BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Tsuga heterophylla | Western Hemlock 



GENERAL BOTANICAL CHARACTERISTICS : 
Western hemlock is a large, native, evergreen tree.  At maturity it is
generally 100 to 150 feet (30-46 m) tall and 2 to 4 feet (0.6-1.2 m) in
trunk diameter [72].  On best sites, old-growth trees reach diameters
greater than 3.3 feet (100 cm); maximum diameter is about 9 inches (275
cm).  Heights of 160 to 200 feet (49-61 m) are not uncommon; maximum
height has been reported as 259 feet (79 m) [57].

Western hemlock has a long slender trunk often becoming fluted when
large and has a short, narrow crown of horizontal or slightly drooping
branches.  The needles are short-stalked and 0.25 to 0.87 inch (6-22 mm)
long, flat and rounded at the tip.  The twigs are slender [72].  The
bark is thin (1 to 1.5 inches [0.39-0.59 cm]) even on large trees; young
bark is scaly and on old trunks it is hard with furrows separating wide
flat ridges [60].  Western hemlock is shallow rooted and does not
develop a taproot.  The roots, especially the fine roots, are commonly
most abundant near the surface and are easily damaged by harvesting
equipment and fire.  Maximum ages are typically over 400 years but less
than 500 years.  The maximum age recorded is in excess of 700 years
[57].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Seed production and dissemination:  Western hemlock is generally a good
cone and seed producer.  Cones may form on open-grown trees that are
less than 20 years old, but good cone crops usually do not occur until
trees are between 25 and 30 years old.  Individuals usually produce some
cones every year and heavy cone crops every 3 or 4 years.  Each cone
contains 30 to 40 seeds.  The number of viable seeds ranges from fewer
than 10 to approximately 20 per cone [18,56].  Seeds are light and
small, ranging from 189,000 to 508,000 cleaned seeds per pound, with an
average of 260,000 seeds per pound (371,000-900,000/kg, average 530,000
seeds/kg) [56,63].

Western hemlock seeds have large wings enabling them to be distributed
over long distances.  In open, moderately windy areas, most seeds fall
within 1,968 feet (600 m) of the parent tree.  Some seeds can travel as
far as 3,772 feet (1,150 m) under these conditions.  In dense stands,
most seeds fall much closer to the base of the tree [56].

Germination:  Germination is epigeal.  Stratification for 3 to 4 weeks
at 33 to 39 degrees Fahrenheit (1-4 deg C) improves germination.  The
optimum temperature for germination is 68 degrees Fahrenheit (20 deg C)
[57,63].  For each 9 degrees Fahrenheit (5 deg C) below the optimum, the
number of days required for germination is nearly doubled [63].  Given
sufficent time (6-9 months) and an absence of pathogens, western hemlock
will germinate at temperatures just above freezing.  Western hemlock
seeds remain viable only into the first growing season after seedfall
[57].  Viability seems to vary between 36 and 55 percent with an average
of about 46 percent [76].

Western hemlock seed appears to germinate well and seedlings grow well
on almost all natural seedbeds whether rotten wood, undisturbed bed duff
and litter, or bare mineral soil.  The principal requirement for
adequate development on any seedbed appears to be adequate moisture.
For drier situations, mineral soils appear to be best for hemlock
seedlings [76].

Seedling development:  Most seedling mortality occurs in the first 2
years after germination [76].  Seedlings are very shade tolerant but are
sensitive to heat, cold, drought and wind [56].  In British Columbia,
the main cause of mortality appeared to be either drought or frost [76].
Initial growth is slow; 2-year-old seedlings are commonly less than 8
inches (20 cm) tall.  Once established, seedlings in full light may have
an average growth rate of 24 inches (60 cm) or more annually [57].  In
inland regions, one study showed partial shade to be beneficial in
reducing mortality caused by high temperatures and drought.  Once
seedlings are over 2 years old, survival appears to be very good [76].

Vegetative reproduction:  Western hemlock will reproduce vegetatively by
layering or 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.  Western hemlock
grafts readily.  Growth of grafted material is better than that of
rooted material [57].


SITE CHARACTERISTICS : 
Western hemlock thrives in humid areas of the Pacific Coast and northern
Rocky Mountains.  Growth is best in mild, humid climates where frequent
fog and precipitation occur during the growing season.  The best stands
occur in the humid coastal regions.  In subhumid regions with relatively
dry growing seasons, western hemlock is confined primarily to northerly
aspects, moist stream bottoms, or seepage sites [10,57,59].  In Alaska,
western hemlock attains its largest size on moist flats and low slopes
[72].

Precipitation and temperature:  In the coastal range, western hemlock
occurs on sites with a mean annual precipitation of less than 15 inches
(380 mm) in Alaska to at least 262 inches (6,650 mm) in British
Columbia.  In the Rocky Mountains it occurs on sites with mean annual
precipitation ranging from 22 inches (560 mm) to at least 68 inches
(1,730 mm).  Mean annual temperatures where western hemlock commonly
occurs range from 32.5 to 52.3 degrees Fahrenheit (0.3-11.3 deg C) on
the coast and 36 to 46.8 degrees Fahrenheit (2.2-8.2 deg C) in the Rocky
Mountains.  The frost-free period within the coastal range of western
hemlock averages less than 100 to more than 280 days. In the Rocky
Mountains the frost-free period is 100 to 150 days [57].

Elevation:  The elevational range of western hemlock is from sea level
to 7,000 feet (2,130 m).  On the coast, western hemlock develops best
between sea level and 2,000 feet (610 m); in the Rocky Mountains, it
develops best between 1,600 and 4,200 feet (490-1,280 m) [57].

Soils:  Western hemlock grows on soils derived from all bedrock types
(except serpentines) within its range [57].  It grows well on
sedimentary, metamorphic, and igneous materials.  Western hemlock is
found on most soil textures.  Height growth, however, decreases with an
increase in clay content or soil bulk density.  This is attributed to
inadequate soil aeration or the inability of roots to penetrate compact
soils.  Western hemlock does not do well on sites where the water table
is less than 6 inches (15 cm) below the soil surface.  The pH under
stands containing western hemlock ranges from less than 3.0 to nearly
6.0 in the organic horizons.  The pH in the surface mineral horizons
ranges from 4.0 to 6.3 and that of the C horizons from 4.8 to 6.2. The
optimum range of pH for seedlings is 4.5 to 5.0.  Western hemlock is
highly productive on soils with a high range of available nutrients.
The produtivity of western hemlock increases as soil nitrogen increases
[57].

In the Coast Range, western hemlock is commonly associated with the
following shrub species:  vine maple (Acer circinatum), dwarf
Oregongrape (Mahonia nervosa), Pacific rhododendron (Rhododendron
macrophyllum), stink currant (Ribes bracteosum), salmonberry, trailing
blackberry (R.  ursinus), Pacific red elder (Sambucus callicarpa),
Alaska blueberry (Vaccinium alaskaense), big huckleberry (V.
membranaceum), oval-leaf huckleberry (V. ovalifolium), evergreen
huckleberry (V. ovatum), and red huckleberry (V. parvifolium)
[12,31,32,57].

In the Rocky Mountains, western hemlock is commonly associated with the
following shrub species:  Oregon grape (Mahonia repens), russet
buffaloberry (Shepherdia canadensis), birchleaf spirea (Spiraea
betulifolia), dwarf blueberry (Vaccinium caespitosum), globe huckleberry
(V. globulare), and grouse whortleberry (V. scoparium) [55,57,59,69,73].

Common shrub associates of both coastal and Rocky Mountain regions are
as follows:  Sitka alder (Alnus sinuata), snowbush ceanothus (Ceonothus
velutinus), oceanspray (Holodiscus discolor), rustyleaf mensziesia
(Menziesia ferruginea), devilsclub, Pacific ninebark (Physocarpus
capitatus), prickly currant (Ribes lacustre), thimbleberry, and common
snowberry (Symphoricarpos albus) [57].

Common herb associates with western hemlock include maidenhair fern
(Adiantum pedatum), ladyfern (Athyrium filix-femina), deerfern (Blechnum
spicant), mountain woodfern (Dryopteris austriaca), oakfern
(Gymnocarpium dryopteris), swordfern (Polystichum munitum), bracken fern
(Pteridium aquilinum), vanillaleaf (Achlys triphylla), wild ginger
(Asarum caudatum), princes-pine (Chimaphila umbellata), queenscup
beadlily (Clintonia unifora), cleavers bedstraw (Galium aparine),
sweetscented bedstraw (G. triflorum), twinflower (Linnaea borealis),
one-sided pyrola (Pyrola secunda), feather solomonplume (Smilacina
racemosa), white trillium (Trillium ovatum), roundleaf violet (Viola
orbiculata), and beargrass (Xerophyllum tenax) [55,57,59,69,73].


SUCCESSIONAL STATUS : 
Obligate Climax Species

Western hemlock is very shade tolerant.  Only Pacific yew (Taxus
brevifolia) and Pacific silver fir are considered to have equal or
greater tolerance of shade than western hemlock.  Western hemlock is
generally considered a climax species either alone or in combination
with its shade-tolerant associates, but it can be found in all stages of
succession [57].  It is an aggressive pioneer because of its quick
growth in full overhead light and its ability to survive on a wide
variety of seedbed conditions [29,57].  It also invades seral stages of
forest succession after a forest canopy has formed [35].  If several
centuries pass without a major disturbance, a climax of
self-perpetuating, essentially pure western hemlock can result [10].  On
drier upland slopes in Glacier National Park, western hemlock often
achieves dominance over western redcedar.  Western hemlock rarely
replaces western redcedar entirely [35].  In Idaho, western white pine
(Pinus monticola) stands are slowly replaced by a western
hemlock-western redcedar climax [52].


SEASONAL DEVELOPMENT : 
The reproductive cycle of western hemlock occurs over 15 to 16 months
from the time of cone initiation in early summer, until seeds are shed
in the fall of the following year. Fertilization and seed development
occur in the second year.  Phenology varies between coastal and interior
regions [56,57,76].  Trees in the interior region or at higher
elevations begin development later in the spring and complete
development earlier in the fall than do trees growing in coastal and
low-elevation regions [56].

At low-elevation coastal British Columbia locations, pollination
commonly occurs in early to mid-April, whereas in the interior of
British Columbia, it may occur from May until mid-June.  Records from
western Washington and Oregon show that pollination may occur from
mid-April until late May [56].  Fertilization occurs in coastal western
hemlock about mid-May.  The time from pollination to seed release ranges
from 120 to 160 days in western hemlock.  It can vary according to
weather and temperature during cone maturation.  Dry, warm weather in
late summer may cause more rapid drying and earlier opening of cones
with consequently, earlier seed release.  Wet, cool weather may delay
cone opening and seed release.  Most seeds are shed in the fall when
cones first open [50,56,76].  Cones may close in wet weather and reopen
more fully with subsequent dry weather.  As a result, seeds may be shed
throughout the winter or even during the next spring.  Mature cones
often persist on the tree throughout the second year but contain few
viable seeds [56,57].

Related categories for Species: Tsuga heterophylla | Western Hemlock