Forest Sei., Vol. 29. No. 1, 1983, pp. 33-46
Copyright 1983, by the Society of American Foresters
This file was created by scanning the printed publication.
Text errors identified by the software have been corrected;
however, some errors may remain.
Initial Shock and Long-Term Stand
Development Following Thinning
in a Douglas-fir Plantation
CONSTANCE A. HARRINGTON
DONALD L. REUKEMA
ABSTRACT. Responses following the application of six precommercial thinning treatments to a
27-year-old Douglas-firplantation (2.4-m spacing, height at age 100 = 24 m) have been monitored
for 25 years. Spacing after thinning ranged from 3.4 m to 8. Im. Immediately followingthinning,
t r e e s exhibited thinning shock; that is, substantial height growth reductions. The severity and
duration of the shock effect were partially related to the severity of thinning. From 15 to 25 years
following thinning, however, height growth was positively related to growing space with the best
height growth at the wider spacings. Diameter growth increased followingthinning, with the range
in diameter growth rates between spacings increasing over time. Basal area and volume growth
per hectare were reduced by treatment, but the differences among spacings are decreasing. FOREST
Sol. 29:33--46.
ADDITIONALKEY wORDS. Pseudotsuga menziesii, precommercial thinning, respacing, thinning
shock, height growth, diameter growth.
THINNING is a c o m m o n and effective m e t h o d for regulating individual tree and
stand development. There are situations, however, where both initial and longterm responses to thinning give unexpected results. For example, initial responses
to thinning are not always positive. Negative responses include reductions in
height or d i a m e t e r growth, chlorotic foliage, and d a m a g e or mortality associated
with increased exposure (e.g., sunscald). These negative responses, individually
or collectively, are sometimes referred to as " t h i n n i n g shock." In addition to the
physiological shock responses, thinning can also result in increased phystcal d a m age or mortality due to wind, snow, or ice.
A generally unexpected, positive long-term response to thinning is increased
height growth. Although it has long been recognized that extremes o f stand density
can influence tree height growth, it is generally a s s u m e d that within the range o f
densities encountered in managed stands, density will have no effect on height
growth (cf. Hagglund 1981). This is a key a s s u m p t i o n underlying the use o f site
index as a measure o f site quality. If height growth differences can result from
variations in stand densities, it m a y be necessary to reevaluate yield predictions
and the associated decisions by land managers on appropriate m a n a g e m e n t regimes.
There is increasing evidence that at least for some poor-site Douglas-fir stands,
both initial stocking levels and thinning can markedly affect height as well as
diameter growth. Short-term decreases in height growth following thinning (Miller
and R e u k e m a 1977, C r o w n and others 1977) and long-term increases in height
t
The authorsare, respectively, Research Forester and Principal Silviculturist, Pacitic Northwest Forest
and Range Experiment Station, Forestry Sciences Laboratory. 3625 93rd Avenue S.W., Olympia, WA
98502. Manuscript received 5 March 1981.
Vt)LUME 29. N t ' r , i , e r I, 1983 / 33
Control
row
Block
1
5.0
4.2
3.4
4.0
i
I
6.0
8.1
8.1
4.0
I
I
Block
2
3.4
6.0
I
5.0
l
i
!
4.2
i
N
©
Control
plots
©
©
Scale
(meters)
FIGURE 1. Relative locations of study plots, control row, and control plots. Numbers inside study
plots refer to approximate spacing in meters after the thinning.
growth associated with plantation spacing (Reukema 1979) have been documented
in recent years. This paper summarizes the 25-year growth of a Douglas-fir plantation precommercially thinned to six stocking levels. Both initial thinning shock
and long-term increased height and diameter growth with increased growing space
were observed.
STUDY DESIGN
The study was designed to determine how stands at various stocking levels develop
to commercial size. The plan was to evaluate the potential of both poorly stocked
stands and o f stands close to o p t i m u m stocking for a wide range oftarget diameters.
The six stocking levels selected were 875, 625, 500, 375. 250, and 125 trees per
hectare. A randomized block design was used, with six 0,2-ha measurement plots
in each of two contiguous blocks; each plot was surrounded by an 8-m buffer strip
(Fig. 1). Treatment assignment within each block was random.
The original study was designed to evaluate the effects of spacing, not ofthinning
per se; it did not plan to compare the thinned plots with the unthinned plantation.
It quickly became clear that the thinning did have some adverse effects and 20
trees in a row in the unthinned stand east o f the measurement plots were designated
to serve as a control for monitoring height growth. To provide a better indication
of the growth trends in the unthinned plantation, data from three 0.02-ha plots,
34 / FOREST SCIENCE
e s t a b h s h e d in 1964 to serve as controls in a ti~rtthzation study, are included m
s o m e o f the figures and tables. T h e s e u n t h i n n e d control plots were located just
south o f the study area in the s a m e plantation. A v e r a g e tree height (in 1964) in
the control plots was a p p r o x i m a t e l y equal to that in the control row: thus these
plots are on a roughly c o m p a r a b l e site.
THE STUDY AREA
T h e study area is on the W i n d R i v e r E x p e r i m e n t a l Forest near Carson, W a s h ington. T o p o g r a p h y is relatively u n i f o r m with a northerly aspect; elevation averages a b o u t 600 m. The area is classified as site V (height at 100 years = 24 m).
T h e soil is a well-drained l o a m with rapid permeability. Soils in the vicinity are
generally considered to be nitrogen deficient; n e a r b y studies involving nitrogen
fertilization and alder interplanting h a v e s h o w n m a r k e d response. T h e study area
was burned in the extensive 1902 Yacolt Burn a n d was reburned by a n o t h e r
wildfire in 1927.
W e a t h e r i n f o r m a t i o n is available f r o m the W i n d R i v e r Ranger Station, which
is a b o u t 200 m lower in elevation than the study area. M e a n annual temperature,
at the weather station, is 9°C. Total annual precipitation averages 2,570 ram:
however, only 250 m m falls during the frost-free growing season which averages
130 days. T h e majority o f precipitation occurs as rain or snow during the d o r m a n t
season. T h e area has a m o d e r a t e w e a t h e r - d a m a g e h a z a r d due to occasional heavy,
wet snows; this wet-snow hazard is c o m m o n at m i d elevations along the west
side o f the Cascades.
T h e study area is part o f a large plantation that was planted in spring 1929 with
1+ 1 Douglas-fir seedlings, raised in the W i n d R i v e r Nursery from an u n k n o w n
~a:cd s(,urce. T h e seedlings were planted at 2,4 by 2.4 m spacing, or a p p r o x i m a t e l y
1,680 per hectare. In 1953, the plantation had a b o u t 1,600 trees per ha, averaging
8.8 m in height and 10 cm in diameter, t Generally, limbs on the lower third o f
the siem were dead and d i a m e t e r i n c r e m e n t had slowed slightly in recent years.
Based on average tree height, site p r o d u c t i v i t y varied within the study area. T h e
two plots in the northwest corner o f the area had higher than average heights: the
Ibur plots in the southwest portion had lower than average heights.
TREATMENT
T h e thinning was conducted to leave the best trees within an acceptable range o f
spacing for each stocking level. T h e residual n u m b e r o f trees per plot varied
s o m e w h a t from the target levels (Table 1). T h e plots in Block l were thinned in
the fall o f 1953. T h e plots in Block 2, however, were not thinned until the following
spring and s u m m e r : all work was c o m p l e t e d by the end o f July 1954. Felled trees
were left on the site.
MEASUREMENTS, COMPUTATIONS, AND ANALYSES
All trees ()n the m e a s u r e m e n t plots and in the control row were n u m b e r e d and
tagged. After the growing seasons in 1954, 1959, 1964, 1969, 1974. and 1979, all
trees were m e a s u r e d for d i a m e t e r at breast height (dbh) and their condition was
noted. Heights were d e t e r m i n e d on a s u b s a m p l e in each plot. In 1954 and 1959,
12 to 18 trees per plot (all 20 trees in the control row) were measured for height.
Beginning in 1964, 20 trees per plot were m e a s u r e d . T h e trees measured for height
' Average diameter m this report is the diameter at breast height (1.4 m) of the tree of mean basal
area (i.e.. quadratic mean diameter).
V()t.t~Mt!29, Nt xtm~ 1. 1983 ' 3 5
TABLE 1.
Spacing-'
Plot characteristics by treatment after thinning, .fall 1954.
Number
Diameter +
Height
Basal area
Cubic volume
m
sph ~
cm
m
m-'/ha
m '/ha
3.4
840
894
12.4
9.7
! 1.3
7.6
10.2
6.4
50.6
21.6
4.0
627
637
11.2
! !.4
9.1
9.4
6.1
6.5
23.8
27.6
4.2
548
563
11.4
10.2
9.4
7.9
5.6
4.6
22.6
16.6
5.0
405
410
i 3.7
10.2
10.7
7.9
6.1
3.3
28.8
11.2
6.0
252
297
i 1.9
10.4
8.8
8.3
2.8
2.5
11.7
9.5
8.1
i 38
168
12.7
11.9
10.0
10.0
1.8
1.9
7.3
8.0
1 1.4
9. I
Average
(all
plots)
--
--
--
+The first line for each spacing is the plot in Block 1, the second line is the plot in Block 2.
-"A p p r o x i m a t e average square spacing o f residual trees.
Quadratic mean d i a m e t e r at breast height.
Stems per hectare.
in each plot spanned the observed range in diameters and were selected so that
two-thirds o f the tree diameters were greater than the average diameter o f the
plot. If a tree which had been designated for height measurement subsequently
had a broken or dead top, t h a t , t r e e was dropped from the height sample and
another tree was substituted for it from then on. In addition, annual leader length
for the growing seasons 1953-59 was measured on five trees per plot (10 in the
control row).
Five-year height growth was c o m p u t e d for each designated tree that was present
at both ends o f a m e a s u r e m e n t period. The plots differed in their average height
in 1954; we assumed these differences represented differences in site quality, and
adjusted the height growth values to reduce this source o f variation. Adjusted
height growth per plot was d e t e r m i n e d for each measurement period by averaging
the individual tree values and then expressing growth as a percentage o f the 1954
plot height. We were unable to adjust each tree value individually (as we did for
d i a m e t e r growth) because we did not have a 1954 height for all the trees eventually
measured for height. Differences in height growth between the thinning treatments
were tested using analysis o f variance. Tukey's test was used to separate means
when treatment effects were statistically significant (p = 0.10).
D i a m e t e r growth was analyzed for both the total stand and for the 100 largest
stems per hectare (sph) using covariance analysis: the covariate was 1954 individual-tree diameter. D i a m e t e r growth was adjusted to reduce the variation in
the data caused by variations in site quality. Analysis o f variance using adjusted
means was followed by Scheffe's test to separate means when the treatments were
statistically significant (/9 = 0.10).
The tarif system (Turnbull and others 1970) was used to determine a local
v o l u m e equation for each m e a s u r e m e n t period. Volume growth per plot was
d e t e r m i n e d by s u m m i n g the v o l u m e increment o f each tree which survived the
growth period. All v o l u m e c o m p u t a t i o n s were made on the basis o f total stem
cubic volume.
36 / FOREST SCIENCE
STAND DEVELOPMENT
Overview.--The immediate effects of the thinning treatments were detrimental.
Opening up the stand resulted in chlorotic foliage, sunscald, and snow and ice
damage in the form of broken tops and branches and leaning trees; one of the
plots at the widest spacing had particularly heavy damage. In addition, height
growth was reduced by thinning as previously reported by Staebler (1956). The
amount of height growth reduction during the first 10 years was roughly proportional to the severity of thinning.
Long-term effects of increased growing space have, however, been beneficial.
Following the initial shock, height growth did recover in the thinned plots. In
fact, height growth is currently greatest at the widest spacings. Diameter growth
increased in response to thinning--the wider the spacing, the greater the rate of
growth. Even at the widest spacings, tree form is good and branch size is quite
satisfactory,.
Height and diameter growth have slowed in recent years in the closer spacings,
and the range in growth rates among spacings has increased over time. Basal area
and cubic volume per tree are greatest at the widest spacings. Due to the many
fewer trees per plot at the wide spacings, however, basal area and cubic volume
per hectare are greatest at the closest spacings.
Height Growth.--Height growth decreased immediately following thinning. The
1954 leader lengths of all m e a ~ r e d trees in Block 1 (thinned fall 1953) averaged
only 50 percent of the previous year's length. Trees in Block 2 (thinned summer
1954) had 1954 leader lengths that averaged 23 percent less than in 1953. The
reductions in height growth were even greater in subsequent years than in the I st
year. The least height growth occurred in 1957 and 1958 when the height growth
of the thinned trees averaged 70 percent less than the 1953 growth. In contrast,
during those two years, the trees in the control row grew 19 percent and 12 percent
more than in 1953.
The 5-year height growth patterns by thinning treatment are presented in Figure
2. Height growth on thinned plots averaged half of that in the control row during
the first 5 years (l.0 m versus 2.3 m) (Fig. 2a); it was significantly less at the
5.0- and 6. l-m spacings than at the three narrower spacings.
During the second 5-year period, height growth on the thinned plots averaged
about 16 percent less than height growth in the control row with the exception
of the widest spacing (Fig. 2b). Height growth at the 8. l-m remained sharply
depressed during this measurement period, and was significantly less than that at
other spacings.
In the third 5-year period (Fig. 2c), the 8.l-m spacing again had the poorest
height growth, but it was not significantly less than any except the 6.0-m spacing.
Average height growth in the thinned plots was similar to that measured in the
control row. Thus, l0 years after thinning, the shock effect was no longer a factor
influencing height growth.
During the fourth and fifth 5-year periods, height growth was roughly proportional to growing space with the greatest height growth at the wider spacings (Fig.
2d, 2e). During the 4th measurement period, height growth at the 3.4-m and 4.0m spacings was significantly less than that at the 6.0-m spacing. In the last measurement period, the 3.4-, 4.0-, and 4.2-m spacings all had significantly less height
growth than the 6.0-m spacing; the height growth at the 3.4-m spacing was also
significantly less than the growth at the 8. l-m spacing. All o f the treatments had
better 10-year height growth than the control row; 10-year height growth in the
control row was very close to the expected height growth of 2.4 m for a stand of
its age and site index (McArdle and others 1961). The widest spacings had 10year height growth increments of 4 to 5 m.
VOLtIME 29. NI~MI~t-R 1, 1983 / 3"7
1954-1959
[ !"3 Unthinned control row I
[~
'~
G Thinned plots
o
u
l
3
I
I
I
1
I
I
I
I
b
I
I
1959-1964
2
m.......Q.=
1
I
3
e
I
I
I
I
I
I
I
o
I
I
1964-1969
2
e
.e
0
1
O)
®
J=
I
I
I
I
I
I
I
I
I
I
L
®
tO
3
d
1969-1974
I
I
I
e
I
I
I
I
I
~
I
I
1974-1979
El'"
I
I
I
I
I
I
I
I
I
I
1
2
3
4
5
6
7
8
9
10
Spacing
F{GURE 2.
(m)
5-year height growth by spacing, (a) 1954-59, (b) 1959-64, (c) 1964-69, (d) 1969-74, (e)
1974-79.
38 / FOREST SCIENCE
10.0
Spacing
I 6.Om
/
9.0
/
/
/
/
/
/
2.4m (control row)
8.0
Y
7.0
/
/
/
6.0
/
/
"
2
/
/
//
5.0
8.1m
3.4m
/
°.
//
/,
/
4.2m
." 5.0m
...;~ 4.Ore
.-'/
."
,
/
•
/
ol
',r
//.././
,~:,"
//
/
4.0
/
/
3.0
2.0
/
/
1.0
T r e e A g e 28
Y e a r 1954
33
38
43
48
53
1959
1964
1969
1974
1979
FIGtJRE 3. Average tree height by spacing and measurement period (heights adjusted to a common
1954 height).
Vt)LUME 29. NtJMm..'R I, 1983 / 39
4:=
o
"rl
0
nl
5.year
diameter
growth
(crn) 4a Total s t a n d
4 b 1 O0 l a r g e s t t r e e s / h a
Spaclng
# # ~ ========== 8.1m
r3
z
r')
4.00
Spacing
nl
,~
3.00
o,,~'N
.
2.00
.~i"
j , , . . ~
~
2.00
""~'~"~ 2.4m
4.2m
=~~~=~,~
1.00
3.00
,, 6.Ore
_ _ .
4.00
,8.1m
"~
• ~
5-year
dlameter
growth
(cm)
(contr¢
plots)
4.0m
~ , ~ , ~~ 4 3.4 m
2.4rn
1.00
(contrc
plots)
I
i
I
i
.I
1954-1959 1959-1964 1964-1969 1969-1974 1974-1979
Measurement periods
!
I
I
I
i
1964-1959 1959-1964 1964-1969 1969-1974 1974-1979
Measurement periods
Fl(;t~Rr_ 4. Five-year diameter growth of surviving trecs by spacing and measurement period (diameter growth adjusted by covariance) (a) for the total
stand and (b) for the I00 largest trees per hectare.
:30A
E
o
1979
25-
1974
m
L
®
E
•
~
£
20-
m
m
1979
1974
15 1969
19s4
m
1969
1964
1959
1954
-
10"
t
L
2.4
3.4
4.0
4.2
5.0
6.0
8.1
(Control plots)
Spacing
treatments
(m)
F~GuRr- 5. Mean diameter of trees surviving the 25-yearperiod by spacing and measurementperiod
(diameters adjusted by covariance).
Total height growth by treatment over the 25-year period was essentially the
same at all except the 6.0-m spacing, which was significantly better (Fig. 3). The
improved growth in recent yefirs has been sufficient to overcome the losses due
to thinning shock. Recent trends indicate that all of the thinned stands will produce
taller trees than the unthinned stand, and that this height will increase with spacing,
up to a spacing o f a t least 6 m. The difference in height in the future will depend
on how long the wide spacings maintain their superior height growth.
Diameter Growth.--Diameter growth of trees in all treatments increased in response to thinning, and the wider the spacing the greater the growth (Fig. 4a).
The relatively poor diameter growth at the 8. l-m spacing in the first measurement
period can probably be considered another symptom of thinning shock. We cannot
tell if this shock effect on diameter growth was limited to the widest spacing;
however, any negative impacts on diameter growth were probably most severe
and longest lasting at the widest spacing.
Overall, the closer spacings had relatively small increases in diameter growth
which lasted for a few years, while the wider spacings had greater increases in
diameter growth which lasted longer. Diameter growth of trees surviving the 25year period averaged more than 80 percent greater at the 8. l-m spacing than at
the 3.4-m spacing (0.60 versus 0.33 cm per year). During the most recent measurement period, growth was almost three times as much at the 8. l-m as at the
3.4-m spacing (0.72 versus 0.26 cm per year). In this last measurement period,
there were significant differences in growth between all of the spacings, with the
one exception that the two narrowest spacings did not significantly differ from
each other. 1 h e added increments in diameter growth rates at the widest spacings
were.greater than anticipated. Rather than observing a diminishing effect of additional growing space on diameter growth as we would have predicted, the reverse
appears to be true. Because the range in diameter growth rates among spacings
has continued to widen over time, the differences in average diameter between
spacings should continue to widen in the future (Fig. 5).
Diameter growth o f the 100 largest trees per hectare generally followed the same
trends as diameter growth o f the total stand (Fig. 4b). Although the range is less,
even for these largest trees in the stand, the wider the spacing, the greater the
diameter growth. Averaged over the 25-year period, the 100 largest trees per
hectare grew about 40 percent faster at the 8. l -m spacing than at the 3.4-m spacing
VOLUME 29, NtJSU~ER 1, 1983 / 41
TABLE 2. Number and diameter of stems lost to mortality by spacing and measurement period.
Mortality by period
Spacing
(m)
2.4 "~
3.4
4.0
4.2
5.0
6.0
8.1
1954-59
sph' Dg(em) 2
.
.
.
1959-64
Dg(cm)
sph
sph
.
1964-69
Dg(cm)
0
--
5
13.7
198
I0
2
0
8.1
--
5
2
12.2
7.6
7
5
5
7
12
12.7
11.7
11.2
12
7
7
16.3
14.5
12.4
7
2
2
1969-74
Dg(cm)
sph
10.9
15.7
!1.7
165
30
15
14.5
14.2
15.0
14.7
i 974--79
sph Dg(cm)
14.0
12.7
49
35
0
18.3
--
10
5
5
5
5
23.9
21.3
20.1
0
0
0
9.4
13.6
15.2
16.8
----
J Stems per hectare.
2 Quadratic mean diameter at breast height.
Unthinned control plots.
(0.67 versus 0.48 c m per year)., During the m o s t recent period, they grew twice
as fast at the 8. l - m as at the 3.4-m spacing (0.81 versus 0.40 cm per year). In the
last m e a s u r e m e n t period, d i a m e t e r growth o f the 100 largest trees per hectare
differed significantly a m o n g the 3 widest spacings and between the three wider
spacings and the three n a r r o w e r ones. Thus, e v e n the largest trees per hectare are
still growing significantly better at the widest spacings.
D i a m e t e r growth within the spacings was quite variable. At the widest spacing,
where theoretically all trees h a v e been free to grow, 25-year d i a m e t e r growth o f
surviving trees ranged f r o m 7.4 to 26.2 cm. T h e range at the 3.4-m spacing was
f r o m 1.5 to 15.0 cm. Even a m o n g the current 100 largest trees per hectare, 25year d i a m e t e r growth ranged f r o m 11.7 to 26.2 cm at the widest spacing and f r o m
8.1 to 15.0 cm at the closest spacing. D i a m e t e r growth within a spacing tended
to be proportional to tree d i a m e t e r at the t i m e o f thinning; however, there was
considerable variation in the rate o f increase and c o m m o n l y the largest (or smallest) trees per plot in 1979 were not the largest (or smallest) trees per plot in 1954.
Mortality and Damage.--Mortality has generally been minor. In the 25 years
following thinning, 85 trees (7.3 percent) died in the 12 study plots; this would
be equivalent to a b o u t 35 trees per hectare. Mortality has been erratically distributed (Table 2); it has not been closely related to spacing, with two a p p a r e n t
e x c e p t i o n s - - t h e initial m o r t a l i t y at the widest spacing and the recent m o r t a l i t y
at the closest spacings. Generally, m o r t a l i t y has been associated with snow breakage, sunscaid, root rot (Phellinus weiriO, or insect damage. Most o f the m o r t a l i t y
in the 10 years following thinning can p r o b a b l y be attributed to the thinning.
D a m a g e , p r e d o m i n a t e l y in the f o r m o f b r o k e n or dead tops following wet snows,
was quite closely linked with mortality. Plots having the greatest m o r t a l i t y usually
also had the greatest top d a m a g e , and severely d a m a g e d trees at one m e a s u r e m e n t
were often dead at a s u b s e q u e n t m e a s u r e m e n t . Trees with m i n o r top d a m a g e or
sunscald did recover, however.
On a percentage basis, the 8 . 1 - m spacing had the m o s t m o r t a l i t y and damage.
Although the n u m b e r o f trees lost to m o r t a l i t y was relatively small, the loss o f
growing stock at the wider spacings was i m p o r t a n t . One o f the plots at the 8. lm spacing had a 20 percent reduction in n u m b e r o f s t e m s during the initial 10year period following thinning. T h e 8. l - m spacing also had a lot o f top d a m a g e
following thinning, especially on one plot. In 1959, 21 trees had their tops out;
42 / FOREST SCIENCE
• N e t b a s a l area (m=lha)
Spacing
¢~pJ2.4m
(control
30.0, I
plots)
~
25.0
.,dl,,A) 3.4m
.
20.0
4.2m
, / I b 5.0m
15.0
...~r ,,p,.. .~-
iiiXl,.
,""""
,oo
or," ,,e , ~
5.0 ~,
I
1954
,,,,,o6.0m
I
1959
FIGURE 6.
"
~oo o~*e"
.,~
I
1964
I
1969
Time
~
I
1974
I
1979
Net basal area by spacing and measurement period.
t
11 were on the 8. l-m plots. In 1964, 38 trees were listed as having damaged tops
or p o o r terminal growth; 20 were on the 8. l - m plots.
In the two most recent 5-year periods, the 3.4-m spacing had the most mortality
and the most top breakage. T h e average d i a m e t e r o f mortality in 1974 and 1979
(at the 3.4-m spacing) was smaller than the average d i a m e t e r of surviving trees;
thus, some o f this mortality was most likely c o m p e t i t i o n related. Most o f the
recent damage at this spacing was also confined to the smaller trees.
Volume a n d Basal Area G r o w t h . - Basal area growth per tree has increased sharply
with increased spacing, but the greater growth rates have not been great enough
to offset thc lesser n u m b e r o f trees. Thus, the initial differences in basal area per
hectare between treatments have widened o v e r time (Fig. 6). The rate o f gross
basal area growth has been i m p r o v i n g o v e r time with increased spacing; in addition, the recent mortality at the closer spacings is reducing net growth. In the
last 5-year period, the net change in basal area was quite similar (2.7 to 3.0 mV
ha) at the five closest spacings (i.e., 3.4- tO 6.0-m). T h e 8. l - m spacing has not yet
caught up to the others in basal area growth.
Total v o l u m e growth per hectare generally followed the same trends as basal
area growth. T h e differences in cubic v o l u m e between treatments have widened
VOLUME 29. Nu.~l,er 1. 1983 / 43
Net total
225
i-
volume
Spacing
(m31ha)
2.4m
(control
plots)
200 -
175
e/"
150
/
l
idl~l~ip3.4m
~lf'
.,~ll,lplil~jlI /4.0m
.
/
125 -
//
oe4.2m
.**e 5.0m
100
"/:/ ..., 8.0m
/
/ Ill
~,~.~
~111
.J
,
l e 8.1m
7" "" ~ **~,p # S#
,~4
25
,,,o
dP~
~
Ill tltitt41P1sttt~111ttl~' ~ ~ g.i ~ ~" .Ib
1954
I
1959
I
1964
I
1969
I
1974
I
1979
Time
FIGURE7. Net total cubic volume by spacing and measurement period.
o v e r t i m e (Fig. 7). A m o n g all but the widest spacing (3.4- to 6.0-m). however.
the range in v o l u m e growth has narrowed. F o r example, although the 6 . 0 - m
spacing has only a third as m a n y trees as the 3.4-m spacing, in the m o s t recent
m e a s u r e m e n t period, its net v o l u m e growth was a l m o s t 90 percent o f that at the
closer spacing. V o l u m e growth rates are still increasing o v e r t i m e at all spacings.
44 / FOREST SCIENCE
DISCUSSION
Precommercial thinning of Douglas-fir can result, at least on some sites, both in
immediate decreases and in long-term increases in height and diameter growth.
Both thinning shock and increased height growth following thinning have been
previously reported; however, this case history is unique in the magnitude of the
height growth responses.
Two other studies have also reported thinning shock in Douglas-fir; both were
on low quality sites (site V), similar to the site used in this study. The first study
(Miller and Reukema 1977) was in a 20-year-old plantation and combined thinning and fertilization of individual trees with nitrogen or nitrogen plus other
elements. After 5 years they found that thinning alone had not increased diameter
growth and had reduced height growth by about 25 percent. Thinning plus fertilization increased both diameter and height growth. The other study (Crown
and others 1977) was in a 24-year-old plantation and included two levels of
thinning and two levels of nitrogen fertilization. In their 3-year data, height growth
was reduced following thinning with greater reductions in height growth in the
heavier thinning treatment. Fertilization reduced thinning shock, and the heaviest
fertilizer application had the least height growth reduction. Thus, in Douglas-fir
stands which are prone to thinning shock and which are nitrogen deficient, it
appears that shock can be reduced or eliminated by fertilization.
The occurrence, duration, and severity of thinning shock are apparently related
to thinning intensity, site quality, and tree species, vigor, and age. Thinning shock
is not limited to Douglas-fir. Height growth reductions following thinning have
also been reported for red pine (Bickerstaff 1946; Berry 1965; Day and Rudolph
1966, ! 972), sand pine (Burns and Brendemucbl 1978), Sitka spruce (Jack 197 l),
balsam fir (Piene 198 l), and other species (Braathe 1957). Overall, it appears that
both the physiological shock symptoms and the physical damage following thinning are most common when stands are heavily thinned later than the recommended time for precommercial thinning (Reukema 1975). In addition, thinning
shock seems to be most common on medium to poor quality sites.
Increasing growing space, either by initial spacing or thinning, can significantly
reduce competition for the site's resources and, on some sites, increase height
growth. Interestingly enough, two of the studies in which thinning shock was
found also showed subsequent increases in height growth. The Douglas-fir plantation reported on by Crown and others (1977), where thinning reduced 3-year
height growth, was later reported on by Hall and others (1980). Six years after
treatment all the thinned treatments (fertilized and unfertilized) had greater annual
height growth increments than the corresponding unthinned plots. The authors
concluded that if the present trends in height growth continued, thinning (as well
as fertilization) would result in increased total height. Day and Rudolph (1966,
1972) reported on a 25-year-old red pine plantation (height at age 50 = 20 m)
thinned by various methods to different residual stocking levels. During the first
3 years following thinning, two of the heaviest thinning treatments had significantly reduced height growth. During the next 4 years, however, height growth
on all the thinned plots was better than on the unthinned ones.
On some sites, height growth is apparently depressed by close spacing. In a 53year-old Douglas-fir spacing trial (located l I km from our study area), heights at
the narrowest spacing (1.2 m) were 37 percent shorter than those at the widest
spacing (3.7 m) (Reukema 1979). Braathe (1957), in his review of the European
literature, provides additional examples of the effects of initial spacing and thinning on height growth.
Young Douglas-tit stands growing on poor quality sites are capable of taking
advantage of additional growing space, and wc believe wider than usual spacings
VoLli,~ie 29. Nt'xtm!r I, 1 9 8 3 / 4 5
are q u i t e p r o m i s i n g . O n s o m e sites t h i n n i n g shock m a y occur, p r i m a r i l y , we t h i n k ,
w h e n s t a n d s o n p o o r sites are h e a v i l y t h i n n e d later t h a n r e c o m m e n d e d . W e b e l i e v e
t h a t the long-term r e s u l t o f p r e c o m m e r c i a l l y t h i n n i n g such s t a n d s will be i n c r e a s e d
height a n d d i a m e t e r g r o w t h .
A l t h o u g h c o n f i r m a t i o n o f t h e s e r e s u l t s at o t h e r l o c a t i o n s is n e e d e d , we b e l i e v e
it has b e e n s h o w n t h a t h e i g h t g r o w t h o f D o u g l a s - f i r is n o t a l w a y s i n d e p e n d e n t o f
s t a n d d e n s i t y . T h e a v a i l a b l e i n f o r m a t i o n m a y be i n s u f f i c i e n t for r e f i n i n g c u r r e n t
h e i g h t g r o w t h p r e d i c t i o n s b y s i l v i c u l t u r i s t s or s t a n d m o d e l e r s ; h o w e v e r , f u t u r e
m a n a g e d s t a n d p r e d i c t i o n s s h o u l d take these effects i n t o a c c o u n t .
LITERATURE CITED
BERRY, A. B. 1965. Effect of heavy thinning on the stem form of plantation-grown red pine. Dcp
For Publ I 126, 16 p. Can For Serv, Ottawa, Ontario.
BIC'KERSTAFF,A. 1946. Effect of thinning and pruning upon the form of red pine. Dominion For
Serv Silvic Res Note 81, 33 p. Can Dept Mines and Resour, Lands, Parks, and For Branch,
Ottawa, Ontario.
BRAATHE, P. 1957. Thinnings in even-aged stands. A summary of European literature. 92 p. Fac
For, Univ New Brunswick, Fredericton.
BURNS, R. M., and R. H. BRENDEMUEHL. 1978. Sand Pine: Early responses to row thinning. USDA
Forest Serv Res Note SE-262, 8 p. Southeast Forest Exp Stn, Asheville, N C.
CROWN, M., R. V. QUENET,and C. LAYTON. 1977. Fertilization and thinning effects on a Douglasfir ecosystem at Shawnigan Lake: 3-year growth response. Can For Serv Rep BC-X-152, 36 p.
Victoria, B C.
DAY, M. W., and V. J. RUDOLPH. 1966. Early growth results of thinning red pine by three methods.
Quart Bull Mich Agric Exp Stn 49(2):183-188. Mich State Univ, East Lansing.
DAY, M. W., and V. J. RUDOLPH. 1972. Thinning plantation red pine. Mich Agric Exp Sm Res Rep
151, 10 p. Mich State Univ, East Lansing.
HAGGLUND,B. 198 I. Evaluation of forest site productivity. For Abstr 42(11): 515-527.
HALL, T. H., R. V. QUENET,C. R. LAYTON,and R. J. ROBERTSON. 1980. Fertilization and thinning
effects on a Douglas-fir ecosystem at Shawnigan Lake: 6 year growth response. Can For Serv Rep
BC-X-202, 31 p. Victoria, B C.
JACK, W. H. 1971. The influence of tree spacing on Sitka spruce growth. Irish For 28(1):13-34.
MCARDLE, R. E., W. H. MEYER, and D. BRUCE. 1961. The yield of Douglas-fir in the Pacific
Northwest. U S Dep Agric Tech Bull 201, 74 p.
MILLER, R. E., and D. L. REUKEMA. 1977. Urea fertilizer increases growth of 20-year-old thinned
Douglas-fir on a poor quality site. USDA Forest Serv Res Note PNW-291.8 p. Pac Northwest
Forest and Range Exp Stn. Portland, Oieg.
PIENE, H. 1981. Early growth responses to operational spacing in young balsam fir stands on the
Cape Breton Highlands, Nova Scotia. Can For Serv lnf Rep M-X-125. 29 p, Fredericton, N B.
REUKEMA, D. L. 1975. Guidelines for precommercial thinning of Douglas-fir. USDA Forest Serv
Gen Tech Rep PNW-30, l0 p. Pac Northwest Forest and Range Exp Sm, Portland, Oreg.
REUKEMA, D. L. 1979. Fifty-year development of Douglas-fir stands planted at various spacings.
USDA Forest Serv Res Pap PNW-253, 21 p. Pac Northwest Forest and Range Exp Sm, Portland,
Oreg.
S'rAEBLER,G. R. 1956. Evidence of shock following thinning of young Douglas-fir. J For 54(5):339.
TURNBULL, K. H., G. R. LZTTLE,and G. E. HOVER. 1970. Comprehensive tree volume tarif tables.
Revised ed, Wash Dep Nat Resour, Olympia, Wash. (Not paged.)
Reproduced by USDA Forest Service,
for o f f i c i a l use.
46 / FOREST SCIENCE