Syn: Abies heterophylla Rafinesque 1832 (Taylor 1993). At the upper elevational limits of its distribution and under stressful conditions, T. heterophylla tends to resemble T. mertensiana, e.g., leaves are less strictly 2-ranked and stomatal bands on the abaxial leaf surfaces are less conspicuous than at lower elevations (Taylor 1993).
Trees to 60 (75) m; trunk to 200 cm dbh; crown narrowly conic. Bark gray-brown, scaly and moderately fissured. Twigs yellow-brown, finely pubescent. Buds ovoid, gray-brown, 2.5-3.5 mm. 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. Ovulate cones ovoid, (1)1.5-2.5(3) × 1-2.5 cm; scales ovate, 8-15 × 6-10 mm, apex round to pointed. 2n=24 (Taylor 1993). Detailed information on pollen, including photomicrography, can be found in Davis (1999).
Canada: Alberta and British Columbia; USA: Alaska, Montana, Idaho, Washington, Oregon and California at 0-1830 m elevation in coastal to midmontane forests. It is a dominant species over much of its range (Taylor 1993). Hardy to Zone 6 (cold hardiness limit between -23.2°C and -17.8°C) (Bannister and Neuner 2001). See also Thompson et al. (1999).
Tsuga heterophylla is a dominant forest tree within its range. In low elevation coastal forests of Washington, Oregon and southern British Columbia, it shares dominance with Picea sitchensis, while in slightly drier environments of the western Cascade Range its primary codominant is Pseudotsuga menziesii (Franklin and Dyrness 1988). Its primacy in the western coastal forests has several causes. Conifers in general do well in the wet Pacific Northwest because their evergreen habit allows them to maintain positive net rates of photosynthesis during the long, wet winters; because many of them are very shade-tolerant, able to survive in a relatively dark understory until death of overstory trees allows them to continue their growth; and because the infrequency of stand-destroying disturbance is readily exploited by these long-lived trees (Waring and Franklin 1979). Tsuga heterophylla does particularly well because the most common disturbance in this ecosystem is windthrow, which may destroy single trees or, more rarely, may wipe out forests across a landscape. Tsuga heterophylla seedlings establish preferentially on organic substrates, especially down wood. This gives hemlock a significant competitive advantage over most other conifers in the area. Notably, Picea sitchensis shares this ability to readily establish on dead wood (Harmon and Franklin 1989).
Height 55 m, dbh 259 cm, crown spread 20 m; at Quisitis Point, Pacific Rim National Park, BC (Stoltmann 1987). Also, height 73 m, dbh 218 cm, crown spread 20 m, in Olympic National Park, WA. Also, height 53 m, dbh 276 cm, crown spread 20 m, in Olympic National Park, WA (American Forests 1996).
This is another one of those species (the others are Picea sitchensis and Pseudotsuga menziesii) where the tallest trees are just trying to keep up with the redwoods. The tallest known, named "Tsunami" (discovered on the day that the 2011 Japanese tsunami made landfall in nearby Crescent City), is 82.83 meters; Mario Vaden discovered it (Taylor 2011). The previous record tallest hemlock was 78.9 m tall, 140 cm dbh, in Prairie Creek Redwoods State Park, CA (R. Van Pelt e-mail 1998.03.18).
M.L. Parker found a specimen from the Caren Range (BC) with 1238 rings (R. Van Pelt e-mail 1998.03.18). The species is capable of establishing in the forest understory and surviving for very long periods (centuries) before entering the forest canopy. This ecological setting characterizes much of the lifespan of the oldest known individuals. Also, the oldest (and largest) specimens tend to be found near the upper elevation limits of the species, within subalpine forests. This may be an artifact of long-term climate change—it is thought that the Pacific Northwest mountains were slightly warmer and drier during the 'Medieval Optimum' of ca. 1100-1250 AD, so optimum growth rates were attainable at higher elevations—or it may reflect reduced susceptibility to disease caused by low annual temperatures and mixed-species stand composition.
As of 1999, more than 20 studies had have been published. About half have dealt with forest ecological problems including primary succession, fire history and tree population dynamics, but there have also been studies of culturally modified trees, the effect of volcanic eruptions, the effect of smelter emissions, wood technology and even forensic uses. Because the species typically grows in a forest interior environment, ring width variation is typically determined primarily by the tree's competitive status. Thus, it is an excellent species for studies of forest structural change during development of late-successional forests. However, because the wood decays rapidly after tree death, most dating studies must rely exclusively on inference from living trees.
The bark has a high tannin content. Native peoples used it as a tanning agent, pigment and cleansing solution. The Quileute, for example, used a hemlock bark extract for tanning hides and soaking spruce-root baskets to make them watertight. Some Coast Salish people used a red dye made from hemlock bark to color mountain goat wool and basket materials, and as a facial cosmetic and hair remover. Various tribes would use the bark to dye fish nets brown, making them invisible to fish. Black and yellow-orange dyes were also derived from the bark. The wood is moderately heavy and fairly easy to carve. It was carved into implements such as spoons, combs and wedges. The Haida made large feast bowls from the wood of bent hemlock trunks. The branches were considered an excellent bedding material, and were also used to collect herring spawn by placing them in estuaries where the herring would use them as a spawning substrate. The pitch and the outer and inner bark were widely used medicinally (Pojar and Mackinnon 1994).
Tsuga heterophylla is the most economically important timber hemlock. The wood is superior to that of other hemlocks for building purposes and it makes excellent pulp for paper production (Taylor 1993). Large portions of the Washington and Oregon Coast Ranges, and the Olympic Peninsula of Washington, are managed primarily for Tsuga heterophylla production. It is also planted for forestry in NW Europe, primarily Britain and Ireland. It is locally naturalised in Britain, with dense regeneration in several forests. It is also used as an ornamental (USDA hardiness zones 5, inland populations, to 7, coastal populations).
The species can be readily found in its range from BC to CA. Outstanding examples can be found throughout the lowlands of Olympic National Park in WA and at Pacific Rim National Park on Vancouver Island, BC.
Western hemlock is the state tree of Washington (Taylor 1993).
Davis, Owen K. 1999. Pollen Grain Morphology Hemlock. URL=http://geo.arizona.edu/palynology/pid00012.html, accessed 2000.02.06.
Harmon, M.E. and Jerry F. Franklin. 1989. Tree seedlings on logs in Picea-Tsuga forests of Oregon and Washington. Ecology 70(1):48-59.
Taylor, Michael. 2011. Winter 2011 Latest News. www.landmarktrees.net, accessed 2011.03.18.
This page edited with the help of M.P. Frankis, 1999.02.
British Columbia Ministry of Forests. 1996. Tree Book: Learning to Recognize Trees of British Columbia. URL=http://www.for.gov.bc.ca/PAB/EDUCATE/TREEBOOK/tree-toc.htm, accessed 6-Feb-2000.
Farjon (1990) provides a detailed account, with illustrations.
Last Modified 2012-11-27