Puget lowland forests

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Puget Sound, Canada Photograph by John Morrison

The Puget lowland forests occupy a north-south topographic depression between the Olympic Peninsula and western slopes of the Cascade Mountains, extending from north of the Canadian border to the lower Columbia River along the Oregon border. The portion of this forest ecoregion within British Columbia includes the Fraser Valley lowlands, the coastal lowlands locally known as the Sunshine Coast and several of the Gulf Islands. This ecoregion is within the Nearctic Realm and classified as part of the Temperate Coniferous Forests biome.

This ecoregion is distinct from the mountainous hydro-riparian systems to the west, as well as the drier areas to the north and east. The Puget Sound Valley is a topographically depressed previously glaciated area consisting of moderately dissected tableland covered by glacial till, glacial outwash, and lacustrine deposits. Topographic relief in the valley is moderate, with elevation ranging from sea level to 460 metres (m), although seldom exceeding 160 m. North of the USA-Canada border, bedrock outcrops of Mesozoic and Palaeozoic origin form undulating hills on the lower elevation mainland that reach an elevation of 310 m above sea level. The Fraser River dominates this ecoregion. The majority of soils in the valley are formed in glacial materials under the influence of coniferous forest. Haplorthods (brown podzolic soils) are most common and contain moderately thick forest floor layers with well-developed humus.

caption Source: World Wildlife Fund This ecoregion has a Mediterranean-like climate, with warm, dry summers, and mild wet winters. The mean annual temperature is 9°C, the mean summer temperature is 15°C, and the mean winter temperature is 3.5°C. Annual precipitation averages 800 to 900 millimeters (mm) but may be as great as 1530 mm. Only a small percentage of this precipitation falls as snow. However, annual rainfall  on the San Juan Islands can be as low as 460 mm, due to rain-shadow effects caused by the Olympic Mountains. This local rain shadow effect results in some of the driest sites encountered in the region. Varied topography on these hilly islands results in a diverse assemblage of plant communities arranged along orographically defiined moisture gradients. Open grasslands with widely scattered trees dominate the exposed southern aspects of the islands, while moister dense forests occur on northern sheltered slopes characterized by Western red cedar (Thuja plicata), Grand fir (Abies grandis), and Sword fern (Polystichum munitum) communities.

Before Native American cultivation and European settlement, the Puget Sound lowland forests were dominated by dense coniferous forests, most commonly composed of Western red cedar, Western hemlock (Tsuga heterophylla), and Douglas-fir (Pseudotsuga menziesii). Mixed stands of Douglas-fir with some Oregon oak (Quercus garrayana), Pacific dogwood (Cornus nuttallii) and Toyon (Arbutus menziesii) are common on drier sites. Moist tracts support stands of Western hemlock and Western red cedar. Periodic flooding and infrequent fires were once the predominant prehistoric disturbance regimes in the region. Long time intervals (on the order of centuries) between large-scale fire events were more typical of moister forest types, with drier forests (Quercus spp., Pinus ponderosa) and prairies experiencing frequent fires.

Biological distinctiveness

caption Bald eagle (Haliaeetus leucocephalus) molting to adult plumage, USA. Source: George Jameson

This ecoregion is intermediate in vertebrate species richness (284 species), and conifer richness (eleven species) compared to the thirty other ecoregions within its Major Habitat Type (MHT). Birds (200 species) make up the largest fraction of vertebrate taxonomic groups represented. Notably, eight of the total faunal species are endemics, and six of these are snail taxa. Although plant communities within the region are similar to others in the T. heterophylla zone of western Washington, large areas once contained prairie, oak woodland, and pine (Pinus spp.) forest types. Other notable features that are uncommon elsewhere in western Washington include the following: (1) Pinus contorta, P. monticola, and P. ponderosa as major constituents along with P. menziesii and Glautheria shallon; (2) Quercus garryana groves (relict examples on the Fort Lewis Airforce Base); (3) extensive prairies often invaded by P. menziesii and associated with groves of Quercus (Fort Lewis); (4) abundant, but poorly drained sites with swamp or bog communities (relict examples in the Seattle area); (5) occurrence of species rarely or never found elsewhere in western Washington such as Juniperus scopulorum, Populus tremuloides, Pinus ponderosa, and Betula papyrifera. Prairies were significant features south of Puget Sound and once included the Tacoma Prairies near Tacoma, Washington and Wier Prairie near Olympia, Washington. Since human settlement, the extent of these prairies has been substantially reduced by urbanization, cultivation, and invasion by Douglas-fir and Oregon Oak communities caused by overly aggressive fire suppression and livestock overgrazing.

Remaining riparian forests in the region provide important spawning areas for salmonids (Oncorhynchus spp.), habitat for amphibians and snails, roosting sites for bats, perching and nesting sites for Bald eagles, and travel corridors for wildlife (e.g., Black-tailed deer, neotropical migratory birds).


Characteristic mammalian fauna includes Creeping vole (Microtus oregoni), Raccoon (Procyon lotor), Southern sea otter (Enhydra lutris), Mink (Mustela vison), Coyote (Canis latrans), Black-tailed deer (Odocoileus hemionus), Pallid bat (Antrozous pallidus), and Harbour seal (Phoca vitulina).


There is a rich diversity of birds which include the Near Threatened Spotted owl (Strix occidentalis), Turkey vulture (Cathartes aura), Bald eagle (Haliaeetus leucocephalus), Blue grouse (Dendragapus obscurus), as well as a gamut of seabirds, numerous shorebirds and waterfowl.


There are a small number of reptilian taxa within the ecoregion: Common garter snake (Thamnophis sirtalis); Western terrestrial garter snake (Thamnophis sirtalis); Northern alligator lizard (Elgaria coerulea); Western fence lizard (Sceloporus occidentalis); Northwestern garter snake (Thamnophis ordinoides); Sharp-tailed snake (Contia tenuis); Yellow-bellied racer (Coluber constrictor); and Western pond turtle (Clemmys marmorata).


The amphibian taxa present in the Puget lowland forests are: Cope's giant salamander (Dicamptodon copei); Monterey ensatina (Ensatina eschscholtzii); Long-toed salamander (Ambystoma macrodactylum); Western redback salamander (Plethodon vehiculum); Northwestern salamander (Ambystoma gracile); Pacific chorus frog (Pseudacris regilla); Coastal giant salamander (Dicamptodon tenebrosus); Rough-skin newt (Taricha granulosa);  the Vulnerable Spotted frog (Rana pretiosa); Tailed frog (Ascopus truei); and Northern red-legged frog (Rana aurora).

Conservation status

Habitat loss

This ecoregion is situated within the most densely human populated area of the state of Washington and province of British Columbia, encompassing the cities of Vancouver, Victoria, Bellingham, Seattle, Tacoma, and Olympia. Consequently, only five percent of the original habitat within the region remains and most remaining areas have been heavily (90 to 100 percent) altered. Small, isolated ecological islands of original habitat (e.g., old-growth forest, bogs, prairie-Oak woodlands) are surrounded by urbanization and agriculture.

Among the plant communities in this region the prairie-Quercus garryana woodlands, riparian forests and wetlands, and old-growth lowland forests (almost completely destroyed) are most threatened.

Remaining blocks of intact habitat

caption Puget Sound, Canada. Source: Meghan McKnight

No sizable blocks of intactThe condition of an ecological habitat being an undisturbed or natural environment habitat are extant in the region. A few relict examples of prairie-oak communities occur on Fort Lewis and are managed by the base to maintain characteristic plant composition. The remaining forests have been largely converted to tree farms or exist as small city or state parks. Remaining areas should be used to restore degraded habitats through the use of prescribed fire (prairies) and long-rotation timber harvest (plantations). Burn’s bog wetland complex, the southern most domed peatbog in western North America, is found adjacent to Vancouver.

Degree of fragmentation

Habitat fragmentation has been extensive throughout the region; almost no native habitat remains.

Degree of protection

Only a very small fraction of the natural communities in this region have been protected. Opportunity exists to use local conservation easements to protect remaining riparian and wetland areas and to restore a portion of the old-growth forests. However, this potential is limited by urbanization, which is actually projected to increase substantially over the period 2015-2020; the region is one of the fastest growing areas in the USA. A case can be made to better manage riparian areas and nearby watersheds to maintain municipal watersheds and open spaces, and to provide biological corridors to adjacent ecoregions.

Ecological threats

The ecoregion was given a critical ranking because of threats to remaining native habitats from urbanization; agriculture; fire suppression; invasive species; flood control and hydroelectric dams; and logging. A serious threat to native plant communities, especially prairie-oak woodlands, is invasion by scotchbroom (Cytisus scoguarius) and encroachment by trees as a result of fire suppression in these prairies.

Suite of priority activities to enhance biodiversity

Within the ecoregion the best opportunities for conservation include the following:

  • Establish one or more forest biological corridors between the Cascade and Coast Range forests south of Olympia (the Skookimchuck River valley is a prime candidate that if restored could act as a connection between populations of northern spotted owls (Strix occidentalis caurina) on the Olympic Peninsula and those in the Cascade Mountains)
  • Establish several riparian habitat corridors along streams draining into Puget Sound from the Cascades (e.g., Nisqually, Skykomish, Nooksack rivers)
  • Protection of Burn’s Bog, British Columbia
  • Maintain the remaining prairie-oak woodlands on the Fort Lewis base through the use of prescribed fire management

Conservation partners

The region is a hotbed for local conservation with several regional and national groups having offices in the Seattle area. Some examples include:

  • Burns Bog Conservation Society
  • Canadian Nature Federation
  • Friends of Caren
  • Galiano Conservancy Association
  • National Audubon Society
  • The Nature Conservancy, British Columbia
  • Nature Trust of BC
  • Northwest Ecosystem Alliance
  • Pender Harbour and District Wildlife Society
  • Tetrahedron Alliance
  • White Rock and Surrey Naturalists
  • The Wilderness Society

Neighbouring ecoregions

The following ecoregions have some tangency with the Puget lowland forests:

  • British Columbia mainland coastal forests, to the north and east
  • Central and Southern Cascades forests, to the southeast
  • Central Pacific coastal forests, to the west

Relationship to other classification schemes

In general, there is a high concordance with Omernik's classification of this ecoregion (Puget lowlands ecoregion #2). Bailey, however, delineated this ecoregion further south of the lower Columbia River to the Willamette Valley of central Oregon and thus the boundary utilised in this analysis corresponds closely to Bailey's northern portion of the Pacific lowland region (i.e., above the lower Columbia River).  Justification for splitting the ecoregion is provided by Franklin and Dyrness, who indicate that the Puget lowlands may be recognised as a separate vegetative zone similar to the Coastal Douglas-fir zone in British Columbia, because both the Puget Sound and British Columbia areas were glaciated and are influenced by large oceanic bodies. Such climatic influences are much less dramatic in the Willamette Valley. Therefore, the delineation herein more closely approximates these suggested changes. This ecoregion is given the ecocode NA0524 by the World Wildlife Fund.

Only a small fraction of this ecoregion lies in Canada. The Lower Mainland of British Columbia (TEC 196) extends westward from the foothills of the Cascade Range at Chilliwack to the Fraser River delta at Richmond and northward to include the narrow Georgia Lowland along the Sunshine Coast. The Georgia-Puget Basin (TEC 195) incorporates the numerous Gulf Islands of the Strait of Georgia off the coast of British Columbia, hence its forest region names: Coastal Strait of Georgia and Southern Pacific Coast (1 and 2).


  • Fred W. Beckey. 2003. Range of Glaciers: the Exploration and Survey of the Northern Cascade Range. Portland, Oregon: Oregon Historical Society Press. ISBN 978-0-87595-243-7.
  • Derek Hayes. 1999. Historical Atlas of the Pacific Northwest: Maps of Exploration and Discovery. Seattle, Washington: Sasquatch Books . ISBN 1-57061-2153.
  • Stuart S. Holland. 1976. Landforms of British Columbia: A Physiographic Outline (Bulletin 48). British Columbia Department of Mines and Petroleum Resources.
  • James P.Ronda. 1984. Lewis and Clark among the Indians. Lincoln, Nebraska: University of Nebraska Press. ISBN 08032-8990-1.;

Disclaimer: This article contains some information that was originally published by the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.




Fund, W., & Hogan, C. (2015). Puget lowland forests. Retrieved from


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