106
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bNITED STATES AND CANADA
Society of American Foresters
F, H. Fyre,
ditor
1980
Nom-H PAan;E'OREST COVER 'l'YPES
WESTERN Foru:sr Covrn 'IYI'n
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Pacific Douglas-Fir
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Definition and composition. -Pacific Douglas-fir
is one of the most well known, extensive and, in vol­
ume of wood, highly productive forest types in the
world. Stands that comprise the type contain at
least 80 percent Douglas-fir. Other species that may
occur in light mixture include western hemlock,
grand fir, Pacific silver fir, white fir, noble fir, Cal­
ifornia red fir, western redcedar, bigleaf maple, Pa­
cific madrone, red alder, tanoak, Oregon white
oak, giant chinkapin, vine maple, redwood, Port
Orford-cedar, ponderosa pine, we5tern white pine,
incense-cedar, sugar pine, and Sitka spruce.
Geographic distribution. -The type is present in
southern British Columbia, but it is widespread
west of the summit of the Cascade Range in Wash­
ington and Oregon south to approximately 43°.
north latitude. Farther south, a drier, less extensive
portion of the type becomes increasingly intermixed
with other types. In discontinuous stands, it contin­
ues south throuJ;!h the Klamath Mountains of Ore­
gon and northern California through the Coast
Range to the vicinity of Mendocino County, Cali.
fornia. Although pure stands o1Douglas-fir occur in
the southern Cascades and the Sierra Nevada, they
are small, disjunct, and probably should be consid­
ered local phases of other types containing this
species.
Elevation of the type is generally from sea level to
about 500 m (1,500 ft.} in the northern part of its
range and to about 2,000 m (6,000 ft.) in t_he south.
ern. Annual precipitation ranges from about 600 to
3,000 mm (23 to 120 in.) (Franklin and Dyrness
1973).
Ecological relationships. -As its wide latitudinal
and elevational_ ranges show, the type is capable of
a apting to diverse site conditions. Where the type
is excluded, the reason may be excessive heat (Sileo
1960, Hallin 1968), drought (Isaac 1938, Tappeiner
1966), cold (Cleary and Waring 1969), too much ;
moisture, or high water tables (Minore 1968). The :
type generally occurs on moderately deep and welldrained soils derived from highly variable parent t
materials (Franklin and Dyrness 1973).
, Natural Douglas-fir stands typically regenerate
after wildfire in partial.shade· cast by fire-killed
trees. Seed is provided by scattered surviving trees :
or islands of trees. The young stands that result '
often are so dense as to crowd out competing vege- i
tation. As stands develop to pole size or larger, mor­
tality from snow or 1ce breakage, small fires, root
pathogens, bark beetles, windthrow,· and suppres­
sion periodically relieves overcrowding. Mature
stands can remain healthy for decades. Finally,
when mortality h caused openings in the canopy,
the climax species, typically western hemlock, west­
ern redcedar, and Pacific silver fir, become estab­
lished. Eventually they take-over the site unless
wildfires renew the cycle.
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In the absence of fire or other ·catastrophes, the
type can be maintained only through the actions of
man in creating mineral seedbeds necessary for
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seedling establishment and in opening dense cano.
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g pies to permit development of a new stand (Isaac
1943, Williamson 1973). W here canopies exceed
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Vi' taiping more shade tolerant species.
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Combining a moderate climate and ample pre
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species creates an extremely productive forest.
Stem-wood volume alone commonly exceeds
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1,3003/ha (about 150,000 bd. ft., Scribner rule) in
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old-growth stands.
Fire is a destructive agent in Do glas-fir, but it
prepares a seedbed for regeneration. However,
other agents, such as the Douglas-fir bark beetle
(Keen 1952) and the black bear (Poelker and Harty.rell 1973). cause mnch stand damaue vet leave the
torest floor undisturbed and do not provide the nee­
. essary mineral soil seedbed. Instead, they preserve a
ing surface better suited to competing conifers
and woody shrubs. U large open areas are created
and no Douglas-fir seed source is present, stands of
red alder and bigleaf maple may capture the area
(Newton et al. 1968)
Variants and associated vegetation.- Douglas-fir
occasionally forms uneven-aged but essentially pure
-stands and apparently exists as a climax in drier
areas occupied by the type. This climax situation
can be caused by topographic rain shadows, soil,
and climate characteristics that lead to droughty,
but not xeric, soil conditions. Such conditions occur
from the east sides of Vancouver Island (JCrajina
1959) southward through the east sides of the Olym­
pic Mountains and Coast Ranges, and through the
intermediate mountain slopes of southwestern Ore­
gon. to the southern limits of the type range (Dau­
bertmire 1978). Sometimes on these sites Douglas-fir
\s mixed with hardwoods, such as Pacific madrone,
. Oregon white oak or tanoak, and with conifers,
principally ponderosa pine. This mixing leads to a
merging with Douglas-fir-tanoak- Pacific rna­
drone and Pacific ponderosa pine- Douglas-fir
types The . former type may replace Pacific
Douglas-fir after wildfire or disturbance by man.
Douglas-fir climax stands often are associated
with oceanspray and salal. Grand fir, western hem­
lock, vine maple, Oregongrape, and swordfem are
common associates in more mesic, seral Douglas-fir
stands (Franklin and Dyrness 1973).
.
RicHARD L. WILLIAMSON
USDA Forest Service
Pacific Northwest Forest and
Range Experiment.Station