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Concept
Tsuga heterophylla
Summary
Tsuga heterophylla, the western hemlock or western hemlock-spruce, is a species of hemlock 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. The Latin species name means 'variable leaves'.
Western hemlock is a large evergreen conifer growing to tall, exceptionally , and with a trunk diameter of up to . It is the largest species of hemlock, with the next largest (mountain hemlock) reaching a maximum height of . The bark is brown, thin, and furrowed (outwardly appearing similar to that of Douglas-fir). The crown is a very neat broad conic shape in young trees with a strongly drooping lead shoot, becoming cylindrical in older trees, which may have no branches in the lowest . 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 long.
The leaves are needle-like, long and broad, strongly flattened in cross-section, with a finely serrated margin and a bluntly acute apex. They are mid to dark green above; the underside has two distinctive white bands of stomata with only a narrow green midrib between the bands. They are arranged spirally on the shoots but are twisted at the base to lie in two ranks on either side of the shoot. The cones appear on trees over about 25 years old; they are small, pendulous, slenderly cylindrical, long and broad when closed, opening to broad. They have 15–25 thin, flexible scales long. The immature cones are green, maturing gray-brown 5–7 months after pollination. They are usually plentiful enough to cover the ground beneath the tree. The abundant seeds are brown, long, with a slender, pale-brown wing measuring long.
Initial growth is slow; one-year-old seedlings are commonly only tall, and two-year-old seedlings tall. Once established, saplings in full light may have an average growth rate of (rarely ) annually until they are tall, and in good conditions still annually when tall.
Official source
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