Temperate rainforest extends along more than 2,000 miles of North America’s northwest coast and offshore islands from lands bordering Prince William Sound, Alaska (roughly 61° N), to a little south of San Francisco Bay, California (about 38° N). In Southeast Alaska and British Columbia the forest forms a band about 100 miles wide, but then it narrows to a width of about 35 miles in the Pacific Northwest of the US. Another narrow band occurs on the windward slopes of the Cascade Mountains from southern Canada to central Oregon. The upper limits of the forest (treeline) varies with latitude—as do the tree species present. In the northernmost outposts of the rainforest, treeline is reached near 650 ft asl, whereas in the south forest transitions into alpine tundra at about 9,000 ft asl.
The regional climate is maritime with no extended periods of great warmth or cold. Precipitation occurs throughout the year, becoming more seasonal farther south where a mediterranean regime (i.e.,winter precipitation) influences rainfall patterns. Frequent fog and near-constant cloudiness keep humidity high and forest fires a relatively rare occurrence. High atmospheric moisture also slows rates of decomposition and leads to thick organic layers on the forest floor and peat bogs in northern parts of the biome.
Interestingly, Pacific salmon are the keystone species in these coastal forests. They sustain marine mammals as well as birds, bears, and even wolves.
This coniferous forest is the largest extent of temperate rainforest on the planet. Tall, spire-shaped trees, often draped with epiphytic lichens, grow in dense stands. Common species are western hemlock (Tsuga heterophylla), western red cedar (Thuja plicata), Douglas fir (Pseudotsuga menziesii), silver fir (Abies amabilis), shore pine (Pinus contorta subsp. contorta), Yellow cypress (Chamaecyparis nootkatensis), and in the south, coast redwood (Sequoia sempervirens).
Major changes in species composition occurs along the 23° latitudinal extent of the rainforest, and ecologists recognize four major forest types:I. Boreal rainforest is the northernmost expression of the biome in northwest North America. It grows amidst alpine glaciers on the shores of Prince William Sound. Wetlands, including muskeg and meadows, increase habitat diversity. Western hemlock and Sitka spruce are dominant and may grow up to 100 ft tall. The low species-richness is believed due to the young age of this forest, only colonized by trees since the relatively recent retreat of Pleistocene glaciers. The Chugash National Forest, just east of Anchorage, conserves part of the boreal rainforest and is an important stopover place for migrating shorebirds.
II. Perhumid Temperate Rainforest receives significant precipitation every month of the year, with an annual total of more than 100 inches. The largest national forest in the US, the Tongass, preserves important old growth forest of this type and is considered the crown jewel of the national forest system. Canada’s Great Bear Rainforest and the Haida Gwaii archipelago offshore similarly protect a large swathe of forest on British Columbia’s “Raincoast”. Western hemlock and Sitka spruce are the most common and largest trees. Hemlock attain heights of 100-150 ft and ages of 200-500 years. Sitka spruce grow even taller, up to 225 ft tall and may live 500-700 years. They are joined by smaller conifers such as mountain hemlock and western red cedar. Shore pine is often scrubby and can be found from sea level to the alpine zone, It often grows in muskeg, where yellow cedar also occurs.
Intact rainforest means watersheds necessary to support salmon fisheries are protected. All six species of Pacific salmon spawn in the Tongass and support some of the largest concentrations of brown bears and bald eagles in North America. Great Bear Rainforest hosts a white phase of the black bear known as “Spirit Bear” or Kermode bear. Great Bear is also home to the marbled murrelet, a robin-sized seabird that nests in clumps of moss high in the tallest and oldest trees. Fishing at sea, its wastes contribute nutrients to the forest.
The salmon fisheries are also important commercially, for recreation, and for subsistence harvesting by the Tlingit, Haida, and Tsimshian peoples of the region.
III. Seasonal Temperate Rainforest occurs from central Vancouver Island to southern Oregon and inland on the western slopes of the Cascades. Coastal areas are frequently bathed in fog and sea spray. Most of the 78-160 inches of annual precipitation falls in autumn and winter. Fires tend to burn and allow for forest replacement every 90-250 years. The forests on the Olympic Peninsula, including the Hoh Rainforest and the Quinault Rainforest (in Olympic National Park) are more diverse than the perhumid rainforests and are dominated by Douglas fir, western red cedar, Sitka spruce, and shore pine. Broadleaf deciduous trees such as bigleaf maple (Acer macrophyllum), red alder (Alnus rubra), vine maple (Acer circinatum), and black cottonwood (Populus trichocarpa) accompany the conifers. Mosses and lichens are abundant on the forest floor and on the trees. The forests on the western slopes of Cascades are dominated by Douglas fir, western hemlock, and western red cedar. Few old growth stands remain in this heavily logged region.
Among native birds inhabiting the Hoh Rainforest is the northern spotted owl (Strix occidentalis caurina). Native mammals include bobcat (Lynx rufus), cougar (Puma concolor cougar), Olympic black bear (Ursus americanus altifrontalis), Roosevelt elk (Cervus canadensis roosevelti), and black-tailed deer (Odocoileus columbianus).
IV. Warm Temperate Rainforest is found from the vicinity of the California-Oregon border to just south of San Francisco Bay and is the domain of the coast redwoods. The highest species richness and highest rate of endemism in all the Northern Hemisphere rainforests occur here. Mild, wet winters and cool, dry summers mark the climate of the region. Annual precipitation in northern redwood forests is greater than 125 inches; less than 5 percent falls in the dry summer months. Conditions are drier in the southern forests. Growing with the redwoods are white fir (Abies concolor), sugar pine (Pinus lambertiana), Ponderosa pine (P. ponderosa), incense cedar (Calocedrus decurrens), Sitka spruce, western red cedar, and western hemlock. There is a diverse understory of hardwood trees, shrubs, ferns, and forbs. The canopy supports rich communities of invertebrates, mosses, lichens, small mammals, and even some salamanders. The soil fauna is also rich.Only remnants of redwood forest remain intact. Most are found in isolated but protected areas such as Muir Redwoods National Monument,:Humboldt Redwoods State Park, King Range National Conservation Area, and Redwoods National Park. These patches amount to less than 4 percent of the original forest. Remaining groves of giant trees on non-public lands are threatened by logging. All stands are threatened by climate change and the associated decline in coastal fog and introduced diseases such as Sudden Oak Death.
Main source: Dominick A. DellaSala et al. 2011. “Temperate and Boreal Rainforests of the Pacific Coast of North America.” In Dominick A. DellaSala, editor. Temperate and Boreal Rainforests of the World: Ecology and Conservation. Island Press.