These forests occupy the outer coastal portions of British Columbia, southeastern Alaska, and northwestern Washington. Their center of distribution is the northern coast of British Columbia, as Thuja plicata approaches its northernmost limit in the southern half of southeastern Alaska. These forests occur mainly on islands but also fringe the mainland. They are never more than 25 km from saltwater; elevation ranges from 0 to 600 m, and below 245 m in Alaska (above 200 m, Chamaecyparis nootkatensis replaces Thuja plicata). The climate is hypermaritime, with cool summers, very wet winters, abundant fog, and without a major snowpack . Fire is absent from this system in Alaska and rare throughout the rest of the range. These forests are more influenced by gap disturbance processes and intense windstorms than by fire. The terrain is mostly gentle to rolling, of low topographic relief, and often rocky. Soils typically have a distinct humus layer overlying mineral horizons or bedrock; where the system is best developed in central British Columbia, the humus layers are very thick (mean 17-35 cm). Soils are often imperfectly drained, but this is not a wetland system. Thuja plicata and Tsuga heterophylla are the dominant tree species throughout, and Chamaecyparis nootkatensis joins them from northern Vancouver Island north. Canopy cover of trees is typically over 60%. Pinus contorta and Tsuga mertensiana can be present in some locations in the central and northern portion of the range. Abies amabilis occurs in British Columbia and northern Washington stands but is not typically found in southeastern Alaska. In Washington, nearly pure stands of Tsuga heterophylla are common and seem to be associated with microsites most exposed to intense windstorms. A shrub layer of Gaultheria shallon, Vaccinium ovalifolium, and Menziesia ferruginea is usually well-developed. The fern Blechnum spicant in great abundance is typical of hypermaritime conditions. Oxalis oregana (absent in Alaska) is important in the understory of moist sites in Washington. Polystichum munitum occurs at the northern end of its range in southeastern Alaska on well-drained sites. The abundance of Thuja plicata in relation to other conifers is one of the diagnostic characters of these forests; the other is the low abundance of Pseudotsuga menziesii (absent in Alaska) and Picea sitchensis. Where these forests are best developed, they occur in a mosaic with forested wetlands, bogs, and Sitka spruce forests (the latter in riparian areas and on steep, more productive soils).
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