Tsuga mertensiana, known as mountain hemlock, is a species of hemlock native to the west coast of North America, found between Southcentral Alaska and south-central California.

Description

Tsuga mertensiana is a large evergreen conifer growing up to tall, with exceptional specimens as tall as tall. They have a trunk diameter of up to . The bark is about thick and square-cracked or furrowed, and purplish-brown to gray in color. The crown is a neat, slender, conic shape in young trees with a tilted or drooping lead shoot, becoming cylindric in older trees. At all ages, it is distinguished by the slightly pendulous branchlet tips. The shoots are orange–brown, with dense pubescence about long. The leaves are needle-like, long and broad, soft, blunt-tipped, only slightly flattened in cross-section, pale glaucous blue-green above, and with two broad bands of bluish-white stomata below with only a narrow green midrib between the bands; they differ from those of any other species of hemlock in also having stomata on the upper surface, and are arranged spirally all around the shoot.

The cones are small (but much longer than those of any other species of hemlock), pendulous, cylindrical, long and broad when closed, opening to broad, superficially somewhat like a small spruce cone. They have thin, flexible scales long. The immature cones are dark purple (rarely green), maturing red–brown 5 to 7 months after pollination. The seeds are red–brown, long, with a slender, -long pale pink–brown wing.

Taxonomy

There are three taxa, two subspecies and a minor variety: Their ranges differ in California, where western hemlock is restricted to the Coast Ranges and mountain hemlock is found in the Klamath Mountains and Sierra Nevada. Unlike western hemlock, mountain hemlock mostly grows at high altitudes except in the far north, from sea level to in Alaska, in the Cascades in Oregon, and in the Sierra Nevada.

Best development of mountain hemlock is on loose, coarse-textured, well-drained soils with adequate moisture, and in British Columbia, on thick and very acidic organic matter and decayed wood. Adequate soil moisture appears to be especially important in California and Montana, where summer drought is most pronounced. In these locations, mountain hemlock typically grows in isolated populations in north-facing glens and cirque basins where snow collects and may remain well into summer. Mountain hemlock is considered a minor climax species on most of its habitats, and pioneers on glacial moraines in British Columbia and Alaska. In many communities of the mountain hemlock forest in British Columbia and Washington and northern Oregon, Pacific silver fir is a major climax species.

References

Further reading

  • Mountain Hemlock from "An Appreciation," by Harriet Monroe, Sierra Club Bulletin, 1916. California Legacy Project.
  • Interactive Distribution Map of Tsuga mertensiana