The Pacific temperate rainforests of western North America is one of the largest temperate rain forest regions on the planet. The Pacific temperate rainforests lie along the western side of the Pacific Coast Ranges along the Pacific Northwest Coast of North America from the Prince William Sound in Alaska through the British Columbia Coast to Northern California, and are part of the Nearctic realm. The Pacific temperate rain forests are characterized by a high amount of rainfall, in some areas more than 300 cm (10 ft) per year and moderate temperatures in both the summer and winter months (10–24 °C or 50–75 °F). This ecoregion is a subregion of the Cascadia bioregion. These rainforests occur in a number of ecoregions, which vary in their species composition, but are predominantly of conifers, sometimes with an understory of broadleaf trees, ferns and shrubs. In the WWF's system, the ecoregions of the Pacific temperate rainforests are the Northern Pacific coastal forests, Haida Gwaii, British Columbia mainland coastal forests, Central Pacific coastal forests, Central and Southern Cascades forests, Klamath-Siskiyou forests, and Northern California coastal forests ecoregions.
Flora The forests in the north contain predominantly Sitka spruce (Picea sitchensis) and western hemlock (Tsuga heterophylla), while those in the coastal forests are home to both species mentioned, as well as coast redwood (Sequoia sempervirens), coast Douglas-fir (Pseudotsuga menziesii), western redcedar (Thuja plicata) and shore pine (Pinus contorta). Notably, many of the world's largest and tallest tree species are found in this ecoregion. Dense growths of epiphytes and mosses cover the trees, and lush vegetation is present everywhere. Hardwood trees such as the bigleaf maple and the alder are also common, especially at lower elevations and along stream banks, and are vital to the ecosystem, in part because of their nitrogen fixing.
History
About 200 million years ago (during the Triassic and Jurassic periods), the landscape was dominated by conifers, which were the most diverse group of trees and constituted the greatest majority of large trees. When flowering plants emerged (in the following Cretaceous period), they quickly prevailed, causing most conifers to become extinct, and those that survived to adapt to harsh conditions. Perhaps the most significant difference in this change is that the primitive conifers invested their energy in the basic food supply for every seed, with no certainty of fertilization; by contrast, flowering plants create the food supply for a seed only after it is triggered by fertilization. The Pacific temperate rainforests now remains the only region on Earth of noteworthy size and significance where, due to unique climatic conditions, the conifers flourish as they did before being displaced by flowering plants. The northern Pacific temperate rain forests are relatively young, emerging in the past few thousand years following the retreat of the ice sheets of the last ice age.
Ecology
The ecosystem of Pacific temperate rainforests is so productive that the biomass on the best sites is at least four times greater than that of any comparable area in the tropics. These forests are more massive than any other ecosystem on the planet with the uncountable mass of both living and decaying material. One of the reasons behind this is the rarity of fire. Unlike drier forests that can catch fire periodically, temperate rainforests experience small-scale disturbances like avalanches and blow-downs. This type of rainforest has a large latitude range from about 40 degrees north to about 60 degrees north. The climate differences from north to south create a few important forest zones, which are distinct due to the different plant and animal species that reside in them:
At the southern limit in northern California is the "coast redwood zone". Beginning at the California/Oregon border, and extending through the north end of Vancouver Island is the "seasonal rain forest zone". The common tree species are Douglas fir, western red cedar, Sitka spruce, and western hemlock. Beyond the northern end of Vancouver Island, is the "perhumid rain forest zone". Douglas fir becomes less of a dominate species, and the forest starts to be mainly composed of western red cedar, Sitka spruce, and western hemlock. The Gulf of Alaska begins where the fjords of southeast Alaska end, and marks the transition into the "sub-polar rain forest". Here the forest occupies only a very narrow strip between the ocean and the icy alpine zone. The cedar trees no longer thrive in this harsher climate, and the dominant trees are limited to Sitka spruce, and western and mountain hemlock. The northern limits of the rainforest are scattered in thin bands in the northern Prince William Sound, Kenai Fjords, eastern Kodiak Island, and western Cook Inlet.
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