Joseph E. Means
Mountain hemlock (Tsuga mertensiana) is usually found on cold, snowy subalpine sites where it grows slowly, sometimes attaining more than 800 years in age. Arborescent individuals that have narrowly conical crowns until old age (300 to 400 years) and shrubby krummholz on cold, windy sites near timberline add beauty to mountain landscapes. Taylor and Taylor (76) thoroughly describe its form. Uses of its moderately strong, light-colored wood include small-dimension lumber and pulp.
Ranges and Cascade Range in Oregon; the Cascade Range and Olympic
Mountains in Washington; the northern Rocky Mountains in Idaho and
western Montana; the Insular, Coast, and Columbia mountains in British
Columbia; and in southeast and south-central Alaska [4,8,32,46]. In
California it is also locally abundant in the Klamath Mountains. The
extreme southern limit of mountain hemlock is near Silliman Lake in
Tulare County, California .
Regional Distribution in the Western United States
This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):
1 Northern Pacific Border
2 Cascade Mountains
4 Sierra Mountains
8 Northern Rocky Mountains
Occurrence in North America
- The native range of mountain hemlock.
Mountain hemlock is a native, slow-growing, coniferous, evergreen tree
usually 75 to 100 feet (23-30 m) tall and 2.5 to 3.5 feet (0.8-1 m) in
diameter . However, it takes on a variety of growth forms to adapt
to subalpine conditions. Below 4,000 feet (1219 m) in the Coast Ranges,
it grows in dense stands reaching diameters of 3 to 4 feet (0.8-0.9 m)
and heights up to 150 feet (46 m) . On exposed ridges at high
elevations, it often grows as a low-spreading shrub or small tree
In the open, mountain hemlock develops a strongly tapered trunk bearing
slender branches almost to the ground; the branches usually droop and
often have ascending tips. The outline of the crown is narrowly conical
beneath a slender drooping leader. Crowns of old trees are often bent
or twisted. In dense stands, the crown covers only the upper half or
less of the tree, and the trunk below develops with a more gradual
taper, and becomes virtually clear of branches . The twigs are
mostly short and slender. The needles are crowded on all sides of short
twigs and curved upward .
The bark is thick and deeply furrowed into scaly plates on old trees
[44,72]. The bark is early broken and rough on young trees . The
root system is shallow and widespreading [38,49,59].
Sierra Nevada Forests
The Limestone salamander is a highly localized endemic of the Sierra Nevada forests foothills conifned to a limited reach of the Merced River. The Sierra Nevada forests are the forested areas of the Sierra Nevada Mountains, which run northwest to southwest and are approximately 650 kilometers long and 80 km wide. The range achieves its greatest height towards the south, with a number of peaks reaching heights of over 4000 meters. Several large river valleys dissect the western slope with dramatic canyons. The eastern escarpment is much steeper than the western slope, in general.
The Sierra Nevada forests ecoregion harbors one of the most diverse temperate conifer forests on Earth displaying an extraordinary range of habitat types and supporting many unusual species. Fifty percent of California's estimated 7000 species of vascular plants occur in the Sierra Nevada, with 400 Sierra endemics and 200 rare species. The southern section has the highest concentration of species and rare and endemic species, but pockets of rare plants occur throughout the range.
Sierra Nevada amphibian endemics are the Yosemite toad, Mount Lyell salamander (Hydromantes platycephalus), the Vulnerable Limestone salamander (Hydromantes brunus), Kern salamander and the Endangered Inyo Mountains salamander (Batrachoseps campi). The non endemic amphibians are: the Endangered Southern mountain yellow-legged frog (Rana muscosa); the Near Threatened Cascades frog (Rana cascadae); Northern red-legged frog (Rana aurora); Pacific chorus frog (Pseudacris regilia); Foothill Yellow-legged frog (Rana boylii); Long-toed salamander (Ambystoma macrodactylum); and the Monterey ensatina (Ensatina eschscholtzii).
A considerable number of mammalian taxa are found in the ecoregion, including the Long-eared chipmunk, Alpine chipmunk, Western heather vole, Walker Pass pocket mouse, and the Yellow-eared pocket-mouse. A diverse vertebrate predator assemblage once occurred in the ecoregion including Grizzly bear (Ursus arctos), Black bear (Ursus americanus), Coyote (Canis latrans), Mountain lion (Puma concolor), Ringtail (Bassariscus astutus), Fisher (Martes pennanti), Pine marten (Martes americana) and Wolverine (Gulo gulo).
There are a small number of reptilian taxa present in the Sierra Nevada forests: sagebrush lizard (Sceloporus graciosus); Northern alligator lizard (Elgaria coerulea); Southern alligator lizard (Elgaria multicarinata); Sharp-tailed snake (Contia tenuis); California mountain kingsnake (Molothrus ater); Common garter snake (Thamnophis sirtalis); Couch's garter snake (Thamnophis couchii); Western gopher snake (Pituophis catenifer); Longnose snake (Rhinocheilus lecontei); and the Common kingsnake (Lampropeltis getula).
A number of bird species are found in the ecoreion including high level predators that include several large owls, hawks and eagles. Other representative avifauna species present are the Blue-headed vireo (Vireo solitarius); Brown-headed cowbird (Molothrus ater); and the Near Threatened Cassin's finch (Carpodacus cassinii).
Mountain hemlock is commonly found on cold, snowy subalpine or boreal
sites where it grows slowly, sometimes reaching more than 800 years of
age. Though pure stands are less common than mixed stands, extensive
pure stands of mountain hemlock do occur in Alaska and in the central
high Cascades of Oregon . In the Siskiyous, mountain hemlock is
generally confined to cool, north-facing, cirquelike topography. It
does not form extensive stands like those in the Cascades because
suitable habitat is found only on the highest peaks. In the Siskiyous,
the lower limit of mountain hemlock is governed by high temperatures and
competition with Shasta red fir (Abies magnifica shastensis) .
Mountain hemlock in western Montana is generally confined to the moist,
upper slopes of the Bitterroot Mountains .
Elevational range: The elevational range of mountain hemlock has been
recorded as follows [8,49]:
Alaska - 0 to 3,500 feet (0-1,067 m)
southern British Columbia - 1,000 to 3,000 feet (300-900 m)
northern Washington - 4,200 to 5,600 feet (1,300-1,700 m)
Rocky Mountains - 5,100 to 6,900 (1,550-2,100 m)
southern Oregon - 5,200 to 7,500 (1,600- 2,300 m)
Siskiyous - 4,000 to 7,000 (1,220 - 2,134 m)
northern Sierra Nevada - 7,900 to 10,000 (2,400-3,050 m)
southern Sierra Nevada - 9,050 to 10,000 (2,750-3,500 m)
Climate: Mountain hemlock generally occurs on sites with mild to cold
winters; short, warm to cool growing seasons; and moderate to high
precipitation . Average annual snowfall ranges from about 32 to 50
feet (10-15 m) .
Soils: Mountain hemlock grows on soils derived from a wide variety of
parent materials, however, it is rare and stunted on soils derived from
calcareous parent materials in the Selkirk Mountains of British
Columbia. It is found on organic soils in the northern portion of its
range more often than in the southern portion. In Alaska it is found on
organic soils bordering muskegs where it may be a major stand component.
Best development of mountain hemlock is on loose, coarse-textured,
well-drained soils with adequate moisture . In British Columbia
best growth is on thick, very acidic organic matter and decayed wood.
In the Siskiyous, soils in the mountain hemlock series are loam to silt
loam and average 40 inches (100 cm) in depth . Adequate soil
moisture appears to be especially important in California and Montana.
Soils are typically acidic with a pH ranging from 3.4 to 5.0 . In
the Coastal Mountains, mountain hemlock can grow on the rockiest soils,
even including recent lava flows, if moisture is adequate . The
nutritional requirements of mountain hemlock are low .
Plant associates: In mixed stands, mountain hemlock usually coexists
with subalpine fir (Abies lasiocarpa), Pacific silver fir, or
Alaska-cedar (Chamaecyparis nootkatensis). In Montana, subalpine fir
and Engelmann spruce (Picea engelmannii) are nearly constant associates
of mountain hemlock . Common understory associates with mountain
hemlock are as follows: beargrass, big huckleberry, grouse
whortleberry, rustyleaf menziesia (Menziesia ferruginea), Cascades
azalea (Rhododendron albiflorum), Alaska huckleberry (V. alaskaense),
ovalleaf huckleberry (V. ovalifolium), long-stoloned sedge (Carex inops
ssp. inops), mertens cassiope (Cassiope mertensiana), copperbush,
mountain heather, deer cabbage, marsh marigold (Caltha biflora), and
skunk cabbage (Lysichitum americanium) [1,19,33,68].
Key Plant Community Associations
Mountain hemlock commonly occurs as a dominant or codominant in
high-elevation alpine or subalpine forests. In western Washington and
Oregon, the mountain hemlock zone is the highest forested zone .
Mountain hemlock is often codominant with Pacific silver fir (Abies
amabilis) [1,21]. One of the most widespread mountain hemlock
communities is the mountain hemlock-Pacific silver fir/big huckleberry
(Vaccinium membranaceum) type found in British Columbia and the Oregon
and Washington Cascades. In the Rocky Mountains, the mountain
hemlock/beargrass (Xerophyllum tenax) habitat type is generally found on
south slopes and is characterized by a high cover of beargrass with big
huckleberry and grouse whortleberry (V. scoparium) as common associates.
A similar Pacific silver fir-mountain hemlock/beargrass association is
found in Oregon . Published classifications identifying mountain
hemlock as a dominant or codominant are as follows:
Forest types of the North Cascades National Park Service complex .
Preliminary plant associations of the Southern Oregon Cascade Mountain
Preliminary plant associations of the Siskiyou Mountain Province .
Plant association and management guide for the Pacific silver fir zone .
Forest habitat types of northern Idaho: A second approximation .
Classification of montane forest community types in the Cedar River
drainage of western Washington, U.S.A. .
Preliminary forest plant association management guide. Ketchikan area,
Tongass National Forest .
Subalpine plant communities of the western North Cascades, Washington .
Alpine and high subalpine plant communities of the North Cascades Range,
Washington and British Columbia .
Fire ecology of western Montana forest habitat types .
Forest vegetation of the montane and subalpine zones, Olympic Mountains,
Natural vegetation of Oregon and Washington .
The forest communities of Mount Rainier National Park .
Plant associations of south Chiloquin and Klamath ranger districts--
Winema National forest .
Vegetation and environment in old growth forests of northern southeast
Alaska: a plant association classification .
Forest habitat types of Montana .
Preliminary classification of forest vegetation of the Kenai Peninsula,
Preliminary forest plant associations of the Stikine area, Tongass
National Forest .
This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):
FRES11 Spruce - fir
FRES22 Western white pine
FRES23 Fir - spruce
FRES24 Hemlock - Sitka spruce
FRES26 Lodgepole pine
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
205 Mountain hemlock
206 Engelmann spruce - subalpine fir
207 Red fir
208 Whitebark pine
210 Interior Douglas-fir
212 Western larch
215 Western white pine
218 Lodgepole pine
256 California mixed subalpine
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
K001 Spruce - cedar - hemlock forest
K002 Cedar - hemlock - Douglas-fir forest
K003 Silver fir - Douglas-fir forest
K004 Fir - hemlock forest
K008 Lodgepole pine - subalpine forest
K013 Cedar - hemlock - pine forest
Soils and Topography
Mountain hemlock is reported on organic soils (Histosols) in the northern portion of its range (9,82) more often than in the southern portion (62,64). In Alaska it is found down to sea level on noncommercial forest land on organic soils bordering muskegs where it may be a major stand component (35). A stunted or prostrate form is often found on these muskegs. It also grows below its usual altitudinal range in British Columbia on poorly drained sites where other species offer little competition (11).
Best development of mountain hemlock is on loose, coarse-textured, well-drained soils with adequate moisture (9,11,62), and in British Columbia (9), on thick and very acidic organic matter and decayed wood. Adequate soil moisture appears to be especially important in California (11,62) and Montana (33)- portions of its range where summer drought is most pronounced. Mature soils typically found under mountain hemlock stands in Alaska, British Columbia, and Washington are Cryaquods and Cryorthods of the order Spodosols. These soils typically have a 13- to 28-cm (5- to 11-in) forest floor with a root mor or mycelial root mor humus layer in British Columbia (9), and a 5- to 10-cm (2- to 4-in) forest floor with a mor or duff mull humus layer in the Washington Cascades (20). In the central and southern Oregon Cascades and in northeastern Washington and northern Idaho, mature soils are generally weakly developed Haplorthods with densely matted felty mor humus layers 2 to 5 cm (1 to 2 in) thick (12,20,90). Mountain hemlock also commonly grows on immature soils (Entisols and Inceptisols). For example, it grows on Andepts (soils derived from volcanic ash) in the Cascade Range in Oregon (38,42). Forest floors and mineral soil surfaces of pumice and ash soils supporting mountain hemlock in Oregon show moderate resistance to wetting when dry (42).
Stands dominated by mountain hemlock typically have very acidic forest floors (pH 3.4 to 5.0, rarely 6.0) and mineral soils (pH 4.2 to 6.2) with low base saturation (9 to 18, rarely 37 percent) in British Columbia (9), Washington, Oregon (77,90), and northern Idaho (12). Mountain hemlock accumulates aluminum in its foliage and fine roots, so it may increase the acidity and speed up podzolization of these soils (85). Total nitrogen in the forest floor (0.4 to 1.13 percent) and mineral soil (0.05 to 0.4 percent) in British Columbia (9), Washington, and Oregon (51,77,90) are comparable to those of other coniferous forests in the region. Levels of available nitrogen in an old-growth stand in Oregon, as indexed by 7-day anaerobic 40° C (104° F) incubations, are extremely low in the mineral soil (1.7 to -2.3 µg N/g) and 100 times higher but still low in the forest floor (45 to 225 µg N/g) relative to levels for lower elevation stands in Oregon and Washington (51). A study of forests on an altitudinal gradient in western Oregon indicates that, as for available nitrogen, a high proportion of soil calcium (98 percent), organic matter (50 percent), and total nitrogen (34 percent) is in the forest floor relative to most lower elevation forest types (77). Because of this, nutrients in the forest floor are very important to the productivity of these forests. These edaphic differences are caused by slower (measured) decomposition rates caused in large part by lower temperatures and, on some sites, by the youth and infertility of the volcanic ash parent material.
Mountain hemlock will grow on most landforms, but individuals typically develop best in mixed stands of the forest subzone on sheltered slopes or in draws. From southern British Columbia south, the tree grows better on northerly exposures (11). The preference for relatively moist, cool sites evidently becomes a necessity as the climate becomes more continental in western Montana (33) and more mediterranean in the central Sierra Nevada (62) at these extremes of its range. In these locations, mountain hemlock typically grows in isolated populations in north-facing glens and cirque basins where snow collects and may remain well into summer.
Limited data on stomatal behavior from Carson Pass, CA, indicate that mountain hemlock is adapted to sites with long-lasting snowpacks. In the spring, mountain hemlocks emerging through 2 to 4 m (7 to 13 ft) of snow were transpiring and, presumably, photosynthesizing (73), whereas nearby whitebark pines (Pinus albicaulis) did not transpire until the soil beneath them was free of snow. Water uptake by seedlings in a greenhouse decreases to near zero at soil water potential of about -2 MPa (versus about -3 MPa for Douglas-fir) because of greater uptake resistance (5). Such decreases suggest that mountain hemlock is less well adapted to droughty sites or sites with high evaporative demand.
Table 1- Climatological data from 14 weather stations within the range of mountain hemlock Temperature Precipitation Location and number of stations
Annual June to August Portion¹ as snowfall °C mm mm pct Alaska², 3 4 -7 13 1681 346 14 British Columbia, 4 3 -3 11 3021 361 29 Washington, 2 4 -3 12 2728 270 51 Northern Idaho, 1 3 -7 15 971 124 65 Oregon, 1 4 -4 13 1643 99 81 California, 3 3 -4 13 1048 51 88 °F in in pct Alaska² (80) 39 20 56 62 13 14 British Columbia (9,45) 38 26 52 104 14 29 Washington (20) 39 27 53 107 11 51 Northern Idaho (79) 37 19 59 38 5 65 Oregon (20) 39 25 56 65 4 81 California (64,77) 38 24 56 39 2 88 ¹Estimated snowfall by assuming 10 cm (4 in) of snow is equivalent to 1 cm (0.4 in) of rain, for all locations but those in British Columbia.
²Stations in Alaska are near sea level. Mountain hemlock grows at higher elevations where temperature and precipitation levels are likely to vary from those recorded. The high snowfall results in snowpacks with maximum depths that range from 245 cm (96 in) in Idaho to 380 cm (150 in) in British Columbia (9). A snowpack may cover the ground for long periods (7 to 10 months in southwestern British Columbia) (9). The relatively short growing season (frost-free period) ranges from 95 to 148 days in southwestern British Columbia (9,45) and from 49 to 63 days in the central Sierra Nevada (64).
Climatic extremes include a temperature range of -29° to 38° C (-20° to 100° F) (11), annual snowfall in excess of 2200 cm (866 in) (66), snowpack up to 750 cm (295 in) (20), and persistence of the snowpack until August or September (9,66). Because there are few weather stations in the range of mountain hemlock, reported extremes are probably often exceeded.
Mountain hemlock grows in an altitudinal band 300 to 1000 m (1,000 to 3,300 ft) wide that increases in altitude from north to south:
Altitude m ft Alaska (83) 0 to 1067 0 to 3,500 Northern British Columbia (46) 300 to 900 1,000 to 3,000 Southern British Columbia (46) 900 to 1800 3,000 to 5,900 Northern Washington (20) 1300 to 1700 4,200 to 5,600 Rocky Mountains (12,33,63) 1550 to 2100 5,100 to 6,900 Southern Oregon (42) 1600 to 2300 5,200 to 7,500 Northern Sierra Nevada (64) 2400 to 3050 7,900 to 10,000 Southern Sierra Nevada (62) 2750 to 3050 9,050 to 10,000 The presence of mountain hemlock in the Rocky Mountains is closely correlated with the eastward penetration of moist maritime air masses (33). On the east side of the Coast Mountains in British Columbia, mountain hemlock is limited to relatively moist sites where snow accumulates early in the fall (46). Krajina (46) proposes that mountain hemlock does not grow on sites with later, thinner snowpacks because it cannot tolerate the frozen soils there.
Throughout most of the range of mountain hemlock local climate differentiates two types of subalpine mountain hemlock forest. A parkland subzone of single trees and small tree clumps (average canopy cover less than 25 percent) extends from treeline or near treeline to the lower forest subzone of relatively continuous forest cover (canopy cover more than 25 percent) (2,9,21,82). Most climatic data (table 1) are from the forest subzone. Detailed microclimatic data (9) and data extrapolated upslope from the weather stations (45) in southwestern British Columbia indicate that the parkland subzone has shorter frost-free and snow-free periods and that maximum snowpack, range of summer temperatures, and actual evapotranspiration are greater than in the forest subzone. Mountain hemlock also grows above treeline in the alpine environment as prostrate krummholz (elfinwood) throughout most of its range (10,20,35).
If climate warms as hypothesized for western North America (24), existing mountain hemlock forests will probably increase in productivity; upper and lower boundaries of the mountain hemlock zone, within which new mountain hemlock forests become established after disturbance, will increase in elevation; and the zone will decrease in area. Near Mount Baker, Washington, ring width of mountain hemlock increases with increasing monthly temperatures in the preceding 12 months, decreasing winter precipitation (37), and decreasing spring snow depth, down to about 1 m (3.3 ft) (27). This implies productivity should increase with predicted temperature increases. Graumlich and others (28) estimated that productivity increased 60 percent in the last century in four high-elevation stands in Washington, three of which contained 48 to 96 percent mountain hemlock. They related this increase most strongly to the increase in growing-season temperature during this period (about 1.5° C or 2.7° F). Thus, further increases in temperature may cause further increases in productivity.
Based on the current elevational distribution of major forest zones in the Oregon Cascades and a mean temperature lapse rate of 4.4° C/100 m (2.4° F/1,000 ft), Franklin and others (24) have hypothesized the effects of two warmer climates. If mean annual temperature increases 2.5° C (4.5° F), the mountain hemlock zone in Oregon may be shifted upwards 570 m (1,900 ft) and decrease in area from 9 to 2 percent; an increase of 5.0° C (9° F) may move it upwards 1140 m (3,700 ft)-above all but the tallest peaks-so it is effectively eliminated.
Habitat & Distribution
Associated Forest Cover
Mountain hemlock usually grows in mixture with other trees, and it has many associates, as is evident from the large number of forest types in which it is found. Though pure stands are less common than mixed stands, there are extensive pure stands of mountain hemlock in Alaska (11) and in the central high Cascades of Oregon (20).
One of the most widespread mountain hemlock communities is the mountain hemlock-Pacific silver fir/big huckleberry (Tsuga mertensiana-Abies amabilis/Vaccinium membranaceum) type found in British Columbia (9) and the Oregon and Washington Cascades (1,20). In British Columbia, the understory is dominated by deciduous ericaceous shrubs: Cascades azalea (Rhododendron albiflorum), Alaska huckleberry (Vaccinium alaskaense), rustyleaf menziesia (Menziesia ferruginea), ovalleaf huckleberry (Vaccinium ovalifolium), and big huckleberry. Also included are strawberryleaf blackberry (Rubus pedatus) and several mosses. Silver fir and Alaska-cedar (Chamaecyparis nootkatensis) are common tree associates in this community in coastal areas, and subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii) are common associates in inland areas (9).
In the Rocky Mountains, the mountain hemlock/beargrass (Xerophyllum tenax) habitat type is generally found on south slopes and is characterized by a high cover of beargrass with big huckleberry and grouse whortleberry (Vaccinium scoparium) as common associates (12,63). Subalpine fir and lodgepole pine (Pinus contorta) are common arborescent associates. A similar Pacific silver fir-mountain hemlock/beargrass association is found in Oregon (20).
The extensive pure or nearly pure mountain hemlock forests in the high Cascades of Oregon are primarily in the mountain hemlock/grouse whortleberry community (38,42,48,72). Except for grouse whortleberry, understory plant cover is generally low, long-stoloned sedge (Carex pensylvanica) being the most commonly mentioned associate.
Mountain hemlock forests in Crater Lake National Park were classified with 89 percent accuracy using Landsat imagery (86), which offers hope for broadscale mapping of this forest type.
Diseases and Parasites
Seedlings of mountain hemlock and associated species recolonize disease-killed areas immediately behind the advancing mortality front. These seedlings are apparently not susceptible to reinfection by P. weiri for 80 to 120 years (89). This may be due to greater vigor caused by higher levels of available nitrogen (up to a 4-fold increase), higher temperatures, and more growing-season moisture in this regrowth zone (7,89).
Other common fungal pests of mountain hemlock include several heart rots (Heterobasidion annosum, Phellinus pini, Fomitopsis pinicola, and Phaeolus schweinitzii) (11,72), of which Indian paint fungus (Echinodontium tinctorum) is perhaps the most common and damaging (42). Several needle diseases and a snow mold (Herpotrichia nigra) also attack mountain hemlock but are not considered serious pests (11).
The mountain hemlock race of hemlock dwarf mistletoe (Arceuthobium tsugense) is a potentially damaging parasite that causes witches' broom, reduction in vigor, and occasionally death (11,50). It is found throughout most of the range of mountain hemlock, but reported infection of mountain hemlock decreases from Washington north (36). Dwarf mistletoe rarely infects mountain hemlock in Alaska, though western hemlock is often infected. Several defoliating insects, bark beetles, and wood-boring insects attack mountain hemlock but do not cause extensive damage (11).
Before effective fire suppression, many mountain hemlock stands south of Alaska probably succumbed to fire, as is indicated by the many young stands (11,20). The species is considered susceptible to fire because it often retains branches almost to the ground, grows in clusters, and, in Oregon, often has well-developed forest floors that dry out in the summer (72).
Wind commonly destroys trees in the coastal strip of British Columbia (45) and Alaska where fire is not important (35). As cutting is increased in mountain hemlock forests, wind damage will probably become a more common cause of mortality of this shallow-rooted species. Periodic snow breakage may remove 2 to 6.5 percent of the foliage mass (29).
After the eruption of Mount St. Helens and deposition of tephra on foliage and soil (18 cm, 7 in), growth slowed but trees did not die (41).
Fire Management Considerations
Fire injury makes mountain hemlock very susceptible to insects and
disease [17,25]. Old-growth mountain hemlock stands 460 years or older
are very susceptible to stand-replacing fires .
In northern Idaho, burning slash produced better stocking of mountain
hemlock natural regeneration compared to leaving slash untreated.
However, manual scarification generally produced better stocking than
did burning. In contrast, slash burning in Oregon increased the time it
took mountain hemlock to reach 60 percent stocking .
Plant Response to Fire
Mountain hemlock is generally slow to regenerate after fire [25,49].
Most burned areas in the mountain hemlock zone on the Olympic Peninsula
do not have adequate stocking for commercial forests even 55 to 88 years
after wildfire . Tree establishment in burned areas is higher
during normal to wet growing seasons .
Tree without adventitious-bud root crown
Secondary colonizer - off-site seed
Mountain hemlock is not well adapted to fire . Fire resistance of
mountain hemlock has been rated as low . Its relatively thick bark
provides some protection, but low-hanging branches, highly flammable
foliage, and a tendency to grow in dense groups make it very susceptible
to fire injury .
Mountain hemlock sites are typically moist with average precipitation
over 50 inches (127 cm), making fire occurrence low (400-800 years)
[7,11,34]. Fuel loading in these sites is often low . In the
Pacific Northwest, the estimated prelogging fire regime in mountain
hemlock forest types is 611 years . Fires in these cool wet forest
types generally occur as infrequent crown fires. When fires do occur in
mountain hemlock forests, they are often severe stand-replacing fires
More info for the terms: climax, series
Obligate climax species
Mountain hemlock is shade tolerant [24,49,4]. It is considered a major
or minor climax species over most of its habitat; however, it is also a
pioneer on glacial morianes in British Columbia and Alaska. Mountain
hemlock is commonly the major climax species in the mountain hemlock
zone south of central Oregon where Pacific silver fir does not occur.
It often succeeds lodgepole pine (pinus contorta) when these species
pioneer on drier sites and tends to replace Engelman spruce .
Mountain hemlock is considered a coclimax species with subalpine fir
where they occur together [17,25]. In the subalpine fir series in the
Lolo National Forest, mountain hemlock and subalpine fir are the only
two trees capable of perpetuating themselves as climax dominants .
Seed production and dissemination: Mountain hemlock begins producing
seed at about age 20. Mature trees 175 to 250 years old produce
moderate to very heavy cone crops at about 3-year intervals in Oregon
and Washington, but crops may be complete failures in other years .
Cones average about 70 to 100 seeds . There are 102,000 to 207,000
seeds per pound . On one study site located at Santiam Pass,
Oregon, the largest number of cones counted was 1,700 on a 20-inch (51
cm) mountain hemlock . Over 1,000 cones per tree were counted on
many other trees during the 5 year study period. Seed production is
better during normal to wet growing seasons than during dry growing
Mountain hemlock's winged seeds are dispersed primarily by wind.
Germination is epigeal and occurs on snow, or mineral or organic soil if
sufficient moisture is available . Germination rates range from 47
to 75 percent [28,49]. Cold stratification of mature seeds shortens
incubation time and may substantially increase germination . Heavy
seeds germinate more rapidly than seeds with low percent dry weight.
Along the eastern high Cascades in Oregon, seed viability of mountain
hemlock varied from 36 to 76 percent over a 2-year period .
Seedling development: Young seedlings grow best in partial shade and
early development is often slow. Seedlings are relatively drought
intolerant . Increasing light intensity and day length increase
seedling height but delay or prevent terminal bud formation under
shelter. Healthy mountain hemlock saplings respond well to release, in
both diameter and height growth. Seedlings and small saplings of
mountain hemlock tolerate heavy snowpacks well .
Vegetative reproduction: Mountain hemlock reproduces vegetatively by
layering [49,73]. This is an effective means of regenerating at
timberline, since layered saplings are sheltered by the growth of the
parent tree and initially receive their nutrients through the
established root system of the old tree . Layering is an important
method of reproduction on muskegs and krummholz areas in Alaska .
Growth Form (according to Raunkiær Life-form classification)
Immediate Effect of Fire
method of killing is root charring and crown scorching . In a
krumholz community of the North Cascades, Washington, all but one
mountain hemlock were killed by fire .
Reaction to Competition
Mountain hemlock is considered a minor climax species on most of its habitats; however, it pioneers on glacial moraines in British Columbia and Alaska (11) where it is nevertheless considered indicative of the climax forest (46); also see references in 19). Pacific silver fir is a major climax species in many communities of the mountain hemlock forest subzone in British Columbia (9) and Washington and northern Oregon (20). Alaska-cedar, western redcedar (Thuja plicata), and western hemlock (Tsuga heterophylla), however, are climax associates on some sites. Mountain hemlock is more commonly the major climax species in the mountain hemlock zone south of central Oregon where Pacific silver fir does not occur.
Mountain hemlock often succeeds lodgepole pine or subalpine fir when these species pioneer on drier sites (20). It also tends to replace Engelmann spruce when the two species occur together, possibly because hemlock is better able to withstand the allelopathic effects of spruce than are other associated species (76).
Two-thirds to three-quarters of the net primary productivity (NPP) is allocated below ground, according to the available data on three stands 130 to 280 years old dominated by silver fir but with significant mountain hemlock components (30,84). This high allocation below ground is probably caused by the need to obtain sufficient nutrients in these infertile soils (30). Mycorrhizae were found to be important sinks for carbon and components of nutrient cycles in one stand (84).
Life History and Behavior
Mountain hemlock has a 2-year reproductive cycle. Pollination occurs in
the spring or early summer of the second year . Mountain hemlock
releases pollen in June in the Cascade Range in Oregon, from mid-June to
mid-July in British Columbia, and from mid-May to late June in Alaska
. Fertilization occurs from about late July to early August in
British Columbia. Reproductive buds can easily be identified in the
late summer and fall. Cones ripen and open from late September to
November [49,56]. In the Bitterroot Mountains of Idaho cones ripen in
August . In Montana cones open and release the wide-winged seeds in
September or October and then abscise .
Mountain hemlock is generally slow to regenerate after disturbances such as logging, site preparation, or wildfire. Most burned areas in the mountain hemlock zone on the Olympic Peninsula do not have adequate stocking for commercial forests (600 trees/ha 1,500/acre) even 55 to 88 years after wildfires (3). Reproduction is greater during normal-to-wet growing seasons, than during dry growing seasons, and greater in areas near live trees at the edge of fires and near trees that survive the fires than in areas farther from seed sources. In the parkland subzone, reproduction is limited to the margins of tree clumps (48,72), except when successive years with earlier than normal snowmelt allow invasion of subalpine meadows (3,23).
Young stands 20 to 40 years old (some in burned areas) in southern Oregon and northern California may be pure mountain hemlock and quite dense (9,900 to 24,700 trees/ha, or 4,000 to 10,000/acre) (11).
In Oregon, mountain hemlock forests typically regenerate slowly after they are clearcut. In a study of 25 clearcuts, 5 to 11 years were required to reach 60-percent stocking on 0.0012-ha (0.003-acre) subplots (56). Establishment of seedlings during the first 2 years in an Oregon shelterwood cut was very low because germinants were few at low residual basal areas (less than 11.5 m²/ha or 50 ft²/acre) and all seedlings died at all higher basal areas (69).
The normally slow restocking process is retarded by slash treatment. On the east side of the Cascade Range in Oregon, treated (generally piled and burned) clearcuts had lower stocking (33 percent based on 0.0004-ha (0.001-acre) subplots) than untreated clearcuts (57 percent) because of destruction of advance regeneration and a 50-percent decrease in the number of subplots stocked with natural, post-harvest reproduction (61). These clearcuts ranged from 3 to 19 years old. Stocking of mountain hemlock and its associates near Willamette Pass, OR, in 13-year-old strip cuts with unburned slash was 95 percent (on subplots of 0.0012 ha or 0.003 acre), compared with 82 percent in units that had been burned. The difference was due to advance regeneration of silver fir and mountain hemlock in the unburned clearcuts. These studies indicate that stocking in clearcuts in the mountain hemlock zone is typically made up of post-harvest naturally seeded trees, such as mountain hemlock, Shasta red fir (Abies magnifica var. shastensis), silver fir, lodgepole pine, and western white pine (Pinus monticola); and advance regeneration of mountain hemlock and Shasta red fir (56,67). Planting has been relatively ineffective in speeding regeneration on these cold, snowy sites compared with advance and post-harvest natural regeneration that slowly provide adequate to abundant stocking.
Healthy mountain hemlock saplings (mean d.b.h. 4.5 cm, 1.8 in) respond well to release, in both diameter and height growth (68). Understory saplings with crown ratios greater than 50 percent and growing fastest before release will likely be the best crop trees (68).
Regeneration of mountain hemlock varies in response to environmental gradients. In six strip cuts at Willamette Pass, OR, it decreases from the south (shaded) side to the north (unshaded) side. In the Cascade Range in central Oregon, it decreases with increasing cover of grass and forbs (67). Near Windigo Pass, OR, mountain hemlock makes up an increasing proportion of tree regeneration as elevation and pumice depth increase (56) and so becomes increasingly important for reforesting these harsher sites. In Oregon, the proportion of mountain hemlock to other species in clearcuts and burned areas typically is lower than that in surrounding mature stands, whereas the proportion of lodgepole pine, western white pine, and Shasta red fir reproduction is relatively higher.
Seedlings and small saplings of mountain hemlock tolerate heavy snowpacks well. Bent boles and branches spring erect after snowmelt. Leader replacement by axial buds is less frequent than in other North American hemlocks, and the drooping leaders take 3 or more years to become erect (40).
Seed Production and Dissemination
Flowering and Fruiting
Growth and Yield
Mountain hemlock grows slowly in height (fig. 1) and in diameter. Three investigators found that stem-dissected trees in Oregon and southern Washington without signs of impeded height growth were only 7 to 28 m (23 to 91 ft) tall at 100 years (39,43,54). Height growth of mountain hemlock is initially slower than that of western hemlock but continues at a moderate rate to greater age. As a result, use of western hemlock site curves in old mountain hemlock stands leads to large overestimates of growth and yield (39).
Figure 1- Height growth curves for two mountain hemlock
communities on the east side of the Cascade range in central
Oregon (adapted from 43). Mean site index (base age 100
years) and form of height growth curve differ markedly be-
Site index (base age 100 years) and the form of the height growth curve are different in different plant communities in the central Oregon Cascades (fig. 1) (43). The mountain hemlock/smooth woodrush (Tsuga mertensiana/Luzula hitchcockii) and mountain hemlock/prince's-pine-pinemat manzanita (T. mertensiana/Chimaphila umbellata-Arctostaphylos nevadensis) communities have ranges in site index of 7 to 14 m (23 to 46 ft) and 13 to 19 m (43 to 62 ft), respectively. In the Coast Mountains of British Columbia, site index (base age 100 years) ranges from less than 6 m (20 ft) on xeric sites to 34 m (110 ft) on the best sites (46).
Mountain hemlock stands at least 200 years old can have high basal areas and volumes; the highest values are in mixed species stands. In the mountain hemlock zone in British Columbia, stands in which that species makes up more than half the volume (59 to 79 percent) have volumes of 125 to 924 m³/ha (1,786 to 13,204 ft³/acre); volumes are much higher (range 588 to 1348 m³/ha, 8,397 to 19,260 ft³/acre) in stands where mountain hemlock makes up less than half the volume (9 to 36 percent) (9). These more productive mixed-species stands usually grow on deeper soils irrigated by seepage and have a slightly longer average snow-free period, whereas the communities in which mountain hemlock forms a majority of the stocking occupy the poorer, colder sites.
A similar pattern occurs in the Cascade Range in southern Oregon (42). The mountain hemlock/grouse whortleberry community (nearly pure mountain hemlock) produces an estimated 1.0 m³/ha (14 ft³/acre) per year, and the Shasta red fir-mountain hemlock/pinemat manzanita/long-stoloned sedge community produces an estimated 3.8 to 9.8 m³/ha (54 to 140 ft³/acre) per year (42). The latter community tends to grow on warmer sites, and most of the productivity is by Shasta red fir, not mountain hemlock. Basal areas of both communities are high, 76 and 62 m²/ha (330 and 270 ft²/acre), respectively. Mountain hemlock communities in the western Cascades of Oregon and Washington commonly have a mixture of other tree species; estimated productivity ranges from 3.8 to 7.6 m³/ha (54 to 108 ft³/acre) per year (8,38). Volume tables are available for mountain hemlock for Alaska (32) and the central Oregon Cascades (6). Volume growth, biomass, and leaf area equations are also available (1,26,71,81,87).
Leaf area index (all sides), like volume productivity, is lower in nearly pure mountain hemlock forest (10 m²/m² or 10 ft²/ft²) (25) than in mixed species forest (35 m²/m² or 35 ft²/ft²) (88) in the Cascade Range.
Molecular Biology and Genetics
Individuals morphologically intermediate between western hemlock and mountain hemlock are occasionally found where the two species occupy the same site. These populations have been given hybrid status (Tsuga x jeffreyi (Henry) Henry) (11, 13,47). A study of leaf pigment chemistry of 43 morphologically intermediate individuals collected from throughout western Washington indicated, however, that only three (all from Corral Pass near Mount Rainier) were chemically intermediate and so were potentially of hybrid origin (74). Six leaf cuticle characteristics of Tsuga x jeffreyi in Britain were similar to mountain hemlock and dissimilar to western hemlock (70), also not supporting hybrid status. The limited overlap in the timing of pollen release (14) and failure of all known controlled pollinations to produce filled seeds (in British Columbia (53) and in Oregon (cited in 74) support the conclusion that true hybrids are probably much rarer than the morphological intermediates on which hybrid status is based, if such hybrids occur at all.
A California form of mountain hemlock has been given specific status (Tsuga crassifolia Flous) and is proposed as a hybrid between Engelmann spruce and mountain hemlock (13). The hybrid swarms expected from backcrosses with the parent taxa have not been observed (13), however, and this specific status and phylogeny are not generally recognized by taxonomists (17,47,57).
All mountain hemlock in the Siskiyou Mountains from the Oregon-California border south were recently given subspecific status as Tsuga mertensiana (Bong.) Carr. ssp. grandicona Farjon, in recognition of the generally larger cones of trees in this region (17,63). This classification is less ambitious than the new species T. crassifolia.
Clausen (10) studied the mountain hemlock phenotypes of erect simple stems and prostrate krummholz (elfinwood) over a gradient in elevation (3050 to 3350 m; 10,000 to 11,000 ft) in the Sierra Nevada and described them as genetic races. It has not been determined, however, whether these growth forms have different genotypes or result solely from different environments.
Barcode data: Tsuga mertensiana
Statistics of barcoding coverage: Tsuga mertensiana
Public Records: 6
Specimens with Barcodes: 7
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: N5 - Secure
Rounded National Status Rank: N5 - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Insects and disease: Mountain hemlock is very susceptible to laminated
root rot (Phellinus weiri) [20,49]. In the high Cascades of central
Oregon mountain hemlock is the most susceptible tree. This fungus
spreads from centers of infection along tree roots so that all trees are
killed in circular areas that expand radially. Laminated root rot moves
faster through a nearly pure stand of mountain hemlock than through a
more heterogeneous conifer stand. Growth and coalescence of laminated
rot root pockets in mountain hemlock have produced infected areas of
more than 100 acres (40 ha). Seedlings are not susceptible to
reinfection by laminated root rot for 80 to 120 years. This may be due
to greater vigor caused by higher levels of available nitrogen, higher
temperatures, and more growing-season moisture in this regrowth zone
Other common fungal and parasite pests of mountain hemlock include
several heart rots, of which Indian paint fungus (Echinodontium
tinctorum) is the most common and damaging; several needle diseases;
snow mold (Herpotrichia nigra); and dwarf-mistletoe (Arceuthobium
Mountain hemlock is an occasional host for the western spruce budworm
(Choristoneura occidentalis) .
Frost tolerance: Mountain hemlock is very frost tolerant .
Wind damage: Because mountain hemlock is shallow rooted it is very
susceptible to windthrow. In the coastal strip of British Columbia and
Alaska, wind commonly destroys mountain hemlock by uprooting it. As
cutting is increased in mountain hemlock forests, windthrow will
probably become a more common cause of mortality .
Silvicultural considerations: Many sites dominated by mountain hemlock
are particularly difficult to reforest following clearcutting. In the
Gifford Pinchot, Mount Hood, and Willamette national forests, field
observations showed that the mountain hemlock/big huckleberry/beargrass
and mountain hemlock/grouse whortleberry associations are particularly
difficult to reforest due to a short growing season in a harsh
environment. Artifical reforestation within 5 years following
clearcutting and burning on these sites may not be possible .
The deep, persistent snowpack; short, cool growing season; and poorly
developed soils make regeneration difficult and productivity low for the
mountain hemlock series of the Siskiyou region of southwestern Oregon.
When mountain hemlock stands are managed for timber production, the
following silvicultural considersations are important :
(1) Advanced regeneration and subsequent natural regeneration may
provide the most reliable source for a new stand in 5 years. Protection
from damage during harvest is essential. Damaged regeneration is very
susceptible to rot.
(2) Natural regeneration after harvest establishes sooner in small
openings than large openings and is often most rapid on the shaded south
edges of clearcuts. Keeping clearcuts small to maximize these edge
effects will probably speed regeneration, but it may still be
unsatisfactory in 5 years. The shelterwood system can provide adequate
regeneration in 5 to 10 years.
(3) Planting has been ineffective on these cold, snowy sites. Timing is
critical for articifical regeneration. Plant soon after snow melts.
In British Columbia, the recommended silvicultural method for harvest of
old-growth mountain hemlock is clearcut followed by natural
regeneration. For young natural stands that have developed after fire
or second-growth stands that have developed after clearcutting, the
clearcut method with natural regeneration, seed tree method, or
shelterwood method is recommended .
Relevance to Humans and Ecosystems
Value for rehabilitation of disturbed sites
Mountain hemlock is important for watershed protection . The
mountain hemlock/blueberry (Vaccinium spp.)-copperbush (Cladothamnus
pyrolaeflorus)/deer cabbage (Fauria crista-galli) association in Alaska
captures runoff from snowmelt . Planted stock of mountain hemlock
does not perform well. In high-elevation regeneration trials in the
Vancouver forest region, its performance was poor compared to that of
the other high-elevation species. Natural regeneration may perform
Importance to Livestock and Wildlife
Mountain hemlock stands provide good hiding and thermal cover for many
wildlife species [8,45]. Sites dominated by mountain hemlock provide
important summer range for deer in Alaska and Vancouver Island because
of abundant nutrient-rich forbs available in the understory [19,49]. In
Montana, mountain hemlock habitat types provide summer range for mule
deer, elk, and bear . Mountain hemlock seeds have been found in the
stomachs of crows and grouse .
Other uses and values
Pacific Northwest and throughout Great Britain [42,49,4]. Its dense,
compact foliage coupled with its slow growth make it ideal as a garden
evergreen . Hemlock species (Tsuga spp.) played a supernatural role
as magical objects in the mythology of the Thompson and Lillooet
Interior Salish of British Columbia .
Wood Products Value
at which it occurs and is unimportant as commercial timber . It is,
however, harvested to a limited extent near its lower limits; the wood
is generally marketed with western hemlock [71,4]. The wood is
moderately strong and light colored and is most often used for
small-demension lumber and pulp . The wood is also used for railway
ties, mine timbers, interior finish, crates, kitchen cabinets, and
flooring and ceilings . Nearly pure stands of mountain hemlock on
Prince of Wales Island have been logged for pulp .
Some mountain hemlock forest types are important deer summer range on Vancouver Island (34). A shrubby plant association (Mountain Hemlock-Copperbush) there provides abundant browse in old stands as well as in early seral conditions, so harvesting does not significantly affect food availability.
Tsuga mertensiana, known as 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. Mertensiana refers to Franz Carl Mertens (1764–1831), a German botanist.
Tsuga mertensiana is a large evergreen coniferous tree growing to 20 to 40 metres (66 to 131 ft) tall, with exceptional specimens as tall as 59 m (194 ft) tall. They have a trunk diameter of up to 2 m (6 ft 7 in). 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 (0.04 in) long. The leaves are needle-like, 7 to 25 mm (0.28 to 0.98 in) long and 1 to 1.5 mm (0.04 to 0.06 in) 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 around the shoot.
The cones are small (but much longer than those of any other species of hemlock), pendulous, cylindrical, 30 to 80 mm (1.18 to 3.15 in) long and 8 to 10 mm (0.31 to 0.39 in) broad when closed, opening to 12 to 35 mm (0.47 to 1.38 in) broad, superficially somewhat like a small spruce cone. They have thin, flexible scales 8 to 18 mm (0.31 to 0.71 in) long. The immature cones are dark purple (rarely green), maturing red–brown 5 to 7 months after pollination. The seeds are red–brown, 2 to 3 mm (0.08 to 0.12 in) long, with a slender, 7 to 12 mm (0.28 to 0.47 in)-long pale pink–brown wing.
The geographic range of Tsuga mertensiana fairly closely matches that of Tsuga heterophylla (western hemlock), likewise mostly less than 100 km (62 mi) 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 and mountain hemlock is found in the Klamath Mountains and Sierra Nevada. Unlike western hemlock, mountain hemlock mostly grows at high altitudes except in the far north, from sea level to 1,000 m (3,300 ft) in Alaska, 1,600 to 2,300 m (5,200 to 7,500 ft) in the Cascades in Oregon, and 2,500 to 3,050 m (8,200 to 10,010 ft) in the Sierra Nevada.
- Tsuga mertensiana subsp. mertensiana. Northern mountain hemlock. Central Oregon northwards. Cones smaller, 30 to 60 mm (1.2 to 2.4 in) long, 12 to 25 mm (0.47 to 0.98 in) broad when open, with 50 to 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 to 80 mm (1.77 to 3.15 in) long, 20 to 35 mm (0.79 to 1.38 in) broad when open, with 40 to 60 scales.
Mountain hemlock is usually found on cold, snowy subalpine sites where it grows slowly, sometimes attaining more than 800 years in age. Arborescent individuals that have narrowly conical crowns until old age (300 to 400 years) and shrubby krummholz on cold, windy sites near timberline add beauty to mountain landscapes. Areas occupied by mountain hemlock generally have a cool to cold maritime climate that includes mild to cold winters, a short, warm-to-cool growing season and moderate to high precipitation.
Best development of mountain hemlock is on loose, coarse-textured, well-drained soils with adequate moisture, and in British Columbia, on thick and very acidic organic matter and decayed wood. Adequate soil moisture appears to be especially important in California and Montana, where summer drought is most pronounced.
Mountain hemlock will grow on most landforms, but individuals typically develop best in mixed stands of forest on sheltered slopes or in draws. From southern British Columbia south, the tree grows better on northerly exposures. The preference for relatively moist, cool sites evidently becomes a necessity as the climate becomes more continental in western Montana and more mediterranean in the central Sierra Nevada at these extremes of its range. In these locations, mountain hemlock typically grows in isolated populations in north-facing glens and cirque basins where snow collects and may remain well into summer.
Mountain hemlock is adapted to sites with long-lasting snowpacks. In the spring, mountain hemlocks emerging through 2 to 4 m (6.6 to 13.1 ft) of snow were transpiring, whereas nearby whitebark pines did not transpire until the soil beneath them was free of snow. Mountain hemlock is well adapted to cope with heavy snow and ice loads, with tough branches and the drooping branchlets shedding snow readily.
Mountain hemlock is tolerant of shade and other forms of competition. It is more tolerant than all its associates except Pacific silver fir, western hemlock, and Alaska cedar. Mountain hemlock is considered a minor climax species on most of its habitats; however, it pioneers on glacial moraines in British Columbia and Alaska. Pacific silver fir is a major climax species in many communities of the mountain hemlock forest in British Columbia and Washington and northern Oregon. Alaska cedar, western redcedar, and western hemlock are climax associates on some sites. Mountain hemlock is more commonly the major climax species in the mountain hemlock zone south of central Oregon where Pacific silver fir does not occur.
Mountain hemlock often succeeds lodgepole pine or subalpine fir when these species pioneer on drier sites. It also tends to replace Engelmann spruce when the two species occur together, possibly because hemlock is better able to withstand the allelopathic effects of spruce than are other associated species.
Outside of its native range, mountain hemlock is grown as an ornamental tree. It is planted as a specimen tree in native plant landscapes in California, and particularly in gardens 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 urban air pollution.
- 'Blue Star'
- Conifer Specialist Group (1998). Tsuga mertensiana. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 12 May 2006.
- Farjon, A. (1990). Pinaceae. Drawings and Descriptions of the Genera. Koeltz Scientific Books. ISBN 3-87429-298-3.
- "Tsuga mertensiana". Flora of North America.
- "Tsuga mertensiana". Gymnosperm Database.
- David Gledhill in The Names of Plants (3rd edition), Cambridge University Press, 2002.
- This article incorporates public domain material from the United States Department of Agriculture document "Silvics Manual, Volume 1, Tsuga mertensiana" by Joseph E. Means.
- Brooke, Robert C.; Peterson, E. G.; Krajina, V. J. (1970). "The subalpine mountain hemlock zone". In Krajina, V. J. Ecology of western North America 2. Vancouver: University of British Columbia Department of Botany. pp. 147–349.
- Habeck, James R. (1967). "Mountain hemlock communities in western Montana". Northwest Science 41 (4): 169–177.
- Krajina, V. J. (1969). "Ecology of forest trees in British Columbia". In Krajina, V. J. Ecology of western North America 2. Vancouver, BC: University of British Columbia, Department of Biology. pp. 1–147.
- Rushforth, K. (1987). Conifers. Helm ISBN 0-7470-2801-X.
- Chase, J. Smeaton (1911). Cone-bearing Trees of the California Mountains. Chicago: A. C. McClurg & Co. p. 99. LCCN 11004975. OCLC 3477527. LCC QK495.C75 C4, with illustrations by Carl Eytel - Kurut, Gary F. (2009), "Carl Eytel: Southern California Desert Artist", California State Library Foundation, Bulletin No. 95, pp. 17–20 (PDF) retrieved Nov. 13, 2011
M.Van Campo-Duplan and H.Gaussen (1948) postulated that this taxon originated by hybridization between Picea and Tsuga . Although this is unlikely, some characteristics such as leaf arrangement and shape, phenolic chemistry, and pollen grain structure lend some support for this hypothesis.
Names and Taxonomy
mertensiana (Bong.) Carriere [38,46,49]. Mountain hemlock in the
Siskiyous from the Oregon-California border south were recently
classified as Tsuga mertensiana spp. grandicona Farjon, in recognition
of the generally larger cones of trees in this region . All others
are classified as Tsuga mertensiana spp. mertensiana. There are no
recognized varieties or forms. Mountain hemlock will hybridize with
western hemlock (T. heterophylla) .
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