Tsuga mertensiana

Tsuga mertensiana

Tsuga mertensiana (Mountain Hemlock) 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 Tulare County, California.

Description

It is a large evergreen coniferous tree growing to 20–40 m tall, exceptionally 59 m, and with a trunk diameter of up to 2 m. The bark is thin and square-cracked or furrowed, and 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 1 mm long. The leaves are needle-like, 7–25 mm long and 1–1.5 mm 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 round the shoot. The cones are small, but much longer than those of any other species of hemlock, pendulous, cylindrical, 30–80 mm long and 8–10 mm broad when closed, opening to 12–35 mm broad, superficially somewhat like a small spruce cone. They have thin, flexible scales 8–18 mm long. The immature cones are dark purple (rarely green), maturing red-brown 5–7 months after pollination. The seeds are red-brown, 2–3 mm long, with a slender, 7–12 mm long pale pink-brown wing.

Distribution and taxonomy

The range matches that of Tsuga heterophylla (Western Hemlock) fairly closely, likewise mostly less than 100 km from the Pacific Ocean apart from a similar inland population in the Rocky Mountains in southeast British Columbia, northern Idaho and western Montana. Their ranges however differ in California, where Western Hemlock is restricted to the Coast Ranges, while Mountain Hemlock is found in the Klamath Mountains and Sierra Nevada. Unlike Western Hemlock, it mostly grows at high altitudes except in the far north, from sea level to 1000 m in Alaska, 1600–2300 m in the Cascades in Oregon, and 2500–3050 m in the Sierra Nevada.

There are three taxa, two subspecies and a minor variety:

  • Tsuga mertensiana subsp. mertensiana. Northern Mountain Hemlock. Central Oregon northwards. Cones smaller, 30–60 mm long, 12–25 mm broad when open, with 50–80 scales.
    • Tsuga mertensiana subsp. mertensiana var. mertensiana. Northern Mountain Hemlock. Leaves gray-green on both sides.
    • Tsuga mertensiana subsp. mertensiana var. jeffreyi (Henry) Schneider. Jeffrey's Mountain Hemlock. Mixed with var. mertensiana; rare. Leaves greener, less glaucous above, paler below; cones indistinguishable from the type. At one time it was thought to be a hybrid with Western Hemlock, but there is no verified evidence for this.
  • Tsuga mertensiana subsp. grandicona Farjon. California Mountain Hemlock; syn. T. hookeriana (A.Murray) Carrière, T. crassifolia Flous. Central Oregon southwards. Leaves very strongly glaucous. Cones larger, 45–80 mm long, 20–35 mm broad when open, with 40–60 scales.

Ecology

Unlike other hemlocks, it is not very shade tolerant, with young plants typically growing up in open conditions in full light. It is only successful in moist sites with very heavy winter snow to protect the soil from freezing and to provide a steady source of meltwater through the spring and summer, typically growing best on north-facing slopes where snow lasts longer. It is very well adapted to cope with heavy snow and ice loads, with tough branches, and the drooping branchlets shedding snow readily.

Cultivation and uses

Outside of its native range, Mountain Hemlock is grown as an ornamental tree in gardens, particularly in northern Great Britain and Scandinavia, where it is appreciated for its blue-green color and tolerance of severe weather. Cultivation is limited by the very slow growth of young plants and its susceptibility to air pollution. A few cultivars have been selected, mainly for intensely glaucous foliage, such as 'Blue Star' and 'Glauca'.

References

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