Associated Forest Cover
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Sitka spruce is commonly associated with western hemlock throughout most
of its range. Toward the south, other conifer associates include
Douglas-fir (Pseudotsuga menziesii), Port-Orford-cedar (Chamaecyparis
lawsoniana), western white pine (Pinus monticola), and redwood
(Sequoia sempervirens). Shore pine (P. contorta var. contorta)
and western redcedar (Thuja plicata) are also associates that
extend into southeast Alaska. Toward the north, conifer associates also
include Alaska-cedar (Chamaecyparis nootkatensis), mountain
hemlock (Tsuga mertensiana), and subalpine fir (Abies
lasiocarpa)-trees that are usually found only at higher elevations
toward the south. In central and northern British Columbia and Alaska,
however, these species are found with Sitka spruce from sea level to
timberline. White spruce (Picea glauca) is also associated with
Sitka spruce in Alaska, and hybrids occur. The most important hardwood
associates are red alder and bigleaf maple (Acer macrophyllum) in
the south and red alder and Sitka alder toward the north. Black cottonwood
(Populus trichocarpa) is an associate throughout the range.
Stands stocked with at least 80 percent Sitka spruce are identified as
the forest cover type Sitka Spruce (Society of American Foresters Type
223) (6). Sitka spruce is also a component of 10 other forest cover types:
221 Red Alder
222 Black Cottonwood-Willow
224 Western Hemlock
225 Western Hemlock-Sitka Spruce
227 Western Redcedar-Western Hemlock
228 Western Redcedar
229 Pacific Douglas-Fir
230 Douglas-Fir-Western Hemlock
231 Port-Orford-Cedar
232 Redwood
Sitka spruce usually grows in mixed stands, less often in pure stands.
Pure stands usually occur in early successional situations and as
tidewater stands influenced by salt spray. The most extensive pure stands
are found on the Kodiak-Afognak Archipelago at the extreme west extension
of the range. Sitka spruce is the only conifer present on this group of
islands. A relatively recent invader there, spruce is expanding its range
to the southwest, invading a tundra complex at the rate of about 1.6 km (1
mi) per century (14).
In Oregon and Washington, common understory species associated with
Sitka spruce include swordfern (Polystichum munitum), Oregon
oxalis (Oxalis oregana), false lily-of-the-valley (Maianthemum
dilatatum), western springbeauty (Montia sibirica), three-leaved
coolwort (Tiarella trifoliata), evergreen violet (Viola
sempervirens), stream violet (V. glabella), Smith fairybells
(Disporum smithii), red huckleberry (Vaccinium parvifolium),
and rustyleaf menziesia (Menziesia ferruginea). On drier
sites, salal (Gaultheria shallon), Pacific rhododendron (Rhododendron
macrophyllum), and evergreen huckleberry (Vaccinium ovatum) are
common. On wetter forest sites, the previously mentioned species are
found, along with devilsclub (Oplopanax horridum), ladyfern (Athyrium
filix-femina), deerfern (Blechnum spicant), mountain woodfern
(Dryopteris austriaca), and Pacific red elder (Sambucus
callicarpa) (11).
In Alaska, the more common understory plants include devilsclub,
skunkcabbage (Lysichitum americanum), ovalleaf huckleberry (Vaccinium
ovalifolium), red huckleberry, Alaska blueberry (V. alaskaense),
rustyleaf menziesia, salmonberry (Rubus spectabilis), five-leaf
bramble (R. pedatus), thimbleberry (R. parviflorus), bunchberry
(Cornus canadensis), stink currant (Ribes bracteosum), and
trailing black currant (R. laxiflorum) (32). Cryptogams are
abundant throughout the range of Sitka spruce. The Olympic Peninsula is
especially noted for mosses, many of which occur as epiphytes on living
trees.
In Oregon and Washington within the Sitka spruce forest zone, important
plant communities include Tsuga heterophylla-Picea
sitchensis/Gaultheria shallon/Blechnum spicant, Tsuga-Picea/Oplopanax
horridum/Athyrium filix-femina, or Tsuga-Picea/Polystichum
munitum-Oxalis oregana (11). Similar communities can be found in
southern British Columbia within the "fog western hemlock/Sitka
spruce subzone" (23). In Alaska, some of the more common communities
include Picea sitchensis/Oplopanax horridum-Rubus spectabilis/Cornus
canadensis, Picea sitchensis-Tsuga heterophylla/Lysichiton
americanum/Sphagnum spp., and Tsuga heterophylla-Picea
sitchensis-(Thuja plicata)/Vaccinium ovalifolium-V.
alaskaense/Rhytidiadelphus loreus (32).
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Climate
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Sitka spruce is restricted to an area of maritime climate with abundant
moisture throughout the year, relatively mild winters, and cool summers.
Summer temperatures decrease northward and lack the extremes found in more
continental locations. In terms of growing degree days, annual heat sums
(based on a threshold of 5° C or 41° F) range from 2511° C
(4,552° F) at Brookings, OR (lat. 41° 03' N.) to 851° C
(1,564° F) at Cordova, AK (lat. 60° 30' N.) (8). The number of
frost-free days varies locally but generally declines northward; averages
range from about 294 days at Brookings, OR, to 111 days at Cordova, AK
Annual precipitation varies within the range of Sitka spruce and is
influenced greatly by local topography. Annual precipitation of 2950 mm
(116 in) at Forks, WA, and 5615 mm (221 in) at Little Port Walter, AK,
contrasts with 635 mm (25 in) at Anacortes, WA, and 660 mm (26 in) at
Skagway, AK Summer precipitation is greater toward the north, where light
drizzle and fog are frequent. At Cordova, AK, from June to September, at
least a trace of precipitation occurs during 22 to 24 days each month. In
contrast, at Otis, OR, a trace or more of precipitation occurs on only 8
to 15 days each month. Toward the south, fog and moist maritime air are
important in maintaining moisture conditions needed for growth; most
winter precipitation is in the form of rain. Depth of snowfall increases
northward. Average annual snowfall at sea level is 1 em (0.5 in) at
Brookings, OR; 58 cm (23 in) at Quatsino, BC; and 340 cm (134 in) at
Cordova, AK
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Damaging Agents
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Blowdown is probably the most serious damaging
agent of Sitka spruce, but the species is attacked by a number of
pests-insects, disease organisms, and animals. In general, problems tend
to be more severe toward the south. The white pine weevil (Pissodes
strobi) is the most serious insect pest in Oregon, Washington, and
southern British Columbia; weevil damage has been the most serious
deterrent to management of Sitka spruce in the southern part of its range.
Damage is most severe on young trees 3 to 6 in (10 to 20 ft) tall. The
weevil is not a problem on the Queen Charlotte Islands or in Alaska,
possibly because there is insufficient summer heat to allow its
development (22). The spruce aphid (Elatobium abietinum) feeds on
Sitka spruce from California to Alaska and is a pest of ornamental trees.
Epidemics are sporadic and short lived. A root-collar weevil (Steremnius
carinatus) girdles l- and 2-year-old seedlings, causing some losses.
The spruce beetle (Dendroctonus rufipennis) periodically damages
stands throughout the range and is a major pest of spruce in British
Columbia. In addition, damage from a number of defoliators and other
insects is common (13).
Sitka spruce is highly susceptible to decay when injured (18). In the
past, most emphasis has been on studies of decay in old-growth stands, but
currently interest is shifting to young, managed stands. Some of the
organisms causing decay in old growth (for example, Heterobasidion
annosum and Armillaria mellea) can also cause root rot in
young stands. Heterobasidion annosum infects freshly cut stump
surfaces, and in Europe the tendency for plantation-grown Sitka spruce to
develop H. annosum butt rot is well known.
Foliage and stem diseases are usually of minor importance. Several rusts
cause occasional light to moderate defoliation, witches' brooms, or loss
of cones. Seed and seedling diseases are probably most important in
production of containerized seedlings in greenhouses.
Sitka spruce is damaged at various locations by animals such as elk,
bear, deer, porcupines, rabbits, hares, and squirrels. In general, these
problems are more serious in the southern part of the range. Deer are
generally more troublesome in the southern part, porcupines in the
northern part (25). Spruce is often less damaged than its associates.
Few growth abnormalities have been reported, although large tumorlike
growths on stems have been reported in Washington, and they occur in
Alaska as well. The causal agent is not known.
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Flowering and Fruiting
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Individual Sitka spruce may occasionally
produce cones before 20 years of age, but cone bearing in stands usually
does not begin until ages 20 to 40 (24). Sitka spruce is monoecious;
female strobili (cones) are usually produced at the ends of primary
branches near tops of trees; male strobili are usually produced at the
ends of secondary branches lower in trees. Both may be on the same branch.
Reproductive buds are initiated in early summer of the year preceding
pollination and seed ripening, and heavy cone crops have been explained in
terms of early summer drought the preceding year. Cones ripen in the year
they were pollinated. Pollen is shed from the last week in April in the
southern portion of the natural range through early June in the extreme
northwest part of the range. Time of flowering is mainly related to
temperature.
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Genetics
provided by Silvics of North America
In addition to the clinal latitudinal difference in growth rate, cone
characteristics such as size, length-to-width ratio, angle of sterigma,
and phylotaxy also vary with latitude (4).
Variation in wood characteristics has been reported by provenance,
region, site, and individual trees. Although no comprehensive heritability
studies have been completed, Sitka spruce shows considerable variation in
wood density, tracheid length, and grain angle. Improvement in these
characteristics through breeding appears feasible. Selection for vigor
tends to favor trees of lower-than-average specific gravity but has no
effect on tracheid length (15).
Provenance studies show that- at a given planting site- northern,
inland, and high-elevation sources are the first and the most variable in
breaking dormancy. Dormancy appears to be influenced by photoperiod, and
northern provenances are the first to enter dormancy. Total seasonal
height growth is positively correlated with the time interval between
flushing and dormancy. When moved north, introduced southern sources make
better height growth, but they may be subject to frost damage if moved too
far or planted on exposed sites. Once dormant, Sitka spruce is able to
endure very low temperatures without damage. Sitka spruce from northern
provenances may be more resistant to freezing than those from southern
provenances. Dormant leaves from a Bellingham, WA, source withstood
temperatures to -30° C (-22° F), whereas a Juneau, AK, source
withstood temperatures to -40° C (-40° F). Twigs of the two
sources withstood temperatures to -40° C and -60° C (-40° F
and -76° F), respectively (27).
Only limited data are available on genetic variation between individual
trees. Assessment of first-year characteristics of progeny from a diallel
cross among six trees showed that characters affecting tree form were
inherited in a predominantly additive fashion; characters reflecting tree
vigor were under "additive, dominance, and maternal control"
(28). Self-pollinated progeny showed growth depression caused by
inbreeding (28).
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Growth and Yield
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Height growth is slow for the first few years
but increases rapidly thereafter. On average sites in southeast Alaska,
trees can be expected to reach about 27 m (90 ft) in height within 50
years after attaining breast height (7). Average site index at elevations
near sea level varies inversely with latitude, declining from 48 m (158
ft) at base age 100 years in Lincoln County, OR, to 33 m (108 ft) in
southeast Alaska, at the rate of about 1 m (3 ft) per degree of latitude
(8). Observations within the natural range of spruce show that growth rate
also declines with increasing elevation.
Height growth of Sitka spruce and western hemlock are nearly equal
during the period of most rapid growth, but spruce grows more rapidly in
diameter. Consequently, thinning from below tends to favor spruce. Spruce
continues to maintain height growth longer than hemlock and lives longer.
Few hemlock live more than 500 years; Sitka spruce may live to 700 or 800
years. Very old spruces eventually assume a dominant position in
old-growth hemlock-spruce stands.
Sitka spruce trees often attain great size. In Alaska, mature trees near
sea level may exceed 61 m (200 ft) in height and 3 m (10 ft) in d.b.h. In
Oregon, a tree 87 m (286 ft) tall was reported (24). The largest tree on
record is located near Seaside, OR. It is 5.1 m (16.7 ft) in d.b.h. and
65.8 m (216 ft) tall and has a crown spread of 28 m (93 ft) (17).
Stands in which Sitka spruce is a major component tend to be dense, and
yields are high (21,30). Stand volumes can be impressive. One plot in a
147-year-old hemlock-spruce stand in coastal Oregon contained, on an area
basis, 188 spruce and 32 hemlock/ha (76 spruce and 13 hemlock/acre). Total
volume was 2380 m³/ha (34,000 ft³/acre). Spruce averaged 64 m
(210 ft) in height and 86 cm (34 in) in d.b.h., and hemlock averaged 44 m
(144 ft) in height and 46 cm (18 in) in d.b.h (24).
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Reaction to Competition
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Sitka spruce is more tolerant of shade
than Douglas-fir but less tolerant than hemlock. Depending on latitude,
Sitka spruce has been described as being in the tolerant and intermediate
shade-tolerant classes. Overall, it probably can most accurately be
classed as tolerant of shade. Since reproduction under mixed stands is
predominantly hemlock, there is a tendency for this more tolerant species
to eventually dominate the site. Few climax stands proceed to pure
hemlock, however; in time, small openings, usually caused by blowdowns,
develop, allowing reproduction of spruce. The combination of greater
stature, greater longevity, and occasional stand disturbance is enough to
assure a scattering of spruce in the overstory of most climax
hemlock-spruce stands.
Sitka spruce is one of the few conifers that develop epicormic branches
along the stem. Production of these sprouts is related to light intensity,
and roadside trees often develop dense new foliage from base to crown.
Thinning stimulates epicormic branching and could decrease the quality of
the wood, although this is not a problem in production of pulp or
dimension lumber. In deep shade, lower limbs soon die, decay, and break
off, but the resinous branch stubs remain for many years.
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Rooting Habit
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Roots will grow where moisture, fertility,
aeration, and mechanical soil properties are favorable. Consequently,
there is great variability in root form-from flat platelike roots to deep,
narrow-spreading roots (12). Where soils are shallow, soil temperature and
fertility low, and water tables high, shallow rooting is by far the most
common form. Deeper rooting does occur, however, where soils have good
drainage and depth to water table. Rooting to depths of 2 m (6 ft) has
been reported (5).
Sitka spruce commonly produces long lateral roots with few branches and
rapid elongation (20). Annual elongation rates of 42 to 167 cm (16 to 66
in) have been reported (3). Lateral roots up to 23 m (75 ft) in length
have been observed in Alaska (15). Root grafting occurs between roots of
the same tree and between adjacent trees. It is fairly common to find
living stumps sustained by root grafts from adjacent trees. Adventitious
roots develop on trees growing along streams where alluvium is deposited
by periodic flooding. Roots are vulnerable, however, to compaction and
lack of aeration. Spruce are frequently killed by permanent flooding
caused by beavers, and often valuable ornamental and roadside trees are
killed when landfill is deposited around them. Containerized nursery stock
has been successfully inoculated with the mycorrhizal fungi, Laccaria
laccata and Cenococcum geophilum (29).
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Seed Production and Dissemination
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Seeds of Sitka spruce are
small, averaging 463,000/kg (210,000/lb) (26). Seeds ripen in southeast
Alaska during late August or early September, and dispersal usually begins
in October. Cones open during dry weather, release seed, and reclose
during wet weather. One study showed that 73 percent of the seed was
released within 6 weeks of the first dispersal date, and the remainder was
released over 1 year (15). Good crops occur at 3- to 5-year intervals in
the southern part of the range and at 5- to 8-year intervals in Alaska.
Cone and seed production in seed orchards can be increased by treating
trees with gibberellin (31). Dispersal distance depends on several
factors, including height and location of the seed source, local
topography, and wind conditions. Reported dispersal distances range from
0.8 km (0.5 mi) when a seed source was on high ground, to 30 m (100 ft)
when seed was released from the edge of a clearcut area (15).
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Seedling Development
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Under natural conditions, seed germinates
on almost any seedbed, but survival may be low. Germination is epigeal
(26). A mineral soil or mixed mineral and organic soil seedbed is usually
considered best for germination, especially under light shade, as long as
drainage is adequate and the soil provides sufficient nutrients for tree
growth. Fine-textured soils combined with a high water table are suitable
for germination but may be unsuitable for seedling establishment because
of frost heaving. Coarse-textured mineral soils in unshaded conditions may
dry out excessively but may improve after invasion by hair mosses that
bind the soil surface and provide shade. Organic seedbeds are suitable in
shade but are unsuitable in the open if subject to severe moisture
fluctuations. On alluvial sites having high water tables and subject to
frequent flooding, where competition from brush is severe, rotten wood may
be the only suitable seedbed.
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Soils and Topography
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Sitka spruce grows on Entisols, Spodosols, Inceptisols, and Histosols,
on soils derived from a wide variety of parent material. The species
requires relatively high amounts of available calcium, magnesium, and
phosphorus, and grows best where soils are derived from rocks rich in
calcium and magnesium (19). Best development is on deep, moist,
well-aerated soils. Drainage is an important factor, and growth is poor on
swampy sites. Sitka spruce commonly occupies alluvial soils along streams,
sandy or coarse-textured soils, or soils having a thick accumulation of
organic material. Soils are usually acidic, and pH values of 4.0 to 5.7
are typical. Spruce is an early pioneer on immature soils recently exposed
by glacial retreat or uplift from the sea. It is more tolerant of ocean
spray than are associated trees and often occupies a prominent position on
exposed headlands and beaches along the outer coast (2). In Oregon and
Washington, spruce follows lodgepole pine (Pinus contorta) in
succession on coastal sand dunes as they become stabilized by vegetation.
On highly disturbed sites, it frequently becomes established concurrently
with red alder (Alnus rubra) or Sitka alder (A. sinuata), gradually
succeeding the alder as stands are eventually overtopped.
Sitka spruce grows from sea level to treeline in Alaska, at elevations
ranging from 910 m (3,000 ft) in southeast Alaska to 300 m (1,000 ft) in
Prince William Sound. High mountains of the coast ranges lie close to the
sea, forming a barrier to moist, onshore winds and providing abundant
moisture during the growing season. Spruce is limited in elevation by the
short growing season at treeline. South of northern British Columbia,
spruce is restricted to low elevations near the sea where moist maritime
air and fog help provide moisture during summer. For the most part, high
mountains that otherwise might offer suitable habitat lie farther inland
where more continental conditions of summer drought and warmer
temperatures are unsuitable for growth. Exceptions are on the Olympic
Peninsula and in valleys in the Cascade Range off Puget Sound in
Washington, and on isolated peaks in Oregon. On the Olympic Peninsula,
Sitka spruce rarely grows above 610 rn (2,000 ft) in elevation (1).
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Special Uses
provided by Silvics of North America
High strength-to-weight ratio and resonant qualities of clear lumber are
attributes that have traditionally made Sitka spruce wood valuable for
specialty uses, such as sounding boards for high-quality pianos; guitar
faces; ladders; construction components of experimental light aircraft;
oars, planking, masts, and spars for custom-made or traditional boats; and
turbine blades for wind energy conversion systems.
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Vegetative Reproduction
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Asexual reproduction by layering occurs
under natural conditions and in plantations, but layering is most likely
to occur on very moist sites at the edges of bogs or near timberline.
Asexual propagation can be done by air-layering or rooting of stem
cuttings. Clones differ in their ability to root or graft, and clones that
graft easily do not necessarily root easily and vice versa. Cuttings from
shoots of the current year root more easily than cuttings from older
branches (15).
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