61
THINNING IN THE PACIFIC NORTHWEST:
RESULTS FROM THE LOGS STUDY
David D. Marshall and Robert O. Curtis
USDA Forest Service, Forest Sciences Laboratory
Olympia, Washington
While it has had a long history in European forestry, interest in commercial thinning in the
Douglas-fir region dates back to the 1940's (Curtis and others 1998). Prior to this time, large
tracts of old forests, cheap stumpage and logging systems based on railroads led to large scale
extraction operations. By the 1940's it was becoming clear that future wood supplies from the
region would have to come from managed second-growth stands and the introduction of tractors
and trucks to the woods made more flexible harvesting operations possible. Yet into the 1960s,
commercial thinning was still not widely practiced in the region, although it was being touted as
having "enormous potentialities" (Worthington and Staebler 1961). By this time, techniques had
been developed to carry out thinning operations, but research trials were few and still relatively
new, limiting information for developing thinning schedules.
Through the 1980's commercial thinning continued in the region on a small scale. This is was
due to the continued availability of old forests, the lack of markets for smaller and poorer quality
logs and the lack of equipment to harvest and process the smaller logs cheaply. In the last decade,
changes in the economy, decreasing supplies of old timber, development of new technologies and
increased social concern for logging older forest and clearcutting operations has rekindled a
broader interest in thinning in the region.
One of the studies that came out of the interest in thinning in the late 1950's and the resulting
need for growth information from thinned stands in order to develop thinning schedules was the
Regional Douglas-fir Levels-of-Growing-Stock (LOGS) Study. At that time, Staebler (1960)
developed a method for calculating thinning schedules and managed-stand yields based on
estimated gross yield of natural stands (Staebler 1955), estimated diameter growth rates in
thinned stands, and two assumptions. The first assumption was that gross cubic-volume yield and
periodic gross increment of a fully stocked unmanaged stand at any age represented full capacity
of the site to produce wood at that age. The second assumption was that approximately full
increment could be produced with widely differing combinations of growing stock, tree size, and
radial increment. To test these assumptions and provide information on the response of Douglasfir to thinning, the LOGS Study was begun in 1961 as a cooperative effort of the USDA Forest
Service, Weyerhaeuser Company, Oregon State University, Washington State Department of
Natural Resources, and the Canadian Forest Service. The stated objective was "to determine how
the amount of growing stock retained in repeatedly thinned stands of Douglas-fir affects cumulative wood production, tree size, and growth-growing stock ratios" (Curtis and Marshall 1986).
62
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15-16, 1999
The LOGS Study Design
From 1961 to 1970, nine LOGS installations were established in young Douglas-fir stands in
western Oregon, Washington and Vancouver Island, British Columbia. These sites included both
plantations and naturally seeded stands and represented site classes II, III, and IV (King 1966).
Each installation consists of 27 one-fifth-acre plots representing three replications of eight
treatments and an unthinned control all selected to be uniform in site productivity and stocking
and experiencing low competition as evidenced by long live crowns.
All plots except the control received an initial calibration thinning to create as uniform stocking
as possible. This calibration period also allowed the trees to adjust to the change in conditions
before the treatments were applied. To provide local control of treatments, the residual stocking
levels after each thinning treatment were defined as a percentage of the gross basal area growth
of the control plot for the previous period (Table 1). This results in eight treatments that are
increasing in basal area, but at different rates. After the calibration cut, thinning treatments were
applied five times at intervals of 10 feet of height growth averaged on 16 spaced crop trees per
plot (80 per acre). The thinnings favored the crop trees by first removing noncrop trees that
represented the range of diameters of all noncrop trees on each plot. The resulting thinnings are
best classified as crown thinnings.
Table 1. LOGS study treatment schedule, showing percent of gross basal area
increment of control plots retained in basal area growing stock for each treatment.
------------------------------------------------------------------------------Treatment
----------------------------------------------------------------Thinning
1
2
3
4
5
6
7
8 Control
------------------------------------------------------------------------------percent
First
10 10 30 30 50 50 70 70 100
Second
10 20 30 40 50 40 70 60 100
Third
10 30 30 50 50 30 70 50 100
Fourth
10 40 30 60 50 20 70 40 100
Fifth
10 50 30 70 50 10 70 30 100
------------------------------------------------------------------------------Results From The LOGS Studies
At the time of establishment, the nine installations ranged in total age from 15 to 33 years and
from 28 to 56 feet in top height. At of the end of 1999, only the two lowest site installations have
yet to complete the five prescribed treatment periods (a total of 60 feet of height growth including the calibration period). These two installations are projected to be completed in 2000 and
2010. Installations that have completed the treatment schedule have been remeasured at five year
intervals. One installation was harvested in 1996 after completing all treatments and two additional measurement periods. There have been 14 published reports summarizing all but two of
the installations (report in preparation) (Williamson 1976; Arnott and Beddows 1981; Marshall et
al. 1986; Curtis 1992; Curtis and Clendenen 1994; Hoyer et al 1996) and there has been one
study wide synthesis published (Curtis and Marshall 1986).
63
Past analysis (Curtis and Marshall 1986) has shown that development and results have been
consistent among installations but proceed at different rates based on site productivity. The
general conclusions of the study will be illustrated here using the Hoskins, Oregon installation,
which is on a high site and is therefore one of the most developed installations. Discussions on
growing stock will be based on basal area, although similar relationships could be shown for a
number of other density measures.
B l
w . While basal area increment is related to growing stock, the gross basal area
increment curves are relatively flat and are becoming more horizontal with age (Figure 1). The
maximum gross increment for the treatments occurs at the higher densities and is similar to the
gross increment of the control. To date, suppression-related mortality has only been a factor on
the control plots. This has caused a reduction in the net increment curves for the control and a
peak in maximum net increment at the higher density treatments (treatment 7).
V lum
w . Gross total stem cubic volume increment is more strongly related to growing
stock than basal area increment, but unlike basal area does not show a maximum within the range
of the growing stock attained in the thinned plots (Figure 2). Like basal area increment, volume
increment has decreased with age, but has maintained a stronger relationship with growing stock.
Competition mortality on the control plots have also reduced net increment causing a maximum
to occur at densities represented by the higher density treatments (treatment 7).
D m
w . Diameter increment on surviving trees is highly related to growing stock and
shows a strong decrease in increment with increasing growing stock (Figure 3).
u l c m
( Al) d p
dc u l c m
(PAl). The gross MAI and PAI
curves show no sign of approaching culmination for total stem or merchantable (to a 6-inch top)
cubic volume with MAI increasing for all treatments and the control (Figure 4). Only net total
stem MAI has culminated at about 42 years. Current treatment PAIs at Hoskins are about 1.6
times MAI. The PAI curves have shown considerable period to period variation from treatments,
measurements and weather.
Cumul v v lum
d
z . The cumulative merchantable (to a six-inch top) cubic volume
production (live stand plus all previous thinnings except the calibration) is exceeding the control
for the high (treatments 7 and 8) and medium (treatment 5) treatments in the later periods (Figure
5). Volume on the thinned plots has been redistributed to large average trees (Figure 6). At
Hoskins, all the treatments have over 85% of their current cubic volume in trees 15.6-inches and
greater compared to 50% on the control.
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15-16, 1999
Interpretations and Conclusions
An important result from the Douglas-fir LOGS study is the strong relationship between volume
increment and growing stock, which is much stronger than the same relationship for basal area
increment. When interpreting these results it is important to understand that Douglas-fir and
many other species in the Pacific Northwest have a growth pattern characterized by prolonged
height growth (this differs from some eastern US species). This prolonged height growth can
have a large impact on the relationship between volume growth and growing stock. This can be
illustrated using a basic volume equation,
V = FGH
where V is cubic volume per acre, G is the basal area per acre, and F is a stand form factor.
Differentiating this equation with respect to time (Hegyi 1969) gives the volume growth
equation,
dV/dt = FG(dH/dt) + FH(dG/dt) + GH(dF/dt)
The contribution of the third term to volume growth is minor (tree form changes slowly), the
second term is proportional to the basal area growth rate and the first term is a product of basal
area and height growth. In species with rapid height growth, differences in the level of basal area
will have a greater effect on the magnitude of the first term. Volume growth would be expected
to be more closely related to basal area growing stock in species, like Douglas-fir, which
maintain rapid height growth over long periods. In species that do not have this height growth
pattern, the major contribution to volume growth will be from the second term and more similar
to basal area growth patterns. This also illustrates the danger of evaluating thinning gains from
just considering basal area growth.
The LOGS study does not support the assumption that full gross cubic-volume increment will be
produced across a wide range of stocking levels, also known as the Langsaeter hypothesis
(Mar:Moller 1954). For young Douglas-fir, with its prolonged height growth pattern, increasing
increment (although at a decreasing rate) was found up to the point where suppression-related
mortality becomes important. In developing thinning prescriptions, one must consider the tradeoff of reducing volume growth and increasing diameter growth with increasingly heavy cuts.
Early results and projected trends of the LOGS studies suggest thinning yields for the medium
and high density treatments are on trajectories to catch up or exceed the yields on the controls
depending on how much growing stock was removed and the rate it recovers. Thinning gains
(yield compared to control) appear to increase with rotation age and the longer rotations required
for this are supported by MAI trends in thinned plots that appear far short of culmination. While
the LOGS studies are at ages where Douglas-fir harvesting is practiced, these trends indicate that
short rotations (less than 40 years for Hoskins) will produce less than maximum potential yields.
65
Currently many of the problems facing forestry operations stem from public opinion that does not
like the appearance, especially clearcut regeneration harvests. Politically it may be advantageous
to minimize visual impacts of forestry operations; thinning and extending rotations is one way to
do this. Extending rotations by maintaining growth rates through thinning may produce similar
levels of total wood production and greater value, and reduce the area in regeneration harvest.
Thinning can also accelerate the trends of understory and structural development that can provide
habitat for many wildlife species and other nontimber values.
The LOGS studies have demonstrated the enormous impact that thinning can have on the
growth of trees and stands and on stand development They have also provided a wealth of high
quality and consistent data that have been used in developing Douglas-fir growth and yield
models for the region.
16
600
14
550
� u: " u e _
12
450
TP-3
350
300
TP-4
0e "" -: �
m < : <�
Ce l
4
TP-2
400
10
8
TP-1
500
250
6
200
TP-5
150
2
TP-6
100
50
0
50
100
150
200
250
300
Mid-Period Basal Area (sq. ft. I acre)
350
Figure 1. Relation of periodic annual gross
basal area increment to mid-period basal
increment to mid-period basal area growing
stock by treatment period (TP).
TP-7
0
50
100
150
200
250
300
M1d-Per1od Basal Area (sq. ft. I acre)
350
TP-8
Figure 2. Relation of periodic annual gross
total stem cubic-foot volume increment
to mid-period basal area growing stock by
treatment period (TP).
66
N v mb
1
15-16, 1999
�00
� £
-
0O6
� �. ; ;! �
500
0O.
400
�� ; �
0:0 a
300
0OA
(: � �
200
100
0O2
0
20
0
50
100
150
200
250
300
Mndd-renod Basa Aera (sqO fO acere
renidmgernaddodvadgmdaFgrreaFaoerdoad
growing stock by
treatment period (TP).
FgrreaFaoerdoad
aegmdermaeadmgerna
(TP
35
40
S tan Age
45
50
55
Figure . ross mean annual increment
((AA)
periodic
annual
increment
Figureand
. gross
ross mean
annual
increment
eFngmaePa.grooarmdaadaandvaeadgmrmaea
(PAA)
trends
for
total
stem
cubic-foot
((AA)
and gross periodic annual increment
MATadanaFgrooadmgernedadaandvaeadgmrmae
volume.
(PAA) trends for total stem cubic-foot
(MATaegmanoasrgaeredvaoemradndedisrrea
volume.
urvnrmP
24
16000
�
12000
20
1
22
14000
18
2
16
r 1 : rE 0 rE
10000
8000
6000
4000
6r
� .!
30
350
Figure . Relation of periodic annual survivor
quadratic
mean diameter
increment
Figure . Relation
of periodic
annualtosurvivor
eFngmaePa.mvdeeraarsadmgernedadaandvaongueurg
mid-period
basal
area
growing
stock to
by
quadratic mean diameter increment
ndngdeedarmdaanedrmemgaeadgmrmaeaer
growing
stock
by
treatment
period
(TP).
mid-period basal area growing stock by
"E !> : � � E
25
2000
14
12
10
8
6
4
2
0
20
25
30
35
40
Stand Age
45
50
55
0
20
25
30
35
40
Stand Age
45
50
55
��� ��
Figure 5. Cumulative (live plus thinnings)
merchantable cubic foot volume (to a
6-inch top) yield for all treatments.
Figure 6. Attained quadratic mean
diameter (all trees) for treatment.
67
References
Arnott, J.T. and D. Beddows. 1981. Levels-of-growing-stock cooperative study in Douglas-fir:
report no. 6-Sayward Forest, Shawnigan Lake. Canadian Forestry Service, Pacific Forest
Research Centre, Victoria, B.C. Information Report BC-X-223. 54 p.
Curtis, R.O. 1992. Levels-of-growing-stock cooperative study in Douglas-fir: report no. 11Stampede Creek: a 20-year progress report. USDA Forest Service, Pacific Northwest Research
Station, Portland, OR. Research Paper. PNW-RP-442. 47 p.
Curtis, R.O. and G.W. Clendenen. 1994. Levels-of-growing-stock cooperative study in Douglasfir: report no. 12-the Iron Creek study: 1966-89. USDA Forest Service, Pacific Northwest
Research Station, Portland, OR. Research Paper PNW-RP-475. 67 p.
Curtis, R.O. D.S. DeBell, C.A. Harrington, D.P. Lavender, J.B. St. Clair, J.C. Tappeiner, and J.D.
Walstad. 1998. Ssilviculture for multiple objectives in the Douglas-fir region. USDA Forest
Service, Pacific Northwest Research Stations, Portland, OR. General Technical Report PNWGTR-435. 123 p.
Curtis, R.O. and D.D. Marshall. 1986. Levels-of-growing-stock cooperative study in Douglas-fir:
Report 8, The LOGS study: Twenty-year results. USDA Forest Service, Pacific Northwest Research Stations, Portland, OR. Research Paper PNW-356. 113 p.
Hegyi, F. 1969. Periodic mean annual increment and the derivative in growth prediction. Forest
Research Laboratory, Department of Fisheries and Forestry, Canadian Forestry Service, Sault Ste.
Marie, ON. Information Report O-X-114. 9p.
Hoyer, G.E., N.A. Andersen, and D.D. Marshall.1996. Levels-of-growing-stock cooperative
study in Douglas-fir: report no. 13-the Francis study: 1963-90. USDA Forest Service, Pacific
Northwest Research Station, Portland, OR. Research Paper PNW-RP-488. 91 p.
King, J.E. 1966. Site index curves for Douglas-fir in the Pacific Northwest. Weyerhaeuser Forestry Research Center, Centralia, WA. Weyerhaeuser Forestry Paper 8. 49p.
Marshall, D.D., J.F. Bell and J.C. Tappeiner. 1992. Levels-of-growing-stock cooperative study in
Douglas-fir: report no. 10-the Hoskins study, 1963-83. USDA Forest Service, Pacific Northwest Research Station, Portland, OR. Research Paper PNW-RP-448. 65 p.
Staebler, G.R. 1955. Gross yield and mortality tables for fully stocked stands of Douglas-fir.
USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, Portland, OR.
Research Paper 14. 20p.
Staebler, G.R. 1960. Theoretical derivation of numerical thinning schedules for Douglas-fir.
Forest Science 6(2):98-109.
68
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15-16, 1999
Williamson, R.L. 1976. Levels-of-growing-stock cooperative study in Douglas-fir; report no. 4Rocky Brook, Stampede Creek, and Iron Creek. USDA Forest Service, Pacific Northwest Forest
and Range Experiment Station, Portland, OR. Research Paper PNW-210. 39 p.
Worthington, N.P. and G.R. Staebler. 1961. Commercial thinning of Douglas-fir in the Pacific
Northwest. USDA Technical Bulletin No. 1230. 124 p.
THINNING IN THE
MAINE FOREST
November 15 - 16, 1999
Augusta Civic Center
Augusta, Maine
Conference Proceedings