and vegetation of unmanaged Douglas-fir forests. U.S. Department of Agriculture, Forest Service, PNW Research Station, GTR-PNW-285.

Additional references used for this web page are:

Arno, S.F., and R.P. Hammerly. 1977. Northwest Trees. Mountaineers Books, Seattle, WA.

Maser, C. 1998. Mammals of the Pacific Northwest: From the Coast to the High Cascades. Oregon State University Press, Corvallis, OR.