Purchased by USDA Forest Service for Official Use
VoL 30. No.1. 1984. pp. 117-124
Copyright 1984. by the Society of American Foresters Form Sci••
Young Stand Development in Coastal
Western Hemlock as Influenced by
Three Harvesting Regimes
LARRY L. JAECK
CHADWICK DEARING OUVER
DEAN S. DEBEll
ABSTRAcr.
Growth patterns of naturally regenerated western hemlock (Tsuga heterophylla [Raf.l
Sarg.) were studied by stem analysis following partial cutting of a dense, second-growth stand on
the Washington coast. Three partial cutting densities comparable to thinning, shelterwood cutting,
and seed-tree cutting were established when the stand was 60 years old; final overstory removal
occurred 8 years after the initial cut (and 4 years after an intermediate cut) in all treatments.
'
Stocking resulting from each treatment was similar and excessive, and ranged from 12,400 to
212,500 stems per hectare. It consisted predominantly of trees established between the initial and
final cuts. Eight years after the final harvest cut, heights, ages, and diameters of dominants in the
regenerated stand were greatest in the light (most open) treatment; trees were shorter, younger,
and smaller in diameter with progressively denser overstory treatments. Although rather dife
f rent
in present appearance, the young stands followed similar patterns of canopy differentiation, dom­
inant height growth, and dominance assertion after the final overstory removal. The dominant
stems consistently asserted themselves over other stems after 2 meters height. FOREST SCI, 30:
117-124.
ADDITIONAL KEY WORDS.
Tsuga helerophy/la. stand dife
f rentiation, natural regeneration, shel­
terwood, advance regeneration. INCREASED TIMBER VALUES and recognition of the species' volume growth potential
have stimulated interest in western hemlock (Tsuga heterophylla [Raf.l Sarg.)
management (e.g., Atkinson and Zasoski 1976). Several major forest owners now
intentionally regenerate the species on cutover lands. Regeneration options may
range from clearcutting with advance regeneration, natural reseeding, or planting
to varying degrees of partial cutting to stimulate reproduction prior to a subsequent
final harvest.
To choose wisely among the options, foresters must know how different har­
vesting methods affect establishment and development of young hemlock stands.
In particular, information is needed on stocking, height and diameter growth
rates, and patterns of canopy differentiation and dominance development in ju­
venile hemlock stands. Such information is also useful in developing prescriptions
for precommercial thinning.
I
The authors are, respectively, former graduate research assistant and Associate Professor of Silvi­
culture, College of Forest Resources, University of Washington; and, Principal Silviculturist, Pacific
Northwest Forest and Range Experiment Station, USDA Forest Service, Olympia, WA 98502. This
study was supported by the U.S. Forest Service through University of Washington-U.S. Forest Service
Cooperative Agreement 16 U.S.C. 581, Supplement No. 131. Fieldwork was done on the Hemlock
Experimental Forest, owned by St. Regis Paper Company and managed in cooperation with the U.S.
Forest Service. Manuscript received 11 March 1982.
VOLUME 30, NUMBER 1, 1984/117
,f
A recent study examined hemlock reproduction following a sequence of partial
cuttings in a 60-year-old stand near the Washington coast (Williamson and Ruth
1976). The study was designed primarily as a test of shelterwood cutting in the
coastal hemlock type; and treatments included 12 residual densities, ranging from
9 to 54 square meters of basal area per hectare. In their final assessment, 2 years
after the overstory removal cut, Williamson and Ruth (1976) found that hemlock
seedlings were abundant in all residual density treatments but that progressively taller trees occurred under the increasingly open overstories. The young stands reproduced in the above mentioned study were well suited
for additional research on stand development as influenced by harvesting method.
Conditions established in some of the residual overstory density treatments were
similar to those which would be created by thinning (54 m2/ha basal area), shel­
terwood cutting (37 m2/ha basal area) and seed-tree cutting (9 m2/ha basal area).
We therefore conducted a follow-up study in the three density treatments men­
tioned above to: (i) assess effects of previous overstory densities on development
of the new stands and (ii) examine stand differentiation processes in juvenile
hemlock stands. Stem-mapping and stem-analysis techniques were used in tracing
the development patterns in the young stands which ranged in average age from
11 to 15. Our findings and their implications for hemlock management are herein
presented. Study Area.-The Hemlock Experimental Forest is approximately 21 km north of Hoquiam , Washington, and is part of the coastal Picea silchensis Zone described
by Franklin and Dyrness (1973). The original overstory was pure western hemlock
which developed after a clearcut and fire about 1900. Rainfall is approximately
250 cm annUally. Soils are Hoquiam clay loam-moderately deep, well-drained,
and acidic (pH 4.6-4.8) in the surface layer (Dimock and Herman 1963).
The previously mentioned shelterwood study was conducted on the Hemlock
Experimental Forest between 1960 and 1971. A 28-ha study area was subdivided
into twelve compartments of about 2.4 ha each; unreplicated partial cuts to dif­
ferent residual densities were randomly assigned to each compartment. The first
partial cut was made in 1960-61; the second in 1964-65; and the remaining
overstory was removed in 1968-69.
Experimental Methods.-Our study was conducted in three of the compartments
8 years after final overstory removal. The compartments included the extremes
of light and heavy residual overstories and an intermediate cutting intensity. Stand
data including number of trees and basal area per acre are in Table 1. In our
paper, "light overstory" refers to a heavy partial cut similar to a seed-tree cut.
The "dense overstory" left many standing trees after two preliminary cuts, similar
TABLE 1. Overstory treatments leading to regeneration on the Hemlock Exper­
imental Forest.
Residual overstory trees
1960-61
__...__....._
Light overstory
Medium overstory
Dense overstory
118 / FOREST SCIENCE
Treatrr
Light
over
Mediu:
over
Dense
over:
&Tre
b
Do,
to co:
simil
relati Tw
METHODS
Treatment
TABL
remor
79
222
457
.
1964-65
treeslha
_.__
42
99
272
Residual overstory basal area 1960-61
. _........_.__..
9
37
54
1964-65 m21ha .---'-"-" 10 17
36
grid \\
were
mapJ:'
condi
tagge(
A ste'
secon
projel
stems
Sta.
from
condi
1.
2.
3.
4.
5.
An
stern
0.1-0
locate
StUl
exarUl
had b
used
partrr:
analy
differ:
I
.,
TABLE 2.
Characteristics o/young hemlock stands 8 years after final overstory
removal. a
.\.
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Stocking
( stems
1.37 m tall)
Treatment
Light
overstory
Mean
___
Range
Slemslha
Age
(2 tallest
stems/plot)
Mean
____
Range
Years
_
Height
(2 tallest
stems/plot)
Mean
_
Range
Meters
Dbh
(2 tallest stems/plot)
Mean
___
Range
Centimeters ........
34,595 A 12,3 60-138,380
15.1 A
13-17
8.4 A
62,7 6 6 A 12,3 60-212,515
13.6B
10-17
5.6B 4.1-7.1
4.30B
2.49-7.51
4.5C 2.8-7.1
3.08C
1.07- 6.64
6.3-11.9
7.79 A 3.17-14.68
Medium
overstory
Dense
overstory
•
51,399 A
17,300-145,795
l1.2C
Treatment means followed by same capital letter
8-1 6b
are
not significantly different at P
:5
0.05.
b Does not include one 49 -year-old stem which was released by the first cutting.
to commercial thinnings. The "medium overstory" was intermediate and was
similar to a shelterwood cutting. Patterns of tree interactions were assessed in
relation to overstory densities.
Twenty-four to 26 milacre (0. 0040 ha) plots had been established on a systematic
grid within a central acre of each compartment. Percent stocking, slope and aspect
were recorded from 1961 until 1971. Location and type of regeneration were
mapped. In 1977 a subsample of ten plots were selected to represent the range of
conditions in each compartment. All stems over breast height (1.37 m) were
tagged, mapped, and related to the "crown influence zone" of neighboring stems.
A stem was in the influence zone of a second tree if a branch or branches of the
second tree overlapped one-quarter or more of the lesser tree's horizontal crown
projection at a point at or above two-thirds of the lesser tree's total height. Tagged
stems were cut at the base and total tree heights recorded.
Stand development patterns were determined by stem analyses of 20-30 trees
from each plot. The stems were selected to fulfill one or more of the following
conditions on each plot:
1. The tallest and second tallest stem.
breast height of each age on the plot.
2. The tallest stem
3. Any stem
17 years old (stems beginning before the first thinning).
4. The tallest stem within the crown influence zone (defined earlier) of each of
the two tallest stems/plot.
5. The shortest stem
breast height within the crown influence zone of the
two "dominating" (tallest or second tallest) stems/milacre.
Annual height growth pattern was traced over the entire length of each selected
stem using bud scars or other morphological features or by sectioning stems at
0.1-0.5 m intervals until the points of cessation of yearly height growth were
located.
Student'S t-tests, correlation analyses, and graphical techniques were used to
examine the data. Despite lack of true replication (each partial cutting treatment
had been carried out in only one compartment), analyses of variance were also
used to examine the data regarding selected treatment effects. The three com­
partments were relatively uniform in site and original stand conditions, and such
analyses were deemed preferable to subjective interpretations regarding apparent
differences in treatment means.
VOLUME 30, NUMBER 1,
1984/ 119
I
.
A
n per
Number of stems
plot
Light overstory
No. of
stems
120
100
8
Ht. em)
5
80
60
40
20
()
��������W
ii
B
140
en 120
E 100
4)
....
80
en
60
....
o
40
G>
20
.Q
E
::r
10
Height of tallest stem
Z
Medium overstory
No. of
stems
Ht. (m)
10 E
4)
....
8 en
....
6
4
!
....
o
....
c
140
120
100 80 60 40 20 .J:.
C)
v(
t1:
Gi
Dense overstory
No. of
stems
Ht. em)
10
8
(e
::c
d
},
ST
6
o
II
4
o
2
57-59 60-61
.6.
Cut 1
a:
e'
68-69
Cut
2
.6.
Cut 3
2-year stem classes
FIGURE1.
Height of tallest stem and total number of living stems in 1977, averaged for 2-year stem
classes (germination years) by treatment. Vertical lines in bar for height indicate standard deviation,
arrows at bottom show mean times of overstory cuttings (cuttings in each case extended for 2-year
periods). No new stems entered after 1971.
RESULTS AND DISCUSSION
Young Stand Appearances. -Stocking was abundant (and probably excessive) and
was not significantly related to overstory treatment (Table 2). Heights and di­
ameters of the dominant (two tallest) stems per milacre plot, however, were
significantly different among the three treatments with the larger dominants oc­
curring in the lighter overstory treatments. Diameters were strongly correlated
(r 0.92) with heights of dominant trees. Age of dominants also differed signif­
icantly among treatments, and age and height of dominants were correlated (r
0. 74). Thus, differences in age of dominant trees may account in part for differences
in heights and diameters among treatments. Dominant trees were YQunger, shorter,
and smaller in diameter with progressively denser overstory treatments.
Current (1977) dominant and codominant stems generally became established
after the first cut but before final overstory removal (1968-69) in all treatments
(Fig. 1). Regeneration originating after final overstory removal and regeneration
predating the first overstory cuttings did not contribute significantly to the de­
=
=
120 I FOREST SCIENCE
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62
64
66
68
10
12
14
76
Year
FIGURE 2.
Average height growth patterns of dominant stems for each overstory treatment.
vel oping stands. (A single plot in the medium overstory contained nearly all of
the stems established during the 1957-59 period.) Seedlings in younger age classes
(established at later years) became increasingly important in both number and
dominance as residual density treatments varied from light to dense overstory.
Most dominant stems in the light overstory originated after the first partial over­
story cut (Fig. 1); and these stems were substantially taller than the tallest stems
of other age classes. Alternatively, in the medium overstory treatment differences
in heights of the tallest trees for each age class were less pronounced. In the dense
overstory treatments, heights of the tallest trees were even less variable among
age classes.
Possible explanations for the differences in age structure of the new stands
established under light and dense overstories include:
n
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1. Low numbers of initial regeneration and high light levels under the light
residual overstory may have allowed the first invading hemlocks to develop
rapidly, effectively capturing the site and thereby suppressing development
of seedlings germinating in subsequent years.
2. Some of the older regeneration that germinated after the initial cut under
denser residual overstories may have been physiologically unable to adjust
rapidly from low light levels to the high levels of light created by subsequent
overstory removals (c.f. Tucker and Emmingham 1977).
3. More logging damage probably occurred to the older age classes of repro­
duction in the m edium and dense overstories, since more trees were felled
and skidded in the second and final cuts. Such damage may have affected
height of seedlings surviving the injury, and also may have released younger
seedlings.
i
I
Height Growth Patterns.-Heightlage curves for the two dominant trees on all
plots of each treatment were constructed to determine whether the overstory
treatment had altered developmental patterns of the stand (Fig. 2). Although
average tree heights differed among overstory treatments at time of final cut and
8 years later, the curves appear nearly parallel after the final cut. The times required
for dominant trees to grow from 4 to 5 m (heights through which dominants in
all treatments passed after overstory removal) were 1.5, 1.4, and 1.3 years for the
light, medium, and dense overstory treatments, respectively. Such times were not,
however; significantly different among the three treatments. Thus, height growth
rates of dominant trees after release are similar when compared at a common size
VOLUME 30, NUMBER 1, 1984/ 12 1
,
.
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.
A
Meters
10.0
8.0
J:
en
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C
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Feet
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Feet
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30
30
Ir
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r
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a
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r
(
62
FIGURE 3.
64
66 68 70 72 74 76
63 65 67 69 71 73 75 77
Year
Year
(
A. Average height growth curves for the tallest stem (solid line) per plot and the tallest
of those stems which interacted with the tallest stem (dashed line)-LIGHT OVERSTORY. B.
Paired sample t-values (dashed line) and corresponding two-tailed values (solid line) for differences
between the selected stems of A. C. Average height growth curves for the tallest stem per plot (solid
line) and the shonest of those stems
2:
breast height which interacted with the tallest stem (dashed
line)-LIGHT OVERSTORY. D. Paired sample I-values (dashed line) and corresponding two-tailed
values (solid line) for differences between the selected stems of C.
.-
or developmental stage. Observed differences in present heights of dominants
among treatments were therefore associated primarily with differences in heights
achieved before overstory removal, and not with differences in height growth
patterns after release.
Canopy DifJerentiation.-Both at the time of final harvest (1969) and at the time
of the study (1977), height differences between the dominant tree and the next
two tallest ones were much greater in the light overstory treatment than in the
medium and dense overstory treatments. Heights of the second and third tallest
stems on each plot were therefore compared among treatments when height of
the dominant stem on each plot had been 4 or 5 m tall. At these heights, neither
the second tallest nor the third tallest stems differed significantly in height among
overstory treatments. These results suggest that observed differences in degree of
canopy differentiation are related primarily to the heights and ages of the young
stands of each treatment, and not to any' direct carry-over effect of overstory
treatment on young stand development.
Dominance Assertion.-The minimum age or size at which a tree shows domi­
nance over or becomes significantly taller than its neighbors is often a consider­
ation in timing of precommercial thinning operations. The dominant stem in each
milacre plot was compared with the tallest and shortest stems within its current
crown influence zone for each year after the stems were over 0.3 m tall by using
122/ FOREST SCIENCE
Iô ·
\..
reconstructed height/age curves (Fig. 3). Paired sample I-tests indicated that dom­
inant trees in the light overstory treatment became significantly taller than the
other stems before the dominant was 2 m tall. These results are similar to dom­
inance assertion patterns "for the medium overstory and dense overstory treat­
ments. Thus, the initial differences associated with overstory density do not affect
subsequent patterns of dominance assertion; and the dominant stems can be
identified after they are about'2 m tall.
Differences in "Effective Age. "-Although marked differences exist in present
appearance of young stands established under various overstory treatments, our
analyses of the historical development of these stands indicate that such differences
were caused primarily by differences in height and age of reproduction at the time
of overstory removal. In essence all stands developed in similar patterns once
they attained a certain height, regardless of prior overstory treatment. Variances
in stand characteristics were associated with differences in effective ages-the ages
of hemlock stands of comparable heights and development patterns which had
not been initially suppressed. Differences in times (after final overstory removal)
required for dominant stems of each treatment to reach a specific height (e.g., 3
m) represent differences in effective ages caused by the different treatments. Dom­
inants in the light overstory treatment were 3 m in height when the final harvest
cut occurred; those in the medium and dense overstory treatments required another
'3.9 and 5.8 years, respectively, to attain that height.
CONCLUSIONS AND MANAGEMENT IMPLICATIONS
On the productive sites in coastal Washington, hemlock stands are most com­
monly regenerated by clearcutting and natural seeding or planting. Our study
confirms that young hemlock stands can be established successfully by partial
cuttings which create overstory densities similar to those associated with thinning,
shelterwood cutting, and seed-tree cutting. Young stands established beneath
overstories associated with such partial cutting methods may differ markedly in
appearance at the time of the final overstory removal and for a period thereafter.
They will develop in similar patterns, however, and differ only in effective age.
Assuming that shade of the light overstory treatment (seed-tree cut) is negligible,
growth patterns of seedlings established naturally or by planting in clearcuts are
probably similar to those in the light overstory treatment. Relative desirability
of any of the above regeneration methods will vary with the specific forest con­
ditions and management objectives of forest owners. The differences in effective
age or growth lag in the young stands must be considered along with volume
available for subsequent harvests, logging costs, and mill needs. We believe partial
cutting methods will have most merit in one or more of the following situations:
1. Where
commercial thinnings are an essential feature of the management
plan;
2. Where incidence of dwarf mistletoe and susceptibility of partially cut stands
to windthrow pose relatively minor problems;
3 . Where one wishes to avoid planting costs (or perhaps "trade" these for later
precommercial thinning costs);
4. Where there is a lack of suitable planting stock for an area;
5. Where aesthetiCs or water quality are concerns and c1earcutting without
having previously established regeneration may be undesirable; and
6. Where brush and other undesirable competition needs to be excluded, and
would create a problem in other regeneration systems.
Advance regeneration are frequently found in mature hemlock stands. Other
studies (Wiley 1975, Oliver 1978) suggest that old hemlock advance regeneration
VOLUME 3 0, NUMBER
1, 1984/123
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may be able to respond to release. Regeneration systems which rely on devel­
opment of advance regeneration after clearcutting are useful in some forest types
(e.g., in many of the eastern hardwoods) and may be used successfully to regenerate
hemlocks in less dense, old-growth stands in the Pacific Northwest. In a very
dense, previously unthinned second-growth western hemlock stand in coastal
Washington, however, Williamson and Ruth (1976) had found a decrease in
advance reproduction, presumably caused by logging damage. Our subsequent
study in the same area revealed very few surviving stems of advance reproduction,
and these tended to be scarred, crooked, and did not grow well. We cannot
therefore recommend use of systems which rely on advance regeneration in such dense, previously unthinned coastal hemlock stands. Diameter growth of stems in the regenerated stands was undoubtedly reduced
by high densities and could be stimulated by precommercial thinning. Our data
suggest that dominant leave trees could be selected in thinning operations when
they are 2 m tall; doing such work as soon as possible thereafter would minimize
treatment costs and maximize the period of increased diameter growth.
•
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1
LITERATURE CITED
ATKINSON, W. A., and R. J. ZAsOSKI, eds. 1976. Western hemlock management. Coli Forest Resour,
Inst Forest Prod Contrib 34, 317 p. Univ Wash, Seattle.
DIMOCK, E. J., and F. W. HERMAN. 1963. A guide to the Hemlock Experiment Forest. USDA Forest
Serv, Pac Northwest Forest and Range Exp Stn, Portland, Oreg. 19 p.
FRANKLIN, J. F., and C. T. DYRNESS. 1973. Natural vegetation of Oregon and Washington. USDA
'�.
Forest Serv Gen Tech Rep PNW-8, 417 p. Pac Northwest Forest and Range Exp Stn, Portland,
Oreg.
OLIVER, C. D. 1978.
i.,
:f
Growth response of suppressed hemlocks after release. In Western hemlock
management (W. A. Atkinson and R. J. Zasoski,
,.
eds), p 266-272. Coli Forest Resour, Inst Forest
.\
'1
Prod Contrib 34, 317 p. Univ Wash, Seattle.
TUCKER, G. F., and W. H. EMMINGHAM. 1977. Morphological changes in leaves of residual western
,.
,
.,
hemlock after clear and shelterwood cutting. Forest Sci 23(2):195-203.
WILEY, K. N. 1975. The ecology of western hemlock: a basis for management. In Global forestry
7i
and the western role, p 133-135. Perm Assoc Comm Proc, Western For and Conserv Assoc,
,
Portland, Oreg.
WILLIAMSON, R. L., and R. H. RUTH.
1976.
i
Results of shelterwood cutting in western hemlock.
USDA Forest Serv Res Pap PNW-201, 25 p. Pac Northwest Forest and Range Exp Stn, Portland,
Oreg.
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