FOREST COVER TYPES OF. THE
UNITE t.r§ ATES AND CANADA
Douglas-fir- Western Hemlock
Society of l\tleF.n�i'rP
ditor
230
Definition and composition. -Coast Douglas-fir
and western hemlock are both present in substantial
amounts in this mixed-species type , and together
comprise at least 80 percent of the stocking.
Douglas-fir usually is predominant (especially on
drier sites), but hemlock may be so on more moist or
less fertile sites.
The most common associated conifer is western
redcedar. Grand fir, western white pine, and Pacif­
ic silver fir are also quite often associated. Sitka
spruce is frequently present at low elevations and
noble fir is sometimes present at elevations above
000 m (2,000 ft.) in the Oregon and Washington
Cascades.' Lodgepole pine may be an associate on
drier sites. Western (Pacific) yew is often present,
but always in a su rdinate position. Several hard­
wood species may be present in small quantities.
These include red alder, bigleaf maple, black cot­
tonwood, giant chinkapin, Pacific madrone, vine
maple, and cascara buckthorn.
Geographic distribution.- The Douglas-fir­
western hemlock type occurs west of the Cascade
Range from Vancouver Island and the British Co­
lumbia coast southward through western Washing­
ton and western Oregon. The northern limit is
around Bella Coola (52° north· latitude). In Oregon
the type is split into two major segments-Coast
Ranges and Cascades. The southern limits, except
for a narrow coastal strip that extends into northern
California· (41 ° north latitude), are the Klamath
Mountains on the coast and the divide between the
North and South Umpqua rivers in the .Cascades.
The type may be found at elevations ranging from
sea level to about 1,100 m (3,600 ft.) (Franklin and.
Dyrness 1973).
Ecological relationships. - This cover type falls
mostly within the Tsuga heterophylla forest zone
defined by Franklin and Dyrness (1973). It also extends into their Picea sitchensis and Abies amabilis
zones. These three forest zones are essentially anal­
ogous to Krajina's (1965) coastal western hemlock
and wet subzone of the Douglas-fir biogeoclimatic
zones to which the type is limited in British Colum­
bia (Packee 1976). The area occupied by this
.
als.o loosely corresponds with Mernam,s humid
transition life zone as defined by Scott (Barrett
.
1962).
.
This type thrives in mild, humid climates. Mean
annual precipitation ranges from about 1,200 to
4,000 mm (50 to 16.0 in.); it may be as much as
6 000 mm (235 in.} at some places in the north.
Commonly less than 10 percent of the precipitation
falls during the summer. At lower elevations there is
very little snow whereas at higher elevations snow­
fall may avera e more than 2 m (7ft.}. Within the
major part of the type's range, mean annual tem­
perature is 7° 'to 10° C. Neither winter nor summer
temperatures are extreme. Soils are derived from a
variety of parent materials, but most are at least
moderately deep and of medium acidity. Surf ce
horizons are well aggregated and porous, With
moderate to high organ,ic content (Franklin and
J)yrness 1973).
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·
g
Rs
t I'I:3
F. H. Fyre,
1980
·Stands of this type can be either mid-phases in
the
natural succession from seral Douglas-fir to clim
ax
hemlock- western redcedar (or hemlock), or
even­
aged forest that became established after fire
or
other disturbance. At advanced ages, the Dougl
as­
fir usually dies out and is replaced by more
shade
tolerant species, but on many drier sites the type
persists. Douglas-fir is a long-lived species, and
ages
in excess of 500 years are not uncommon.
Both
Douglas-fir and western hemlock succumb to crow
n .
fires. but the thick bark of older Douglas-fir res
ists
surface fires (Fowells 1965). A new
understory of
hemlock, western redcedar, and true
firs often be­
comes established under the survivin
g old-growth.
Douglas-fir. These understory
species require a
moist site for proper development.
Following denudation by fire or
logging, dense,
nearly pure, even-aged stands of
Douglas-fir com­
monly develop on areas previously
occupied by the
Douglas-fir-western hemlock
type. This conver­
sion to pure Douglas-fir is now
being further en­
couraged by extensive planting of
Douglas-fir after
logging. Hemlock, however, ofte
n seeds in at about
the same time and thus tends to
recreate the mixed
type Sometimes these hemlocks beco
me a substan­
tial component of the main canopy
. At other times
particularly where they ar(i stres
sed by high tern:
perature or low soil moist\ll'e, hem
locks, although
numerous, remain in the understo
ry (Packee 1976).
Variants and associated vegetatio
n. -There are
many variations, involving both the
relative impor­
tance of the two component spec
ies of the type and
associated tree and understory
species. For ex­
m:ple, there is a rather regular incr
ease in Douglas­
frr Importance from north to sou
th in response to
decreasing moisture (Franklin
and Dyrness 1973).
This type may merge with several
other types, in­
cluding the Pacific Douglas-fir type
on the drier
sites and the western redcedar-w
estern hemlock
type the wetter sites. At these transitions
it may
.
be diffiCult to assig
n type names with accuracy
.
Within other types, Douglas-fir
and western hem'lock often occur together, but
generally they are
overshadowed by other species.
Shrubs commonly associated
with the type in­
clude salal, Oregongrape, red
huckleberry, 0\·alleaf
huckleberry, salmonberry,
thimbleberry, devils­
club,and rhododendron. Ass
ociated herbaceous
species include swordfern, dee
r fern twinflower
'
'
and Oregon oxalis.
.
·
·
DoNALD L. BEuKEMA
USDA Forest Service
Pacific Northwest Forest
and
Range Experiment Station