Repl'i.ntecl from the
JOURNAL OF FORES'I'RY, Vol. 60, No.2, February 1962
Purchasec1 by tlte U. S. Forest Service for Official Use.
Some Effects of Thinning on Increment
in Douglas-Fir in Western Washington
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NOl'man P. WOl'thington,
Donald L. Reukema, and Geol'ge R. Staeblel'
THINNING YOUllg Douglas-fir is rap­
50.4' F.; the April to September
ent cutting intensities and frequen­
idly becoming a practical silvicul­
average is 57.80 F.
cies on gross increment of the resi­
tural measure in the Douglas-fir
free growing season is 161 days.
region,
particularly
'vVashington.
llormally
in
western
These thinnings are
commercial
operations
which produce merchantable prod­
ucts having a value equal to or
greater than the cost of extraction.
'1'0
study and observe effects of
thinning on increment of Douglas­
til', the 220-acre Voight Creek Ex­
perimental Forest! was established
in eastern Pierce County, Washing­
ton, in December 1947. This article
reports results during the first 6
years of a study involving various
intensities of thinning.
Logged originally in 1900, the ex­
perimental forest area supports a
stand that is fairly representative
of young Doug'las-fir forests that
have
followed
timber.
logging
of virgin
The stand became estab­
lished about 1912 and is uniformly
stocked,
with
about
80
percent
Douglas-fir by cubic volume.
The
area lies on an upper slope above
the Carbon River at an elevation
of 830 to 1,140 feet. Soils, derived
from a
piedmont
glacier of the
Cascade Range, are primarily Bar­
neston gravelly sandy loam and
Indianola sandy loam. Site qual­
ity averages III for Douglas-fir.
The
nearest
weather
station,
at
Buckley, shows an average annual
precipitation of 48 inches, with 15
inches falling during the months of
April to September.
nual
temperature
Average an­
at Buckley is
THE AU'I'HORS: Norman P. Worthington
anc1 Donalc1 L. Reukema arB research for­
esters, Olympia (Wash.) Research Oen­
ter, Pacific Northwest FOl'est anc1 Range
Expt. Sta., Forest Sel'vice, U. S. Dept.
Agric.; George R. Staebler is silvicul­
turist, Weyerhaeuser 00., Forestry Re­
scm'clt Center, Oentralia, ·Wash.
Average frost­
dual stand.
Logging on the experimental for­
est is carried out by independent
contractors.
A crew usually con­
sists of two men, with a single
horse used for skidding (Fig. 1).
All logs except those from larger
trees are skidded tree length and
bucked at roadside. Skidding dis­
tances are under 600 feet. Logs are
loaded with a forklift loader or, in
some cases, a homemade portable
loader.
Marking guides follow a prior­
ity system: first, merchantable dead
or dying trees and trees making
negligible growth compared with
average; second, excessively limby
dominants whose
release
better
removal
trees;
third,
would
trees
whose removal would improve spac­
ing of remaining trees (Fig. 2);
and lastly, merchantable diseased,
misshapen, and broken trees that
do not fall into any of the first
three priorities. How far marking
proceeds through these four classes
depends upon severity of thinning
and stand character. Usually, very
few trees of the first priority are
present in a young stand.
During
the first
thinnings
at
Voight Creek, products cut were
poles, 48 percent; sawlogs, 36 per­
cent; and miscellaneous, 16 per­
cent. In 1958, sawtimber amount­
ed to 21 i)ercent and pulpwood, 79
percent.
The proportion of saw­
logs has varied with the price of
short logs, running as high as 61
percent in 1956.
Currently, the
smallest utilizable tree is 6.0 inches
d.b.h.
treatments are
every 3 years, (3) medium thin­
ning every 6 years, and (4) heavy
thinning every 9 years. Compar­
able volumes
will have been re­
moved from each thinned area over
a period of 18 years.
Each of the four treatments is
replicated three times.
Thus, the
experimental forest is divided into
three blocks and each block into
compartments
(Fig.
3 ).
four
Blocks are 70 acres and compart­
ments
171;2
roads.
Treatments were assigned
acres,
exclusive
of
at random to the compartments.
This report is restricted to the first
6 years results on Blocks .A and B.
Both check areas in these two blocks
happened to fall on the lowest sites.
Selection of a suitable criterion
for controlling the intensity of a
single
thinning'
Fixed-volume
proved
removals
difficult.
obviously
could not be used because the orig­
inal stands differed widely among
compartments.
For the first thin­
ning in Block A, cutting to a spe­
cified stand-density index was tried.
This measure was not satisfactory,
however, because cutting is not uni­
formly distributed over the entire
range of diameter classes and be­
cause the lower size limit of mer­
chantable trees changes with mar­
kets.
The scheme finally selected calls
for removal of a fixed percentage
of the volume in the first thinning'.
'l'hen in subsequent thinnings, pe­
riodic increment is used as a basis
for adjusting the fixed-percentage
removal to compensate for site dif­
Design of Study
Beg'un in the fall of 1948, the
"Maintainec1 by the Pacific Northwest
Forest an(l Rnnge Expt. Stn. in coopera­
tion with the St.Regis Paper Co.
The
(1) no thinning, (2) light thinning
thinning study ,vas designed spe­
cifically to test the effects of c1iffer115
ferences
am0ng
compartments.
'rhus, the thinnings are not de­
signeel to leave specified amounts
of growing stock in the stand at thp
cOllclusion of each operation.
JOURNAL OF FORES'l'RY
116
11.0 inches on 1/20 acre, and trees
over 11.0 inches 011 the entire 1 5
stand's diameter range are meas­
ment when the study was begun in
acre.
In the spring of, 1952, the
Records of gross increment, mor­
1948.
1/20-acre subplot was eliminated
tality, and volume cut are compiled
Fifteen
Ys-acre
circular
plots
were established in each compart­
These plots contained sub­
/
ured on each plot.
plots of 1/20 acre and 1/80 acre,
and tags were put on all trees 5.1
by
and all had a common center. Trees
inches and larger throughout the
Stand inventories, however, are ex­
on 5 of the plots were tagged and
1/5 acre..
1/10-inch
diameter
classes.
pressed in I-inch classes. Data are
trees on 10 were left untagged. In
All plots are remeasured at in­
the present analysis, only the more
tervals of 3 years to correlate meas­
then
summarized
to
determine
amount cut, residual gTowing stock
the
urements with the shortest thin­
at beginning' of each period, and
tagged plots were used. Originally,
ning cycle (Fig. 4). Diameters to
growth and mortality during each
trees 1.6 to 5.0 inches d.b.h. were
the nearest 1/10 inch and heights
period in terms of basal area and
tallied on 1/80 acre, trees 5.1 to
of five or six trees throughout the
cubic-foot volume.
precise
measurements
from
VOIGHT CREEK EXPERIMENTAL FOREST
+
•
+
+
+
+
+
FIG. I.-Skidding logs to roadside during the first cut in a
medium-thinned compartment.
•
•
'00
800
FIG. 3.-Layout of the Voight Creek Experimental FOl'est,
showing blocks, compartments, and thinning treatments. Blocks
A and B are included in the present analysis.
FIG. 2.-Marking guides follow a priority system. This tree is
being marked for cutting to improve spacing and release more
promising trees.
FIG. 4.-Trees on the heavily thinned compartment of Block A
were measured in 1952, 3 years after thinning. All plots are
measured at intel'vals of 3 yeal·S.
FEBRUARY 1962
117
Volume tables applicable to the
measurement.
entire area were constructed by
solving a formula of the type:
Log V
=
a + blS + b.
b3S (log D)
were used to determine the con­
were
total height and
used
d.b.h.
at
age
37
years
compartments. Even among total
for height on the 40 plots involved
characteristics
stand
varied both among plots and among
Approxin:J.ately 200 trees measured
The
Results
original
of
each tree, and each plot site index
to the nearest 10 feet. Site index
was based on average height and
age of four dominant and codomin­
ant trees on each plot at time of
areas designated for the four thin­
ning treatments, initial basal area
per acre varied from a high of 177
square feet (medium thinning) to
a low of 146 (no thinning). Site
index in the unthinned stand was
also noticeably lower (Table 1). At
the beginning of the second 3-year
measurement
among
period,
treatments
differences
were
even
establishment. Volumes were read
greater, with an orderly progres­
from standard volume tables. Solv­
ing for each site index, the equa­
on the heavily thinned stand to a
tion reduces to the form:
Log V
=
k + b.
(log D)
sion from 116 square feet per acre
high of 158 on the unthinned. By
the time the heavily thinned stand
where constants k and b4 vary with
is ready to be treated a second
site index. This simplified form of
time, however, the range in basal
the equation was then solved for
area among the three intensities of
d.b.h. by I-inch classes. Total stem
thinning will have again narrowed
volumes
markedly.
classes
by
O.l-inch
were
diameter
determined
by
straig'ht-line interpolation.
a
Gross annual increment in basal
Tables
area was very similar among treat­
were made for both absolute vol­
ments
ume and volume growth.
riods (Table 1). During the sec­
Due to elimination of the 1/20­
ond
in
both measurement
3-year period,
however,
pe­
the
acre subplot, data for the first 3­
level of gross increments was con­
year period had to be adjusted to
sistently higher than in the first
the final sampling scheme. To make
period. For both periods combined
the
adjustments,
data
were
re­
(6 years) gross basal-area incre­
corded by both sampling proce­
ment averaged 4.9 square feet an­
dures at the time of the second
nually for the thinned stands and
measurement. Data
5.0 for the unthinned.
covering
the
5.1- to 11.0-inch size class for the
Analysis of variance showed that
first 3-year period were then ad­
differences in gross basal-area in­
justed in proportion to the dif­
crement between the four treat­
ferences obtained in
ments (three thinned and one un-
the
second
tested at Voight Oreek do not alter
.Feet
126
154
145
144
At
start of
seeoncl Fhst Second
3-year 3-year 3-year 6-year
period' pel'iod period period
---- Square feet
146
168
177
167
146
150
124
105
(2, 10, 13).
gross increment
Although average
gross
basal­
area increment was not affected by
kind of thinning, it still could con­
ceivably be related to amount of
growing stock left after thinning.
To test this possibility, individual
plots were used as observations in
regressions
of
basal-area
growth
over basal-area growing stock. Al­
together, 30 plots were available
for thinned stands and 10 plots
for
unthinned.
This
analysis
showed a tendency for growth to
increase with increasing amounts
of growing stock (Fig. 5). This is
contrary to
much evidence that
growth is constant over
a wide
(1, 6, 7).
range of growing stock
On the other hand, results of some
(4, 11, 12)
investigators
are con­
sistent with those from this anal­
ysis. Gruschow's
(4)
data on south­
ern pine isolate instances of in­
creasing,
growth
level,
with
and
decreasing
increased
growing
stock. If the postulated growth­
growing stock relation of Lang­
saeter, as discussed by Hawley and
Smith (5, p. 355) is accepted, these
seeming inconsistencies can be ex­
plained on the basis of initial stock­
ing.
Possibly the Voight Oreek
plots
were
all
below
the
broad
range of stocking which produces
the level plateau of increment. It
is also possible that a curve, con­
vex upward, of g'l'owth stock would
actually fit the data collected, but,
of
the
wide
scatter
of
tistically.
Gl'OSS annual
basal area inerement
Basal area
Unthinned
Light
Medium
Heavy
significant.
nings within the range of those
because
Treatment
not
points, this cannot be verified sta­
TABLE I.-BASAL AREA AND GROSS BASAL-AREA INOREMENT, BY THINNING TREATMENT (ACRE BASIS)" After
iil'st
Site Original thinindex stand ning
were
Other studies have shown that thin­
Basal area.-Basal area in the
(log D)
+
where V = tree volume
S = site index
D = diameter breast high
stants.
thinned)
158
144
126
116
4.6
4.8
4.4
4.0
TABLE 2.-CUBIC-FoOT VOLUME AND GROSS CUBIc-F OT
INCREMENT, BY THINNING TREATMENT (ACRE BASIS)
Cubie-foot volume
----
5.3 5.8 5.0 5.5 5.0
5.3
4.7
4.8
"All trees 1.6 inches d.b.h. and larger, based on 10 1/5-aere
plots pel' treatment.
"Failure of all tl'eatments to show an orderly increase in
basal area between first and second periods was caused by (1)
actual losses due to cutting (in lightly thinned compal·tments)
or mortality, (2) ingl'owth through trees reaching or passing
the 1.6 or 5.1 minimum d.h.h. limits, or (3) incongruity in­
her8llt in the sampling procedme.
Treatment
Unthinned
Light
Medium
Heavy
Gross annual
eubie-foot inerement
At
After start of
first second First Second
Site Original thin- 3-year 3-year 3-year 6-year
index stand
ning period' period period period
Feet
126
154
145
144
4,328
5,844
5,821
5,560
4,328
5,114
4,029
3,383
C1tbio feet
4,836
5,006
4,174
3,828
163
188
162
147
190
228
196
210
177
208
178
178
"Entire-stem volume of all trees 1.6 inches d.b.h. and larger, hased on 10 1/5-acre plots per treatment. 'See footnote 2, Table 1. 118
JOURNAL OF FORESTRY
In the first 3-year period, growth
III
the
thinned
and
moved in thinning. That site index
unthinned
has
little
effect
on
second period, however, the thin­
ned stands grew 211 cubic feet as
basal-area
stands was very similar at com­
growth has been reported before
compared
parable
(3, 8, 9).
thinned. Thus, both thinned and
levels
(Fig. 5).
of
growing
stock
In the second period,
C1tbio volttme.-Results of the
with
190
for
the
un­
unthinned stands made more rapid
however, levels of growth in the
cubic-volume analysis closely par­
growth in the second period, but
thinned stand were substantially
allel those for basal-area.
the thinned stands increased their
above those of the unthinned. Fur­
Average cubic-foot volume of all
rate of growth more than the un­
ther analysis of the second-period
treatment
data showed that at the average
was 5,388 cubic feet per acre, rang­
gross
growing-stock level of 136 square
ing from 4,328 to 5,844 (Table 2).
aged 188 for thinned stands and
177 for unthinned.
feet,
the
better
thinning
thinned.
For
the
cubic-foot
6-year
period,
increment
aver­
the
Such differences are common in a
37-year-old Douglas-fir stand. Fol­
Analysis of variance for each 3­
(0.6 square foot) was statistically
lowing the first thinning, residual
year period and for the 6 years
significant.
determined
volumes in thinned stands aver­
combined
by a covariance analysis based on
aged 4,175 cubic feet compared to
treatment on gross cubic-volume in­
was
of
before
thinned stand over the unthinned
This
growth
areas
showed
the
effects
of
the equation:
4,328 for unthinned stands. This
crement to be nonsignificant. Thus,
Gb
difference
whether stands were thinned or un­
3.53 00 + 0.0141B
,Vhel'e Gb = g l'OSS annual
=
B
incl'ement
l'esidual basal al'ea
=
after
a
cubic
feet)
was
very small because the unthinned
thinned, all treatments resulted in
compartments were on poorer sites
much the same gross increment in
terms of both basal-area and cubic­
and lower in original volume.
This equation was decided upon
only
(153
Average annual increment dur­
multiple-regression
foot volume.
analysis, in which all plots-thin­
ing the first period was practically
Regression analysis also provided
ned and unthinned-were pooled,
identical for thinned and unthin­
results very similar to those for
showed that growth ," as unaffected
ned compartments-166
by either site index or amount re-
cubic feet, respectively. During the
and
basa1
163
GROSS ANNUAL BASAL-AREA
GROSS ANNUAL VOLUME
INCREMENT PER ACRE
INCREMENT PER ACRE
( Square feet
( Cu bic feet)
1
275
x
-
'225
<>
0
<>
.
<>
•
....
0
o
. . "
....
.
<>
xx
x
0
200
<>
o
.,.
.... ,.I'I
P
", .... i1 1 '"
1
x
0
<>
175
X
150
<>
o
125
100
YI:
0
o
,.
0 ;'
1 1
.... "",,,oN0
<>
y:::l. 2.31 + O.OI62X
o THINNED:
X UNTHINNED:
1.54 + 0.0211 X
N
1 I '-
p
NeD
.
0
�':
o THINNED:
0
"
0
0
.. '"
o
x
2
0 0, ... ,
N EP
.
MEP0,o
v 1M '" '11 0
°
'"
275
.
... g..'It'
k;: ....
/
,1-- .....
'0
,
0
0
0
250
225
0.
200
xx
175
°
0
,,.
150
125
o THINNED:
X UNTHINNED:
Y= 3.21 + 0.0116 X
j"
2.40+ 0,0164 X
I
oJ.
,
, .....
60
80
100
120
140
160
160
200
FIG, 5.-Gross annual increment in basal area in relation to
basal al'ea of l'esidual stand for thinned and unthinned stands.
(All trees 1.6 inches d.b.h. and larger.)
0
....
1
y= 63.0+0'
32X
SECOND 30-YEAR PERIO
0
/
x
-.,;
'. "'x
,. "
.,/
.....
......10-';-
0
to-'''''
x
x
?
o THINNED:
1
100
220
(Square feet
"iiiM
O 1
.
r
y= 76.7tO.0213X
y= III.OtO.0232X
X UNTHINNED: V= 87.0+0.0214 X
2,000
BASAL AREA OF RESIDUAL STAND PER ACRE
0 ",e .
0
I
r
x UNTHINNE
300
0
x
0
x
%
I
0
0
x
.
L
XV
75
SECOND 3-YEAR PERIOD
cubic-foot
FIRST 3-YEAR PERIOD
250
7
'When
0
0
FIRST 3-YEAR PERIOD
<>
area.
growth was related to cubic-foot
3,000
4,000
5,000
6,000
VOLUME OF RESI DUAL STAND PER ACRE
I
7,000
Cubic teet)
FIG. 6.-Gross annual inCl'ement in cubic feet in relation to
cuhic-foot yolume of l'esic1ual stand for thinned and unthinned
stands. (Enti re -stem yolume of all trees 1.6 illClhes d.h .h. and
large . )
r
119
FEBRUARY 196
un­
thinning during the first 6 years
which is a rough expression of tree
thinned stands showed nearly the
following treatment. Initial results
height.
same level of increment for the
of twice-replicated thinning treat­
first 3-year period (Fig. 6). Dur­
ments that will range from light
growing stock,
thinned
and
ing the second 3-year period, how­
(and frequent) to heavy (and less
ever, the level of increment was
frequent) were analyzed.
Gross
sUbstantially greater on the thin­
ned plots.
The
increment
per
acre
on
thinned stands was as great as that
second-period
data
for
on unthinned. Also, the desirabil­
thinned and unthinned plots com­
ity of one thinning treatment over
bined were further studied in a
another was not demonstrated with­
multiple-regression analysis to test
in the range of residual growing
effect
stocks
of
the
variables-growing'
stock, site, and volume removed­
represented.
Apparently,
over 40 percent of cubic volume
Growing
would have to be removed to pro­
stock and site were found to sig­
vide a measurable reduction in in­
nificantly affect cubic-foot incre­
crement during the first 6 years
on
volume
increment.
ment. The influence of volume re­
after treatment. Lack of difference
moved, however, was nonsignificant.
The prediction equation from this
in gross increment between treat­
analysis is:
should be considered in deciding
Gy
=
38 + 0.628 + 0.017V
IV-here Gy = gross annual increment
8 = site index
V = l'esidual cubic volume
Multiple covariance analysis of
these data showed again that the
thinned stands were more efficient
producers of cubic-foot increment,
the analyses shown in
verifying
Figure 6 (where differences in site
index
were
not
accounted
for).
,V-hen adjusted to a common grow­
ing-stock level-4,461 cubic feet­
and a common site index-142 feet
-the thinned stands produced 214
cubic feet per acre annually com­
pared with 184 cubic feet for the
unthinned.
This difference of 16
percent in favor of thinning is sig­
nificant at the 5-percent level.
ments
shows
that
other
factors
upon the best treatment.
For all treatments, gross growth
during the second 3-year period
was greater than during the first.
Oovariance analyses for this sec­
ond period further showed that for
a
given
thinned
more
level
of
stands
efficiently
gro·wing
produced
than
stock
wood
unthinned
stands. This greater difference in
increment between thinned and un­
thinned stands for the second pe­
riod suggests that
redistribution
of growth capacity to remaining
trees
was
achieved
following
a
short time lag.
Analyses of basal area and cubic
volume provided
similar
results,
with one exception: for basal area,
residual volume was the only fac­
Summary and Conclusions
tor that influenced increment sig­
A thinning study established in
nificantly; for cubic-volume incre­
a 37-year-old Douglas-fir stand in
ment, both site index and residual
western ,V-ashington to investigate
volume were significant. Oubic vol­
the effects of three intensities of re­
ume, which depends both on tree
moval showed that gross increment
diameter and height, would logi­
was not
cally be more dependent on site,
measurably affected
by
Literature Cited
1. BRAATHE, PEDER. 1957. Thinnings
in even-aged stands; a summary of
European litel'a ture. New Brunswick
Univ. Faculty of Forestry. 92 pp.
Illus.
2. BRIEGLEB, PHILIP A. 1952. An ap­
proach to density measurement in
Douglas-fir. Jour. Forestry 50: 529­
536. Illus.
3. BUCKMAN, R. E. 1959. The growth
and yield of l'ed pine in Minnesota.
(Abstl'act) Diss. Abs. 20 (5), p.
1512.
4. GRUSCHOW, G. F., and T. O. EVANS.
1959. The relation of cubic-foot vol­
ume growth to stand density in
young slash pine stanc1s. Forest Sci.
5: 49-55. Illus.
5. HAWLEY, RALPH 0., anc1 DAVID M.
SMITH. 1954. The practice of silvi­
culture. J. Wiley anc1 Sons, New
York. Ed. 6, 525 pp. Illus.
6. HOLMSGAARD, E RIK.
1958.
Basic
growth concepts. In Silvicultural as­
pects of wooc11anc1s. New York State
Univ. 001. Forestry Bul. 42. pp. 10­
21. Illus.
7. MOLLER, O. M. 1954. The influence
of thinning on volume increment.
In Thinning pl'oblems and practices
in Denmark. New York State Univ.
001. Forestry Wodc1 Forestry Ser.
Bul. 1. pp. 5-44. Illus.
8. SIMMONS, E. M., anc1 G. LUTHER
SCHNUR. 1937. Effect of stanc1 den­
sity on mortality anc1 growth of lob­
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