The Puget lowland forests occupy a north-south depression between the Olympic Peninsula and western slopes of the Cascade Mountains, extending from across the Canadian border to the lower Columbia River along the Oregon border. In British Columbia, this ecoregion includes the Fraser Valley lowlands, the coastal lowlands locally known as the "Sunshine Coast" and several of the Gulf Islands.
It is distinct from the mountainous hydro-riparian systems to the west and the drier areas to the north and east. The Puget Sound Valley is a depressed glaciated area consisting of moderately dissected tableland covered by glacial till, glacial outwash, and lacustrine deposits. Relief in the valley is moderate with elevation ranging from sea level to 460 meters (m) and seldom exceeding 160 m. North of the U.S.-Canada border, bedrock outcrops of Mesozoic and Palaeozoic origin form rolling hills on the Lower 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.
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 high as 1,530 mm. Only a small percentage of this precipitation falls as snow. However, rainfall amounts on the San Juan Islands can be as low as 460 mm because of 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 topographically determined 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 cultivation and European settlement, the Puget Sound lowland forests were dominated by dense coniferous forests most commonly made up of western red cedar, western hemlock (Tsuga heterophylla), and Douglas-fir (Pseudotsuga menziesii). Mixed stands of Douglas-fir with some Garry oak (Quercus garrayana), Pacific dogwood (Cornus nuttallii) and arbutus (Arbutus menziesii) are common on drier sites. Moist sites support stands of western hemlock and western red cedar. Periodic flooding and infrequent fires were once the predominant disturbance regimes in the region. Long intervals (centuries) between large-scale fire events were more typical of moister forest types, with drier forests (Quercus spp., P. ponderosa) and prairies experiencing frequent fires.
Characteristic wildlife includes raccoon (Procyon lotor), sea otter (Enhydra lutris), mink (Mustela vison), coyote (Canis latrans), black-tailed deer (Odocoileus hemionus), and harbour seal (Phoca vitulina). There is a rich diversity of birds which include turkey vulture (Cathartes aura), bald eagle (Haliaeetus leucocephalus), blue grouse (Dendragapus obscurus), seabirds, numerous shorebirds, and waterfowl.
In general, this ecoregion is intermediate in species richness (429 species), and conifer richness (11 species) compared to the 30 other ecoregions within its Major Habitat Type (MHT). Birds (200 species) make up the majority (47 percent) of taxonomic groups represented. Notably, eight of the total species are endemics and six of these are snails. 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) Q. garryana groves (relict examples on the Ft. Lewis Airforce Base); (3) extensive prairies often invaded by P. menziesii and associated with groves of Quercus (Ft. Lewis); (4) abundant and 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, P. 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 settlement the extent of these prairies has been substantially reduced by urbanization, cultivation, and invasion by Douglas-fir and Garry Oak communities caused by fire suppression and livestock grazing.
Remaining riparian forests in the region provide important spawning areas for salmonids (Oncorhynchus spp.), habitat for amphibians and snails, roost sites for bats, perching and nesting sites for bald eagles, and travel corridors for wildlife (e.g., Black-tailed deer, neotropical migratory birds).
This ecoregion lies within the most densely populated area of Washington and British Columbia, encompassing the cities of Vancouver, Victoria, Bellingham, Seattle, Tacoma, and Olympia. Consequently, only 5 percent of the original habitat within the region remains and most remaining areas have been heavily (90-100 percent) altered. Small, isolated 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-Q. garryana woodlands, riparian forests and wetlands, and old-growth lowland forests (almost completely gone) are most threatened.
Remaining Blocks of Intact Habitat
No sizable blocks of intact habitat remain in the region. A few relict examples of prairie-oak communities occur on Ft. 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
Degree of Protection
Very little 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 that is projected to increase substantially over the next decade; the region is one of the fastest growing areas in the United States. A case can be made to better manage riparian areas and nearby watersheds to maintain city watersheds and open spaces, and to provide connections to adjacent ecoregions.
Types and Severity of 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 Conservation
Within the region the best opportunities for conservation include the following:
- Establish one or more forest connections 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 Ft. Lewis base through the use of prescribed fire management
The region is a hot-bed 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
- Sierra Club
- Sierra Club of British Columbia
- Tetrahedron Alliance
- White Rock and Surrey Naturalists
- The Wilderness Society
- World Wildlife Fund, Canada
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 we used in this analysis corresponds more 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 recognized 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, our delineation more closely approximates these suggested changes.
Only a small portion 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).
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