Puget lowland forests
The Puget Lowland Forests (PLF) ecoregion, located in the Pacific Northwest of North America, is characterized by its temperate coniferous forests and significant human population. Covering approximately 8,683 square miles, this area is bordered by the Olympic Mountains to the west and the Cascade Mountains to the east, extending into parts of Canada and down to Oregon. The region enjoys a mild maritime climate, with substantial rainfall, which supports the growth of lush old-growth forests primarily composed of species like western hemlock, western red cedar, and Douglas fir.
Historically, the landscape has been shaped by glacial activity and has undergone extensive changes due to urbanization, agriculture, and logging practices since European settlement. Indigenous cultures, such as the Muckleshoot and Nisqually tribes, have long utilized the forest's resources for sustenance, emphasizing the ecological and cultural importance of these forests.
Today, the PLF is home to diverse wildlife, including species dependent on old-growth habitats. Conservation efforts have intensified, particularly to protect the northern spotted owl and other species reliant on these ecosystems. However, challenges such as population growth, climate change, and urban sprawl pose ongoing threats to the fragile balance of this unique environment.
Puget lowland forests
- Category: Forest Biomes.
- Geographic Location: North America.
- Summary: This ecoregion, dominated by temperate coniferous forest, is now a major population center.
The Puget Lowland Forest ecoregion (PLF) of the Pacific Northwest is home to awe-inspiring old-growth coniferous forests and to millions of people. The mild temperature and ample rainfall of this biome promote forest development. Primarily in Washington state, it is bordered by the Olympic Mountains to the west and the Cascade Mountains to the east. The PLF extends north to Canada, where it includes the Fraser Valley lowlands, the coastal lowlands, and several of the Gulf Islands. To the south, it extends to the border between Washington and Oregon.
The PLF ecoregion covers an area of approximately 8,683 square miles (22,488 square kilometers). The are is home to over 4 million people, and its population is expected to increase to 5.7 million by 2030. The PLF ecoregion contains the Canadian cities of Vancouver and Victoria, British Columbia; and Bellingham, Seattle, Olympia, and Tacoma in Washington state. The San Juan Islands that divide Puget Lowland from the Strait of Georgia in British Columbia are also in this ecoregion.
The Puget lowland is a structural downwarp that was subsequently modified by glacial scour and fill from the Cordilleran ice sheet during the Fraser Glaciation. When the ice sheet retreated 14,000 years ago, meltwater flowed southward, depositing gravelly outwash plains. Portions of these outwash plains are prairies, some of which display mounded topography or Mima mounds, consisting of more or less evenly spaced and circular mounds 1–7 feet (0.3 to 2.1 meters) high and 8–40 feet (2.4–12.1 meters) in diameter.
Climate
The Pacific Ocean buffers the PLF climate from temperature extremes, and provides moisture carried by the prevailing winds. As a result, the Puget lowland has a mild maritime climate with a mean annual temperature of 48 degrees F (9 degrees C), ranging to 59 degrees F (15 degrees C) in the summer and 38 degrees F (3.5 degrees C) in the winter.
Annual precipitation averages 31–35 inches (800–900 millimeters). Areas in the Olympic Mountains rain shadow may receive only 18 inches (460 millimeters) of rain a year, while other areas may receive as much as 60 inches (1,530 millimeters) of precipitation annually. More than 75 percent of yearly precipitation falls between the beginning of October and the end of March.
Elevation in the region ranges from sea level to 1,509 feet (460 meters), yet typically is below 525 feet (160 meters). One of the most influential factors in the Puget Sound area is the limited solar radiation available to plants because the sky is overcast for approximately 229 days a year.
Historical Biome Changes
Although grass and oak (Quercus garryana) prairies and savannas were once extensive features of the southern portion of the PLF, the natural tendency of even this landscape is toward forest. Native prairie vegetation, formerly maintained by Native American burning, has been reduced significantly in size since European settlement. Major threats to prairies include urbanization, agriculture, and invasion of nonnative and woody vegetation resulting from fire suppression and livestock grazing.
The earliest artifacts of inhabitants date to approximately 8,000 years ago. There is evidence that indigenous cultures had a well-developed marine-based lifestyle by 1,000 BCE. Tribes in the Puget Sound include the Muckleshoot, Nisqually, Skagit, and Snoqualmie, which speak a variant of the Salish language family.
While marine waters traditionally provided the majority of the food for the tribes, the lowland forests and prairies complemented this diet with plant foods including salal (Gaultheria shallon), huckleberries (Vaccinium spp.), salmon berries (Rubus spectabilis), and bracken fern (Polystichum munitum). Open-canopied habitats, including the prairies that were anthropogenically managed through burning, provided habitat for a variety of game and shade-intolerant food and medicinal plants, including camas (Cammasia quamash).
In the 1900s, logging transformed the landscape in the Puget Lowlands. Although planting of clear-cuts with trees has been practiced on federal and other lands since the first half of the 1900s, the Washington State Department of Natural Resources has required such planting within three years of harvest on all private holdings since 1975. Today, 95 percent of the PLF ecoregion has been culturally modified. Small, isolated islands of original habitat such as old-growth forest, bogs, and prairie-oak woodlands are surrounded by urbanization.
Before European settlement, old-growth forests of the lowlands were primarily covered with western hemlock (Tsuga heterophylla), western red cedar (Thuja plicata), and Douglas fir (Pseudotsuga mensiesii). Periodic flooding and infrequent fires were historically the predominant disturbance regimes. Typically centuries passed between large-scale fire events in the moist forest types. Drier forests (Quercus spp., P. ponderosa) and prairies, however, experienced more frequent fires. Today, forest remains the primary land-cover class, at 48.4 percent across the biome.
Vegetation
Three vegetation zones occur in the PLF and are named for their dominant trees at late-successional stages: the Western Hemlock Zone, from sea level to the lower montane slopes of the basin; the Pacific Silver Fir (Abies amabilis) Zone, in the midmontane altitudinal belt; and the Mountain Hemlock (Tsuga mertensiana) Zone, at the higher elevations. The most prevalent tree species in the lowland forests today is Douglas fir. Typical understory species include salal, western swordfern (Polystichum munitum), several huckleberry species, Oregon grape (Mahonia spp.), and Pacific rhododendron (Rhododendron macrophyllum).
The PLF has towering evergreen trees that led early European explorers and settlers to describe and sketch the landscape as a primeval landscape. The old-growth stage of lowland conifer forest succession is at its prime at 350 to 750 years, yet it begins to resemble old-growth starting at 175 to 250 years. In the old growth stage, trees vary in size from seedlings and saplings to large trees with massive trunks that are well spaced. The shaded understory retains enough light to support a substantial understory of tree seedlings, shrubs, and forbs.
Biodiversity
The old growth in the PLF is exemplary of an ecosystem in which all parts are connected. The living forest depends on dead logs, stumps, and other litter to retain water through the summers’ drought and provide habitat for microorganisms and invertebrates, which in turn recycle nutrients for new plant life. The canopy of the forest is often covered with lichen, ferns, mosses, and algae that collect moisture and particulate matter from the air.
The riparian forests in the PLF continue to provide important spawning areas for salmonids (Oncorhynchus spp.), amphibians, bats, bald eagles (Haliaeetus leucocephalus), and much more wildlife. Many species of birds and mammals are characteristic indicators of old growth in the PLF. Seven bird species rely on this old-growth habitat, and the forest canopy supports six mammal species, including the northern flying squirrel (Glaucomys sabrinus) and tree vole (Arborimus longicaudus). Larger mammals of the forest include cougars (Puma concolor) and black-tailed deer (Odocoileus hemionus). The northern spotted owl (Strix occidentalis caurina) nests in the cavities of old trees.
Conservation Efforts
Major support for preserving old growth since the 1990s has come from the urgency to preserve the habitat of the spotted owl and other old-growth-dependent species. The Northwest Forest Plan was developed in the 1990s and banned timber harvest in 10 million of the 17 million acres (4 million of the 6 million hectares) of national forests in the Pacific Northwest.
Urbanization and urban sprawl remains one of the largest threats to the region. Populations continue to grow. Climate change in the Puget Lowland Forest biome and all forests in the Pacific Northwest has caused warm and dry conditions. This will increase the frequency and extent of fires. It will also cause drought and insect outbreaks. Regeneration will be more difficult in hotter, drier conditions.
Bibliography
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Kruckeberg, A. R. The Natural History of Puget Sound Country. Seattle: University of Washington Press, 1998.
Reilly, Matthew J. et al. “Fire Ecology and Management in Pacific Northwest Forests. In: Greenberg, C.H., Collins, B. (eds) Fire Ecology and Management: Past, Present, and Future of US Forested Ecosystems.” Managing Forest Ecosystems, vol 39. 2021, doi.org/10.1007/978-3-030-73267-7‗10. Accessed 15 Nov. 2024.
Rickets, T. H., E. Dinerstein, D. M. Olson, and C. J. Loucks, eds. Terrestrial Ecoregions of North America: A Conservation Assessment. Washington, DC: Island Press, 1999.
Sorenson, Daniel G. “Summary of Land-Cover Trends—Puget Lowland Ecoregion.” United States Geological Survey, May 2011. . Accessed 15 Nov. 2024.