Tsuga heterophylla

Western hemlock

Pinaceae

The Basics

Taxonomy: Kingdom - Plantae (plants). Subkingdom - Tracheobionta (vascular plants). Superdivision - Spermatophyta (seed plants). Division - Coniferophyta (conifers). Class - Pinopsida. Order - Pinales. Family - Pinaceae (pine). Genus - Tsuga Carrière. Species -Tsuga heterophylla (Raf.) Sarg.

Ecology: Western hemlock, also called Pacific hemlock and west coast hemlock, thrives in humid areas of the Pacific coast and northern Rocky Mountains. Western hemlock forests are among the most productive forests in the world. The successional role of western hemlock is clear; it is a climax species either alone or in combination with its shade-tolerant associates. Western hemlock is rated to be very tolerant of shade. Western hemlock responds well to release after a long period of suppression. Advance regeneration 50 to 60 years old commonly develops into a vigorous, physiologically young-growth stand after complete removal of the overstory.

Identification

Trees to 50m; trunk to 2m diam.; crown narrowly conic. Bark gray-brown, scaly and moderately fissured. Twigs yellow-brown, finely pubescent. Buds ovoid, gray-brown, 2.5--3.5mm. Leaves (5--)10--20(--30)mm, mostly appearing 2-ranked, flattened; abaxial surface glaucous with 2 broad, conspicuous stomatal bands, adaxial surface shiny green (yellow-green); margins minutely dentate. Seed cones ovoid, (1--)1.5--2.5(--3) 1--2.5cm; scales ovate, 8--15 6--10mm, apex round to pointed.

Threats

In the WFDP: Competition for water and soil resources, especially during drought years, is a likely cause of mortality for western hemlock in the WFDP. Pathogens and pests affecting western hemlock in the WFDP include: laminated root rot (Phellinus weirii), Armillaria root rot (aka Honey fungus; Armillaria spp.), brown felt blight (aka Snow mold; Herpotrichia spp.), and fir engraver beetle (Scolytus ventralis).

Fire effects: Western hemlock has a low degree of fire resistance. It has thin bark, shallow roots, highly flammable foliage, and a low-branching habit which make it very susceptible to fire. Western hemlock tends to form dense stands and its branches are often lichen covered, which further increases its susceptibility to fire damage. Even light ground fires are damaging.

Reproduction

Flowering and Fruiting - Western hemlock is monoecious; male and female strobili develop from separate buds of the previous year. Female strobili occupy terminal positions on lateral shoots, whereas the male strobili cluster around the base of the needles. Cone scales of western hemlock open and close in response to dry and wet atmospheric conditions. Under wet conditions, seed may be retained in the cones until spring.

Seed Production and Dissemination - In the Rocky Mountains, western hemlock consistently produces more seed than its associates. Western hemlock seed falls at a rate of 80 cm per second. Released in a strong wind, it can be blown more than 1.6 km. In a wind of 20 km per hour, seed released at a height of 61 m traveled up to 1160 m; most fell within 610 m of the point of release.

Seedling Development - Provided adequate moisture is available, seed germination and germinant survival are excellent on a wide range of materials. Seeds even germinate within cones still attached to a tree. Initial growth is slow; 2-year-old seedlings are commonly less than 20 cm tall. Once established, seedlings in full light may have an average growth rate of 60 cm or more annually.

Vegetative Reproduction - Western hemlock can be propagated by layering and from cuttings. Seedlings that die back to the soil surface commonly sprout from buds near the root collar. Sprouting does not occur from the roots or the base of larger saplings.

Species Distribution

Citation

USDA Plants Database
USDA, NRCS. 2016. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.

USFS Plant Database
Habeck, R. J. 1992. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

Silvics of North America
Burns, R.M., and B.H. Honkala. 1990. Silvics of North America (Volume 1: Conifers, Volume 2: Hardwoods). USDA Forest Service Agricultural Handbook 654.

Flora of North America
Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 19+ vols. New York and Oxford.

The Jepson Herbarium
The Jepson Manual: Vascular Plants of California. B.G. Baldwin, D.H. Goldman, D.J. Keil, R. Patterson, T.J. Rosatti, and D.H. Wilken [editors]. 2012. 2nd edition, thoroughly revised and expanded. University of California Press, Berkeley, CA. $131.95, hardcover; 1600 pages. ISBN-13: 978-0520253124.

Burke Museum of Natural History and Culture
Burke Museum. 2016 [Online]. University of Washington.