  Tsuga mertensiana growing by a lake in Washington [C.J. Earle]. | Tsuga mertensiana (Bongard) Carrière 1867Common NamesMountain hemlock, black hemlock, hemlock spruce (5), pruche de Patton (1).Taxonomic notesThe sole member of Tsuga subgenus Hesperopeuce (Lemmon) Ueno, told from subgenus Tsuga by the long cylindric cones with pubescent scales, and the less flattened leaves with stomata on both sides.Syn: Pinus mertensiana Bongard 1832; Hesperopeuce mertensiana (Bongard) Rydberg; A. pattoniana Jeffrey ex Balfour, Tsuga pattoniana (Balfour) Sénéclause, Pinus pattoniana (Balfour) Parlatore, H. pattoniana (Balfour) Lemmon (1); see subsp. grandicona for further synonymy. Two subspecies, the type and T. mertensiana subsp. grandicona Farjon (7, 8). One poorly-differentiated variety, T. mertensiana subsp. mertensiana var. jeffreyi (Henry) Schneider [syn. Tsuga pattoniana var. jeffreyi Henry]. The variety has also been considered a hybrid with T. heterophylla, as T. × jeffreyi (Henry) Henry, but the cones show no sign of influence of that species, and hybridisation has never been verified experimentally (1, 8, 9). DescriptionTrees to 40 m tall and 150 cm dbh; crown conic. Bark charcoal gray to reddish brown, scaly and deeply fissured. Twigs yellow-brown, densely pubescent. Buds oblong, 3-4 mm. Needles 10-25(30) mm, mostly spreading in all directions from twigs, curved toward twig apex, thickened centrally along midline, somewhat rounded or 4-angled in cross section, both surfaces glaucous, with ± inconspicuous stomatal bands; margins entire. Seed cones purple ripening mid to dark grey-brown, oblong-cylindric, 3-6 × 1.5-2.5 cm (open); scales pubescent, broadly fan-shaped, 8-l1 × 8-11 mm, apex rounded to pointed. 2n=24 (1, 10).Var. jeffreyi differs in sparser leaves, greener with fewer stomata above, possibly an adaptation to lower altitude habitats, but it is as yet poorly researched. In the past, the greener leaves have been interpreted as being due to hybridisation with T. heterophylla, but the cones and growth habit are indistinguishable from typical T. mertensiana (10). RangeCanada: British Columbia; USA: Alaska, Washington, Idaho, Montana, Oregon, California and Nevada at 0-3050 m. The elevational range of the species is quite variable with latitude, ranging from 0-1070 m in Alaska, to 1600-2300 m in southern Oregon (where subsp. mertensiana grades into subsp. grandicona), to 2750-3050 m in the southern Sierra Nevada. Throughout its range, mean annual temperature is approximately constant at 3-4°C and precipitation, though highly variable (970 to 3020 mm), does not show a latitudinal gradient. However, the portion of precipitation falling as snow decreases greatly with latitude, from 88 percent in California to 14 percent in Alaska, reflecting a translation from Mediterreanean to subarctic maritime climates across 25 degrees of latitude. Its habitat is relatively coastal; it does not occur in the rainshadow of the Coast-Cascade-Sierra Nevada mountain chain except locally in the Rocky Mountains where coastal airmasses can penetrate and produce high winter snowfall. It has been suggested that the species affinity for areas with persistent winter snowpacks is because it cannot tolerate frozen soils. Throughout most of its range, T. mertensiana defines the upper portion of the subalpine forest and is conventionally divided into two elevational zones: the forest subzone of continuous closed-canopy forest, and the parkland subzone of tree clumps separated by fellfields, wet meadows or low subalpine shrubs. Within the parkland subzone, the discontinuous forest canopy typically reflects reduced seedling establishment success associated with a deep and persistent winter snowpack. At the highest elevations, it grows as a timberline tree in krummholz form (1, 2, 6).Var. jeffreyi has been reported from a few scattered sites within the range of the type (8). Big TreeThe official 'Big Tree' is a specimen of subsp. grandicona 34 m tall with a dbh of 224 cm and crown spread of 13 m, in Alpine County, CA (3). The largest known specimen of subsp. mertensiana is 59.1 m tall, with a dbh of 127 cm, in Olympic National Park, WA (4).OldestDendrochronologyAs of Feb-99, there are about 25 published studies dating as far back as 1923. The great majority of these studies have examined climate or some factor closely related to climate, such as timberline fluctuation or glacier expansion. The utility of the species for climate studies is due to its occurrence at the alpine timberline and its strong interaction with snowpack accumulation. For instance, it is by now generally accepted that widespread invasion of subalpine meadows in the Pacific Northwest happened during the 1930s and 1940s in response to a prolonged episode of reduced winter snowpacks (11). Graumlich and Brubaker (12) looked at the relationship between climate and ring width for some stands in the Cascade Range of WA, and I did some (unpublished) exploratory studies focusing on population age structures and competitive interactions in subalpine parklands of the Cascade Range of BC, WA and OR.EthnobotanyThe wood of Tsuga mertensiana is somewhat inferior to that of T. heterophylla as timber and as pulp, a fact that has little retarded extensive logging of subalpine forests. It is adaptable to a wide variety of climatic conditions and is widely used as an ornamental (6) (USDA hardiness zone 5).ObservationsBecause this species grows to the alpine timberline, its most picturesque qualities are displayed in high mountain areas. Notably good sites are the parklands, where the landscape is covered by a mosaic of meadow and tree clumps. Examples include the high country of Garibaldi Provincial Park in BC, the Seven Lakes Basin of Olympic National Park in WA, and Jefferson Park in the Mount Jefferson Wilderness of OR. It can also be found growing nearly everywhere in the high alpine lake country of the Sierra Nevada, including areas in Yosemite, Kings Canyon and Sequoia National Parks.RemarksM.Van Campo-Duplan and H.Gaussen (1948) postulated that this taxon originated by hybridization between Picea and Tsuga. This hypothesis has recently been disproved by DNA studies; X-R Wang (in press, to be cited when details become available).Citations(1) Ronald J. Taylor at the Flora of North America web site.(2) Arno & Gyer 1973. (3) American Forests 1996. (4) E-mail communication from Robert Van Pelt, who measured this tree; 18-Mar-1998. (5) Peattie 1950. (6) Burns & Honkala 1990. (7) A. Farjon. 1988. Taxonomic notes on Pinaceae 1. Proc. Konin. Ned. Akad. Wetensch. ser. C Bot., 91: 31-42. (8) Farjon 1990. (9) R.J. Taylor 1972. The relationship and origin of Tsuga heterophylla and Tsuga mertensiana based on phytochemical and morphological interpretations. American Journal of Botany 59: 149-157. (10) M.P. Frankis, personal communication, 6-Feb-1999. (11) Jerry F. Franklin. 1988. Pacific Northwest forests. P. 103-130 in M.G. Barbour and W.D. Billings, eds., North American terrestrial vegetation. Cambridge: Cambridge University Press. (12) Lisa J. Graumlich and Linda B. Brubaker. 1986. Reconstruction of annual temperature (1590-1979) for Longmire, Washington, derived from tree rings. Quaternary Research 25:223-234. See also: This page edited with the help of M.P. Frankis, Feb-1999. |
[Tsuga] [Pinaceae] [home] This page is from the Gymnosperm Database |