BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Tsuga mertensiana | Mountain Hemlock 



GENERAL BOTANICAL CHARACTERISTICS : 
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 [59].  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) [4].  On exposed ridges at high
elevations, it often grows as a low-spreading shrub or small tree
[38,4].

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 [38].  The twigs are
mostly short and slender.  The needles are crowded on all sides of short
twigs and curved upward [4].

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 [59].  The
root system is shallow and widespreading [38,49,59].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
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 [49].
Cones average about 70 to 100 seeds [28].  There are 102,000 to 207,000
seeds per pound [70].  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 [28].  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
seasons [49].

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 [49].  Germination rates range from 47
to 75 percent [28,49].  Cold stratification of mature seeds shortens
incubation time and may substantially increase germination [62].  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 [49].

Seedling development:  Young seedlings grow best in partial shade and
early development is often slow.  Seedlings are relatively drought
intolerant [2].  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 [49].

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 [4].  Layering is an important
method of reproduction on muskegs and krummholz areas in Alaska [49].


SITE CHARACTERISTICS : 
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 [49].  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) [8].
Mountain hemlock in western Montana is generally confined to the moist,
upper slopes of the Bitterroot Mountains [33].

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 [49].  Average annual snowfall ranges from about 32 to 50
feet (10-15 m) [4].

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 [49].  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 [7].  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 [49].  In
the Coastal Mountains, mountain hemlock can grow on the rockiest soils,
even including recent lava flows, if moisture is adequate [4].  The
nutritional requirements of mountain hemlock are low [41].

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 [33].  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].


SUCCESSIONAL STATUS : 
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 [49].
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 [33].


SEASONAL DEVELOPMENT : 
Mountain hemlock has a 2-year reproductive cycle.  Pollination occurs in
the spring or early summer of the second year [55].  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
[49].  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 [62].  In Montana cones open and release the wide-winged seeds in
September or October and then abscise [44].

Related categories for Species: Tsuga mertensiana | Mountain Hemlock