Mountain Hemlock

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Mountain Hemlock

Mountain Hemlock in the
Emigrant Wilderness Area, California

Conservation status

Least concern

Scientific classification

Kingdom:	Plantae
Division:	Pinophyta
Class:	Pinopsida
Order:	Pinales
Family:	Pinaceae
Genus:	Tsuga
Species:	T. mertensiana

Binomial name

Tsuga mertensiana
(Bong.) Carr.

The Mountain Hemlock (Tsuga mertensiana) 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.

Mountain Hemlock cones; subsp. mertensiana

Mountain Hemlock foliage; subsp. grandicona

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 Tulare County, California. The range matches that of 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.

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, 10-20 mm long, soft, blunt-tipped, slightly flattened in cross-section, pale glaucous blue-green above, and with two broad bands of 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.

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.

There are three taxa, two subspecies and one variety:

Tsuga mertensiana subsp. mertensiana var. mertensiana Northern Mountain Hemlock.

Central Oregon northwards. The type of the species. Cones smaller, 30-60 mm long, 12-25 mm broad (open).

Tsuga mertensiana subsp. mertensiana var. jeffreyi Jeffrey's Mountain Hemlock.

Central Oregon northwards, mixed with the type; rare. Leaves greener above, less glaucous; 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 California Mountain Hemlock.

Central Oregon southwards. Leaves very strongly glaucous. Cones larger, 45-80 mm long, 20-35 mm broad (open).

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 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.

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.