Reproduced from Proceedings of western Hemlock Management Conference,
of Forest Resources,
U.S.
/.his
University of Washington,
Department of Agriculture,
'
College
by the Forest Service,
May 1976,
for official use.
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NATURAL REGENERAT ION OF WESTERN HEMLOCK
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Introduction
Ruth and Berntsen (1955) examined seed fall
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in an 81-acre clearcut in the Cascade Head
Experimental Forest on the coast of Oregon.
They measured seed fall on a promontory in
the middle of the unit (about 14 cabins
from the edge) at the highest point in the
unit and found an average annual seed fall
of 2 43,000 per acre over a 4-year study
period. This average includes I year
considered a failure. Average annual pro­
duction in the adjacent uncut stand was
about 5,500,000 seeds per acre per year.
In 1941, A, P. MacBean of the British
Columbia Forest Service concluded, on the
basis of his studies, that western hemlock
(Tsuga heterophyZZa [Raf.l Sarg.) can be
satisfactorily regenerated under either some
form of selection cutting or clearcutting.
I've seen nothing in the literature since
that time nor In my own experience that
Ind®ed, the
contradicts that conclusion.
silvlcal characteristics of western hemlock
are those of a typical climax species and
suggest that Mr. MacBean's conclusion is
logical.
On the other hand, Hetherington (1965)
concluded that natural regeneration could
not be relied on because he thought that
4 of his 5 recorded years of seed fall on
Vancouver Island were failures. He used the
heavy seed fall for the 1959-1960 seed year,
his first year of observation, as the basis
for comparison for subsequent years. Thus,
Hetherington considered the crop for the
1962 seed year, for instance, a failure
because seed fall that year was only about 2
percent of that of 1959. Hetherington did
not report the actual number of seeds that
fell in the 1959 seed year, but records from
another area (Harris 196]) indicate the
number was at least 12 million viable seeds
per acre in uncut stands. Two percent of 12
million is 2 40,000 viable seeds per acre,
almost a pound of seed, probably an adequate
number for reforestation.
I will further substantiate Mr. MacBean's
findings by discussing the species' seeding
habits, seed bed requirements, seedl ing
density--both advance and postlogging--and
natural regeneration responses to site
preparation.
Seeding Habits
Amount and frequency of crops
Western hemlock is a frequent and prolific
producer of seed. Seed production generally
starts when trees are about 20 to 30 years
old (Ruth 1974). Records from southeast
Alaska show that a heavy seed crop can be
expected every 5 to 8 years, with some seed
11acBean
almost every year (Harris 1976).
(1941) examined two records of seed fall in
the Quatslno region of Vancouver Island,
Canada, one for 9 years and the other for 8
years. Neither record showed any seed
failures in the period 192 7-1938. Haig et
al. (1941) found fair-to-good cone crops of
western hemlock in the Inland Empire 7 years
out of 8 in the period 1927-1934.
Few of us, I am sure, realize how utterly
fantastic a heavy seed crop is in western
hemlock. Bumper crops in southeast Alaska
can yield as much as 82 pounds of sound seed
per acre (Harris 1969). Even a small per­
centage of such an enormous crop provides a
lot of seed. Hetherington cautions against
relying on natural regeneration in western
hemlock, but I think the consensus derived
from publ ished information indicates other­
wise and is valid.
My own records of seed fall in a shel terwood
study near Hoquiam, Washington, show only
two failures In the 6-year period from 1961
to 1966.
Including these 2 years of failure,
average annual viable seed fall under shelter­
, wood stands ranged from about I to 2 miIIion
¯er acre per year (Will iamson and Ruth 1976), even though cone crop reports publi­
shed by the Washington State Department of Natural Resources indicate that no crop for
the years 1961-1966 was considered a bumper
crop.
Patterns of seed fall
Dissemination of western hemlock °eed occurs
primari Iy in late fall and winter months
during dry, northeasterly or easterly winds
(Ruth and Berntsen 1955; Harris 1969;
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166
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Author
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PICIIARD L, I';ILLIAMSO,
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Forestry Sciences Laboratory- I :
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Hetherington 1965).
Some dissemination
seems to occur throughout the year over most
of the natural range of western hemlock,
although only 5 percent or less occurs
(Harris 1967; Williamson and Ruth 1976).
These areas of nonstocking are predictable,
and their treatment is possible either
through burning of slash, spraying, or
mechanical disturbance of brush.
during the summer months.
Along the Oregon
coast Ruth and Berntsen (1955) found that
dissemination of seed consistently started
during the last 10 days of October with half
the seed down by the middle of November and
90 percent by the first of February.
Seedling Survival
In the coastal hemlock zone Soos and Walters
(1963) and Ruth (1967) found that partial
shade had no Influence on germination or
subsequent seedl ing survival. My observa­
tions in the hemlock shelterwood study near
Hoquiam showed that light level does not
influence the number of seedlings until it gets down to about 2 percent of the 1ight in
.the open.
Seed losses to small animals
Gashwiler (1970) discovered that deer mice
consumer about 22 percent of western hemlock
seed fall in the Oregon Cascades.
This
consumption occurred just before or during
the germination process.
Predation of
western hemlock seed does not seem to be as
heavy as predation of seed of other species
with larger seed, such as Douglas-fir.
According to Gashwiler's results, animal
predation of hemlock seed does not appear to
be much of a deterrent to naturaI. regenera­
tion.
Most kinds of seedl ing mortal ity occur
primarily in the first 2 years of seedling
life.
Hardly any occurs from then until
seedlings are old enough to experience
Intense competition among themselves; this
competition leads to natural suppression
In the British Columbia Univer­
mortality.
sity Forest near vancouver, the main causes
of mortality appear to be either drought or
frost heaving (Soos and Walters 1963). S"U'
and Walters· also reported that al I new
germinants on burned humus were dead by mid­
July of the same growing season after germi­
nation.
This implies that heat injury is probably involved in death of very young
seedlings under clearcut and burn conditions.
Germination
Viability
Haig et al. (1941) found that seeds are
viable only over the first winter after they
are disseminated.
The v[able percentage of
total seed fall seems to be correlated with
the size of the seed crop; that is, it is
better in good years than in poor ones (Ruth
and Berntsen 1955).
Viability in an average
good seed year seems to vary between 36 and
55 percent, with an average of about 46
percent.
In drier and hotter inland regions Haig et
a!. (1941) found partial shade beneficial in
reducing mortality caused by high tempera­
tures and drought.
They found 55 percent
mortality due to these factors under partial
shade versus 95 to 100 percent mortal ity due to these same factors in the open or under
dense shade.
I infer similar results from
the shelterwood study near Hoquiam, where
death of germinants was much greater under
open overstory conditions than it was under
shade conditions. Seed bed requirements
In the coastal hemlock zone, western hemlock
seed appears to germinate well, and seedI ings grow well on almost all natural seed
beds whether they be rotten wood, undistur­
bed duff and I itter, bare mineral soi I, or
any mixtures of these.
The principal re­
quirement for adequate development on any
seed bed appears to be adequate moisture
(Soos and Walters 1963; Ðarris 1967).
In
drier situations such as in the Inland
Empire or Oregon, mineral soils appear to be
best for hemlock seedlings, presumably
because they are cooler and probably more
moist than organic matter seed beds.
Berntsen (1955) studied hemlock regeneration
wiÑh the objective of discovering why the
reproduction is typically clumpy.
He found
that the gaps were consistently attributable
to dense patches of brush or very deep slash
which eliminated seed bed conditions.
Underbrush, such as salmonberry, apparently
must be weI I established in the mature
stands before the harvest cut to prohibit
hemlocÒ regeneration after the harvest cut
167
Once seedl ings are over 2 years old, survival
appears to be very good with consequent adequate restocking.
MacBean (1941) found
in his survey of clearcuts on Vancouver
Island that 91 percent of the cutovers had
at least 1,000 seedlings per acre 4 years
after the harvest cut.
In his study area In
Oregon, Berntsen (1955) found a large clear­
cut 81 percent stocked 6 years after the
harvest cut.
WiII iamson and Ruth (1976)
found the largest numbers of seedlings in the shelterwood study near Hoquiam under
overstories of moderate densities where numbers of seedl ings were at least 25,000
per acre.
Farr and Harris (1971) found
simi lar numbers of seedlings after partial cutting of 96-year-old hemlock-spruce stands in southeast Alaska. The extremes of shelter­
wood treatment, the very dense and the very
open, resul ted in about 1,000 to 5,000
seedlings per acre.
Thinning regimes in western hemlock will
probably establish overstories similar to
the densest shelterwood stands and can be
expected to result in similar numbers of
advance seedlings. Far from being concerned
about inadequate numbers of seedlings in the
natural regeneration process in western
hemlock, foresters generally should be much
more concerned with holding down the numbers
of seedlings so as to avoid, as much as
possible, expensive precommercial thinning.
under open stands to increased soi I moisture
stress from Increased solar radiation.
Such stress along the coast is more.1 ikely
in the lower latitudes of Oregon than in
the higher latitudes of Washington, British
Columbia, and southeast Alaska.
Effects of Broadcast Burning
In the coastal hemlock zone both Berntsen
(1955) and Hetherington (1965) found more
seedlings in burned areas than in unburned
areas. This increase in number of seed­
lings was due to elimination of brush compe­
tition and reduction of dense patches o f
slash. O n the other hand Soos and Walters
(1963) found all new germinants on burned humus
dead by mid-July near Vancouver, B.C. Harris
(1966) found fewer seedlings on burned than
on unburned seed bed conditions, and burning
resulted in better seedling distribution.
Obviously, effects of burning on seedling
numbers vary according to aspect, slope,
latitude, geographical region, climate, etc.,
but most reports do not consider all these
variables. Thus, slash burning may or may
not help natural regeneration.
Advance Regeneration
In their concern over newly established
seedlings in the reforestation process of
western hemlock, foresters sometimes forget
In surveying
about advance regeneration.
cutovers on Vancouver Island, MacBean (1941)
found that advance regeneration constituted
45 percent of all regeneratIon 2 years after
He also found that new regene­
harvest cuts.
ration continues to come in after the har­
vest cut for at least 10 years, thus dilu­
ting the proportion of advance regeneration
in the total regeneration' picture. Harris
(1967) found an average of 4,000 advance
regeneration per acre after harvest cutting
in a decadent old-growth stand in Alaska.
The decadence and thus openness of the stand
probably contributed to this relatively
large number of advance regeneration seed­
lings. He found that although there were
some decreases in numbers of advance regene­
ration over time, these seedlings maintained
dominant positions throughout the regene­
ration period relative to newly established
seedlings.
In addition, MacBean (1941) and Harris
(1966) found that burning retards.repro­
duction for a period equivalent to 5 to 10
years because burning eliminates the
advance regeneration.
Summary
Wil I iamson and Ruth (1976) found about 2,000
advance regeneration seedlings (2 feet tall
or less) per acre in the shelterwood study
area near Hoquiam before any cutting. A fter
the first shelterwood cut, 650 advance
regeneration seedlings per acre survived.
These seedlings responded to release right
away and were consistently taller than newly
established seedlings during the 10 years of
observation in this study area.
Our opinion
was that advance regeneration wil I continue
to maintain height superiority over younger
If advance regeneration in other
seedlings.
places is Infected by mistletoe, it might
not demonstrate this height superiority.
Western hemlock is a prolific and consistent
It reproduces success­
caster of seed.
fully under a wlde range of seed bed condi­
tions and overstory densities ranging from.
about one-third removal, by basal area, to
clearcut. The species appears amenable
to the selection system. Established lesser
vegetation and deep slash inhibit regenera­
tion, but these conditions a±e readi Iy
observable and controllable by herbicides
or fire. Advance regeneration is usually
present in sufficient numbers at harvest
and wil I probably be dominant in the
stand throughout the next rotation if not
destroyed.
Forester should recognize that
if they plant genetically superior or common
stock, these seedl ings wi I I have a lot of
naturals to compete with.
Williamson and Ruth (1976) also found that
height growth of seedlings, both advance
and newly established, Increased with de­
creasing overstory density in the shelter­
woog study.
In contrast, Ruth (1967) had
found, in another shelterwood study in
Oregon, that height growth of seedlings
under the most open shelterwood stands de­
creased relative to height growth o f seed­
I ings under moderate overstory densities.
He ascribed this decrease in height growth
I
168
L ITERATURE C I T E D
Berntsen, Carl M.· 1955.
Seedling d Istri­
bution on a spruce-hemlock clearcut.
U. S. D. A. For. Servo Pac. Northwest For.
and Range Exp. Stn. Res. Note No. 119,
7 pp.
Portland, Oregon.
Hetherington, J. C.
1965.
The dissemination,
germination, and surviVml of seed on the
west coast of Vancouver Island from
western hemlock and associated species.
B. C. For. Servo Res. Note 39, 22 pp. ,
iI Ius.
Victor I a, B.C.
Farr, Wi Ibur A. , and A. S. Harris. 1971.
Partial cutting of western hemlock
and Sitka spruce In southeast Alaska.
U. S. D. A. For. Servo Res. Pap. PNW-124,
10 pp.
Pac. Northwest For. and Range
Exp. Stn. , Portland, Oregon.
MacBean, A. P.
1941.
A study of the
factors affecting the reproduction
of western hemlock and its associates
in the Quatsino Region, Vancouver
Island.
B. C. For. Serv. , Victoria,
B. C.
36 pp.
Gashwiler, Jay S.
1970.
Further study of
conifer seed survival in a western
Oregon clearcut.
Ecology 51 (5) :849­
854. Ruth, Robert H. 1974.
Tsuga ( Endl. ) Carr.
Hemlock.
In Seeds of woody plants in
the United States, pp. 819-827.
U. S.
Dept. Agric.
Agric. Handbk. No. 450.
Washington, D. C.
. Haig, Irvine 1., Kenneth P. Davis, and
Robert H. Weidman.
1941.
Natural
regeneration In the western white
pine type.
U. S. D. A. Tech.
Bull.
No. 767, 99 pp.
Washington, D. C.
Ruth, Robert H. , and Carl M. Berntsen.
1955.
A 4-year record of Sitka spruce
and western hemlock seed fall on the
Cascade Head Experimental Forest.
U. S.D.A. For. Servo Pac. Northwest
For. and Range Exp. Stn. Res. Pap.
No. 12, 13 pp.
Portland, Oregon.
1966.
Effects of slash
Harris, A. S.
burning on conlfer regeneration in
southeast Alaska.
U. S. D. A. For.
Servo Res. Note NOR-18, 6 pp.
Northern For. Exp. Stn. , Juneau,
Alaska.
Ruth, Robert Harvey;
1967.
Differential
effect of solar radiation on seedling
establishment under a forest stand.
Ph. D. thesis, Oregon State Univ.,
Corvallis.
165 pp.
Harris, A. S.
1967.
Natural reforestation
on a mile-square clearcut in southeast
Alaska.
U. S.D. A. For. Servo Res. Pap.
PNW-52, 16 pp.
Pac. Northwest For.
and Range Exp. Stn. , Portland, Oregon.
Soos, J., and J. Walters.
1963. Some factors affecting the mortality of western hemlock and western red cedar germinates and seedlings. Univ. B.C. , Fac. For. Res. Pap. No. 56, 12 pp.
Vancouver, B. C. 1969.
Ripening and dispersal
Harris, A. S.
of a bumper western hemlock-Sitka spruce
U. S. D. A.
seed crop in southeast Alaska.
For. Servo Res. Note PNW-I05, I I pp.
·
pac. Northwest For. and Range Exp.
Stn. , Portland, Oregon.
1976.
Williamson, Richard L. , and Robert H. Ruth. Results of shelterwood cutting 1976.
in western hemlock. U. S. D. A. For. Servo Res. Pap.
PNW-20I, 25 pp. Pac. Northwest For. and Range Exp. Stn. , Portland, Oregon. Personal communication. 169
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