The Western Hemlock (Tsuga heterophylla) is a large evergreen coniferous tree growing to 50-70 m tall, exceptionally 78 m, and with a trunk diameter of up to 2.7 m. It is the largest species of hemlock, with the next largest (Mountain Hemlock T. mertensiana) reaching a maximum of 59 m.
It is native to the west coast of North America, with its northwestern limit on the Kenai Peninsula, Alaska, and its southeastern limit in northern Sonoma County, California. It is closely associated with temperate rain forests, and most of its range is less than 100 km from the Pacific Ocean. There is however an inland population in the Rocky Mountains in southeast British Columbia, northern Idaho and western Montana. It mostly grows at low altitudes, from sea level to 600 m, but up to 1800 m in the interior part of its range in Idaho.
The bark is thin and furrowed, and brown in color. The crown is a very neat broad conic shape in young trees with a strongly drooping lead shoot, becoming cylindric in older trees; old trees may have no branches in the lowest 30-40 m. At all ages, it is readily distinguished by the pendulous branchlet tips. The shoots are very pale buff-brown, almost white, with pale pubescence about 1 mm long. The leaves are needle-like, 5-20 mm long, strongly flattened in cross-section, mid to dark green above, and with two broad bands of white stomata below with only a narrow green midrib between the bands.
The cones are small, pendulous, slender cylindrical, 18-28 mm long and 7-8 mm broad when closed, opening to 18-25 mm broad. They have thin, flexible scales 7-13 mm long. The immature cones are green, maturing gray-brown 5-7 months after pollination. The seeds are brown, 2-3 mm long, with a slender, 7-9 mm long pale brown wing.
It is a very shade-tolerant tree, with young plants typically growing up under the canopy of other conifers such as Sitka Spruce and Douglas-fir, then eventually replacing them in climax forest as those species cannot grow in the dense shade cast by Western Hemlocks. However, storms and (rarely) wildfires will create larger openings in the forest where these other species can then regenerate. The tallest specimen, 78.9 m tall, is in Prairie Creek Redwoods State Park , California (USA). It is long-lived, with trees over 1200 years old known.
Outside of its native range, Western Hemlock is of importance in forestry for timber and paper production, and as an ornamental tree in large gardens, in northwest Europe and southern New Zealand. It is naturalised in some parts of Britain and New Zealand, though not so extensively as to be considered an invasive weed tree.
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