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5
¥ i a
o= Growth and Yield of

ressece Western Larch in Response
reensien §=©tQ Several Density Levels

wwenees, and Two Thinning Methods:
15-Year Results

K.W. Seidel

| E32 0 1997

ST AN
EXPERIMEN? ou RANGE

Stati
ON UaRARY copy

Abstract The 15-year growth response from a levels-of-growing-stock study in an even-aged

’ western larch (Larix occidentalis Nutt.) stand, first thinned from above and below at
age 55, was measured in northeastern Oregon. Basal area and volume growth in-
creased with stand density for both thinning methods, whereas diameter growth
decreased. Attacks of the larch casebearer (Coleophora /aricella (Hibner)) for about
3 years reduced height growth because of top dieback, but diameter and volume
growth were not severely impacted. Thinning from above reduced net volume growth
because of considerable mortality caused by windthrow and snow or ice damage,
although surviving trees responded well to increased growing space. Thinning from
below is recommended in previously unmanaged larch stands.

Keywords: Increment (stand volume), even-aged stands, stand density, thinning ef-
fects, growing stock (-increment/yield, western larch, Larix occidentalis.

Introduction Long-term levels-of-growing-stock and spacing studies provide information needed

i to design thinning regimes to attain desired rates of diameter or volume growth to
meet timber production and multiple use objectives. They also are needed to sup-
ply growth and yield data for the development of managed yield tables and to
verify simulation models designed to predict growth and yield for various manage-
ment alternatives. Western larch (Larix occidentalis Nutt.) is an important species in
the mixed-conifer forests of eastern Oregon, and many stands require thinning to
produce salable trees in a reasonable time.

This paper reports 15-year results from a levels-of-growing-stock study begun in
1970 in the Blue Mountains of northeastern Oregon.’ The purpose of the study
was to obtain information on the growth and mortality of even-aged larch stands
thinned to several density levels by two methods. It supplements earlier reports of
results for the first 5 and 10 years (Seidel 1975, 1980).

Study Area and The study site is on Boise-Cascade land about 6 miles northwest of Elgin, Oregon;

Methods the site and the timber stand are described in an earlier report (Seidel 1980).
Study results are applicable to larch stands of similar site quality throughout north-
eastern Oregon and southeastern Washington.

VA cooperative research effort between the Boise-Cascade Cor-
poration and Pacific Northwest Research Station.

K.W. SEIDEL is a research forester at the Silviculture Laboratory,
Pacific Northwest Research Station, 1027 N.W. Trenton Avenue,
Bend, Oregon 97701.

The study consists of a 2 by 4 factorial randomized complete block design
replicated two times for a total of sixteen 0.286-acre plots. It is designed for thin-
ning at 10-year intervals, with remeasurement every 5 years. The first factor, densi-
ty, consists of four levels: 50, 90, 130, and 170 square feet of basal area per acre.
These levels correspond to 21, 38, 55, and 72 percent of the basal area of normal
(fully stocked) larch stands at age 55 and site index 80 given by Schmidt and
others (1976).-2/ The second factor is the thinning method: from above (cutting the
largest trees—dominants and codominants) and from below (cutting the smallest
trees—suppressed, intermediate, and small codominants). Split-plot analyses of
variance were used to test significance of treatment effects for three 5-year periods
(1970-74, 1975-79, and 1980-84). Tukey’s test was used to determine significant dif-
ferences among treatment means. Regression analyses related diameter, basal
area, and volume growth to residual basal area for each period.

All plots were well stocked, ranging from 191 to 226 square feet of basal area per
acre before treatment (table 1). Trees were spaced from 8.5 to 10.3 feet apart;
average d.b.h. (diameter at breast height) ranged from 8.2 to 9.8 inches before thin-
ning. After thinning from above, all plots contained 2 to 8 percent of Rocky Moun-
tain Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), grand fir
(Abies grandis (Dougl. ex D. Don) Lindl.), or Engelmann spruce (Picea engelmannii
Parry ex Engelm.), except one plot where 22 percent of the residual basal area
was grand fir and Douglas-fir. All plots thinned from below were pure larch.

Plots were thinned with a Drott “feller-buncher’-*/ before growth began in 1970. This
machine uses shears and a grapple mounted on a 25-foot boom with a crawler
tractor undercarriage. Operation of this equipment required prior removal of all
trees (clearcut) in swaths 20 feet wide. Swaths were spaced 50 feet apart and
oriented east and west through the stand. The feller-buncher moved along these
clearcut strips, reaching 25 feet into the thinning strips to cut and remove the
entire tree. Some variation in residual stocking levels between replications and be-
tween thinning methods for a given density level existed because a few trees mark-
ed for cutting were missed and some leave trees were accidentally pushed over by
the feller-buncher. In plots thinned from above, the variation in residual basal area
levels in 1970 ranged from a 4-percent undercut to a 10-percent overcut, whereas
in plots thinned from below, variation from desired levels was only +1 to

-3 percent.

2/Site index of plots (83 feet at age 50) is based on curves in
“Ecology and Silviculture of Western Larch Forests” (Schmidt
and others 1976). These curves use total age (age at ground
line).

3/Mention of companies or products is for the convenience of the
reader. Such mention does not imply endorsement by the U.S.
Department of Agriculture to the exclusion of other products or
services that may be suitable.

Table 1—Stand characteristics per acre of western larch before and after the
1970 and 1980 thinnings and in 1975 and 1985-//

Dens Ity
level 2/

Thinned

WN

Thinned

WN

Thinned
1

2
3
4

Thinned

PWN

Thinned

WH

Thinned

WN

ThInned

Number
of

Basal

area trees

Square

_feet_

from above:

2211.5
191.5
ZB
202.0

from below:

211.0
205.5
221.5
226 .0

from above:

50.6
93.8
126.0
153.0

from below:

48.9
87.7
131.8
169.0

from above:

50.4

from below:

55.5
96.5
141.4
183.6

from above:

219

111
239
220
378

58
115
196
219

110
221
220
348

Quadratic

Average mean
spacing dlameter

Eeet Inches

Feet

Average
helght 3/ Total

Cub Ic
feet

BEFORE INITIAL (1970) THINNING

aoaowwo
Or oe OF
Woh oO

omwao
Cin On 5
OWN O

18.1 8.4
13.0 8.2
az 10.0
10.3 8.4
27.4 12.4
19.5 11.9
14.9 est
14.1 11.9
1975 5/
19.8 oot
S02) 8.7
14.1 10.5
10.7 8.8
27.4 Uaioe!
19.5 12.5
14.9 11.5
14.1 12.4

ire)
oO
On a) on 4
ow

ive)
oOoNOOD

oOuUOW

&
aiectesy
ro-—

BEFORE 1980 THINNING 5/

19.9 9.9
14.2 9.5
14.1 Uo
eZ 9.2

See footnotes at end of table.

83.4
81.7
88.5
84.4

Sroiues

3
Ate
7
4

Uoiuy
6,032
6,883
6,285

6,892
6 ,688
7,192
7,402

1,647
2,906
4,091
4,670

1,686
2,974
4,448
5,801

1,668
3,105
4,346
4,876

1,925
BAI
4,775
6,274

1,919
Sh SEE)
5,022
5,002

Volume 4/

Merchantab le,
Including Ingrowth

Cubic

Board
feet

26 ,816
18,540
27,020
18,045

24,555
20,898
19,479
26 , 446

2,816
5,687
13,990
10,054

8,331
13,503
16,790
24,440

4,309
7,273
15,880
13,260

10,005
16,141
19,732
28,440

5,382
8,868
19,103
15,294

Table 1—Stand characteristics per acre of western larch before and after the
1970 and 1980 thinnings and in 1975 and 1985’ (continued)

Volume 4/
Number Quadratic
Density Basal of Average mean Average Merchantab le,
level 2/ area trees spacing diameter helght 3/ Total Including Ingrowth
Square Cubic Cubic Board
afeet-=,))) | Eset’ linchesis |) Eset feoiaa foot aammEetootas
BEFORE 1980 THINNING 5/ (continued)
ThIinned from below:
1 64.3 58 27.4 14.3 101.7 2,288 2,218 11,636
2 107.2 WU 198) 13.2 96.8 3,729 3,591 18,869
3 (Di 196 14.9 11.9 96.9 5,136 4,998 21,478
4 196 .3 219 =14.1 12.9 97.3 6,740 6,493 32,225
AFTER 1980 THINNING 5/
Thinned from above:
1 535 98 21.1 10.0 80.4 157/26 1,656 4,921
2, 96.2 214 314.5 9.2 78.1 3,050 2,841 7,561
5} 140.0 209 +=14.4 11.1 85.3 4,623 4,336 17,118
4 160.6 SY Nilo? 9.2 84.0 4,958 4,543 15,082
Thinned from below:
1 54.9 46 30.8 14.8 102.4 1,955 1,895 10,171
2 99.2 OZR Zhe 13.7 98.3 3,409 3,290 17,425
3 139.7 175) 16.0 1225 98.3 4,782 4,659 20,402
4 183.6 190 15.3 13.4 100.4 Ope Op, lIZ 31,991
1985 5/
Thinned from above:
1 58.4 82 eZSrn 11.4 81.5 1,998 1,894 8,311
2 99.2 168 16.1 10.4 81.5 3,328 3,098 11,986
3} 157.0 189 15.2 12.3 86.5 5,527 5,184 24,773
4 156 .3 278 812.7 10.1 87.2 5,315 4,869 20,086
Thinned from below:
1 65.6 46 31.0 16 .3 105.4 2,500) 25,295 13,782
2 109.6 OB Zila 14.6 99.2 4,015 3,874 21,946
3 140.0 149 «17.1 13.2 100.3 4,913 4,785 23,252
4 203.2 18275 91526 14.4 103.0 7,396 7,145 40,107

i/ Based on plots without clearcut strips.

2/ 1 Is lowest; 4, highest.

3/ Measured with a dendrometer (about 15 trees per plot).

4/ Total cubic-foot volume--entire stem, Inside bark, all trees. Merchantable
cubic-foot volume--trees 5.0-Inch d.b.h. and larger to a 4-Inch top d.1.b.

Board-foot volume--International 1/4-Inch rule, trees 10.0-Inch d.b.h. and
larger to a 6-Inch top d.!I.b.

5/ Basal area, number of trees, and volume per acre should be reduced by 29 percent
If clearcut strips are Included In plot area.

Results

Mortality and Damage

In April 1980, 10 years after the first thinning, plots were thinned for the second
time with chain saws. Plots were not thinned to the original density levels after the
1970 thinning but were marked to allow an 8-percent increase in basal area, ap-
proximately the normal increase in stand density with age for fully stocked stands.
The adjusted density levels after the second thinning were 54, 97, 140, and 184
square feet of basal area per acre, again corresponding to 21, 38, 55, and 72 per-
cent of the density of normal stands at age 65.

Diameter at breast height of all plot trees was measured to the nearest one-tenth
inch in the spring of 1970 and in the fall of 1974, 1979, and 1984. In addition, about
15 trees per plot covering the range of diameters were measured with an optical
dendrometer in 1970, 1974, 1979, and 1984 to derive an equation expressing volume
of the entire stem inside bark as a function of diameter for each plot. The volume
equations developed from the 1970 measurements were used to compute plot
volumes (cubic feet and board feet, International 1/4-inch rule) at the beginning and
end of the first 5-year period. New equations developed from the 1979 measure-
ment were used to compute plot volumes at the end of the second and third 5-year
periods. Height growth was measured by dendrometer only on trees chosen to pro-
vide data for volume equations.

Because of the mechanized thinning equipment used in this stand, 29 percent of
the total area was occupied by clearcut strips, which resulted in a reduction in
volume growth compared with a thinned area completely occupied by trees.
Therefore, growth per acre is presented two ways—based on the 0.286-acre plot
completely occupied by trees and on a larger 0.4-acre plot that includes the clear-
Cut strips.

Examination of data on basal area and volume growth for the plot containing 22
percent of the basal area in fir revealed an unusually high growth rate because of
the more rapid growth of the fir. Therefore, data from this plot were not used in the
growth and statistical analyses.

Considerable mortality and damage occurred during the 15 years of this study
because of wind, snow, and ice damage and attacks of the larch casebearer (Co/-
eophora laricella (HUbner)). During the first 10 years of the study, all mortality oc-
curred in plots thinned from above and was caused by either windthrow or shock
after release. Of the 570 trees in these plots, 12 percent died—7 percent during the
first 5-year period and 5 percent during the second. Most of these trees were in
the intermediate and suppressed crown classes and thus had not developed suffi-
cient windfirmness or large enough crowns to keep pace with the increased
respiratory rate after release. During the third period, an additional 15 percent of
the trees in plots thinned from above died as the result of a severe ice storm in
January 1984 that broke tree boles below the live crown. Also, 6 percent of the 377
trees in plots thinned from below died during the third period from storm damage.
To summarize, 20 percent or 167 of the 855 trees present at the beginning of the
study in all plots died during the 15 years of this study. Eight-five percent of the
mortality (142 trees) occurred in plots thinned from above but only 15 percent

(25 trees) in plots thinned from below.

Diameter Growth

Wind or ice also damaged 12 percent of the trees in plots thinned from above com-
pared with 3 percent in plots thinned from below during the 15 years. This damage
consisted of trees leaning from 10 to 35 degrees from the vertical or several feet of
the top broken off. In 1976, the larch casebearer moved into the study area and at-
tacked larch in all plots. Dieback of terminals occurred on many trees during the
second period from 1976 to 1979. No casebearer damage was observed during the
third period.

Diameter growth from 1970 through 1984 was greatest at the lowest density level
for both thinning methods (table 2). As stand density increased, diameter growth
generally decreased, although this trend was not completely consistent. The growth
rate at the lowest density was about twice that at the highest density (0.2 vs. 0.1
inch per year) when averaged over all periods and thinning methods. Diameter
growth was significantly greater (P<0.01) at the lowest density level, but no statisti-
cal differences were found among the other three levels (0.14, 0.11, and 0.10 inch
per year).

Diameter growth rates also changed over time. Significant diameter growth dif-
ferences (P<0.01) were found among all periods (fig. |). Only small differences in
growth existed between the first and second periods, but during the third period
growth accelerated considerably. Averaged over all density levels and thinning
methods, diameter growth was 0.10 inch per year in the first period and 0.13 inch in
the second, but growth increased to 0.20 inch per year in the third period. The
more rapid diameter growth during the third period can be attributed to the com-
bined effects of the second thinning in 1980 and the decline in larch casebearer
populations about that time. A significant (P<0.01) period-density interaction occur-
red because of the relatively greater growth differences among periods at the
lowest density level than at the other levels.

The thinning method did not affect diameter growth. Differences in diameter growth
between thinning methods were not significant; they averaged 0.13 inch per year in
plots thinned from above and 0.15 inch in those thinned from below.

After 15 years of growth and two thinnings, the average stand diameter in plots
thinned from below at the lowest density level was 16.3 inches compared with 11.4
inches in plots thinned from above (table 1). This difference of almost 5 inches is
caused primarily by removal of larger trees in plots thinned from above versus
removal of smaller trees in plots thinned from below. Annual diameter growth in
these plots during the 15-year period was 0.13 inch (above) and 0.15 inch (below).

Table 2—Periodic annual increment and mortality per acre of western larch by age,
density level, and thinning method after thinning at ages 55 and 65

Merchantab le
volume growth,

Basal area growth Total volume growth Including Ingrowth
Residual
Density basal Diameter
level 1/ area growth 2/ Net Mortality Gross Net Mortality Gross Net Mortality Gross Ingrowth
Square Board
feet inches pe NQUOLO=T OST ae ee CUD LCETEOT =) == ===) Board’ feet =) — = feet Percent

AREA WITHOUT CLEARCUT STRIPS, AGE 55-60 (1ST PERIOD)

Thinned from above:

1 51 0.10 -0.04 Netz 1.08 4 34 38 299 0 299 225 phir
2 94 -08 1.08 ~85 1.93 40 25 65 317 0 317 163 51.4
5 126 08 1.38 Voue4 2.50 51 37 88 378 132 510 202 39.6
4 153 +05 1.04 +91 1.95 4) 24 65 641 0 641 456 ar
Thinned from below:
1 49 16 1.32 0 1.32 48 0 48 335 0 335 33 9.8
2 88 AZ 1.76 0 1.76 68 0 68 528 0 528 109 20.6
| 132 -08 1.92 0 1.92 65 0 65 588 0 588 278 47.2
4 169 -09 2.92 0 2.92 95 0 95 800 0 800 248 31.0
AREA WITH CLEARCUT STRIPS, AGE 55-60 (1ST PERORD)
Thinned from above:
1 %6 +10 -0.03 81 .78 i) 24 27 213 0 213 161 75.6
2 67 -08 -78 -61 1.39 29 18 47 227 0 227 116 49.8
3 89 -08 +98 -80 1.78 36 26 62 268 94 362 143 39.6
4 109 -05 74 +65 1.39 29 18 47 458 0 458 326 71.2
Thinned from below:
1 35 16 +94 0 94 34 0 34 239 0 239 24 10.0
2 63 012 1.25 0 1.25 48 0 48 377 0 377 78 20.7
3 94 -08 Ee 0 1.36 47 0 47 420 0 420 198 47.1
4 121 .09 2.07 0 2.07 68 0 68 571 0 571 177 31.0
AREA WITHOUT CLEARCUT STRIPS, AGE 60-65 (2D PERIOD)
ThInned from above:
1 50 cle 1.78 06 1.84 50 2 52 215 0 215 83 38.6
2 99 -10 1.22 1.10 2.352 46 33 79 319 32 351 188 53.6
3 133 14 3.84 0 3.84 135 0 135 645 0 645 267 41.4
4 158 -06 75 1.72 2.47 25 50 75 407 84 491 295 60.1
Thinned from below:
56 -20 1.76 0 1.76 73 0 ls) 326 0 326 0 0
2 97 «12 2.13 0 2.13 83 0 83 545 0 545 28 5.1
3 141 -08 2.03 0 2.03 72 0 72 350 0 350 96 27.4
4 184 +09 2.55 0 2.55 93 0 93 757 0 757 113 14.9
AREA WITH CLEARCUT STRIPS, AGE 60-65 (2D PERIOD)
Thinned from above:
1 % 17 1.27 05 Wosy 36 1 37 153 0 153 59 38.6
2 71 -10 86 78 1.64 33 23 56 228 23 251 133 53.0
3 95 14 2.73 0 2.73 96 0 96 458 0 458 190 41.5
4 113 06 Ah} 1.22 Nave) 18 36 54 289 60 349 209 59.9
Thinned from below:
1 40 -20 1.25 0 1.25 52 0 52 233 0 233 0 0
2 69 12 1.52 0 Wob¥e 60 0 60 390 0 390 20 5.1
3 101 .08 1.45 0 1.45 52 0 52 250 0 250 69 27.6
4 131 +09 1.83 0 1,83 66 0 66 541 0 541 81 15.0
AREA WITHOUT CLEARCUT STRIPS, AGE 65-70 (3D PERIOD)
ThInned from above:
1 53 +27 1.02 1.54 2.56 55 49 104 678 109 787 401 51.0
2 96 +19 +59 2.90 3.49 56 87 143 926 54 980 370 37.8
3 140 +20 3.40 1.56 4.9% 159 45 204 1,531 0 1,531 510 33.3
4 161 +12 -.86 4.37 3.51 26 131 157 1,071 84 1,155 326 28.2
Thinned from below:
55 +29 2.13 0 Zed 82 0 82 722 0 722 0 0
2 99 +20 2.08 ei Ze5o) 125 10 131 1,094 0 1,094 63 5.8
3 140 15 0.06 2.59 2.65 26 98 124 746 226 972 96 9.9
4 184 -18 3.91 - 86 4.77 168 31 199 1,623 102 ie} 343 19.9
AREA WITH CLEARCUT STRIPS, AGE 65-70 (3D PERIOD)
Thinned from above:
1 38 fd «72 1.09 1.81 39 35 74 481 ih 558 285 51.0
2 68 19 42 2.06 2.48 40 62 102 657 38 695 263 37.8
3 99 -20 2.41 1.11 Supe.) 115 32 145 1,087 0 1,087 362 33505
4 114 12 61 3.10 2.49 18 93 11 760 60 820 231 28.2
Thinned from below:
1 39 +29 1.51 0 1.51 58 0 58 513 0 513 0 0
2 70 +20 1.48 +22 1.70 86 7 93 777 0 777 45 5.8
3 99 15 +04 1.84 1.88 18 70 88 530 160 690 68 9.9
4 131 -18 2.78 -61 3.39 119 22 141 1,152 72 1,224 244 19.9

V/ 1 Is lowest; 4, highest.

2/ Arithmetic diameter growth of trees |iving through three 5-year perlods (1970-74, 1975-79, 1980-84).

Height Growth

0.35

Thinned from above

- --- Thinned from below

0.30

0.25

0.10

Periodic annual diameter growth (inches)

0.05

0.00
40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0

Basal area (square feet per acre)
Figure 1—Periodic annual diameter growth by thinning method

and growth period as a function of stand density; number in
parentheses are growth periods.

Height growth was not affected by changes in stand density, but significant

(P <0.05) growth differences were found among periods and thinning methods. In-
crement ranged from a high of 0.88 foot per year during the first period to a low of
-0.15 foot during the third period (fig. 2). Averaged over all density levels and thin-
ning methods, height growth decreased significantly from 0.76 foot per year in the
first period to 0.13 foot in the second period because of dieback caused by the
larch casebearer. Height growth recovered slightly during the third period to 0.32
foot per year but was still significantly less than first period growth because of ice
damage. Average annual height growth for the 15 years in plots thinned from
above was about 0.30 foot per year compared with 0.51 foot in plots thinned from
below, a significant difference.

Growth period

1st 2d 3d 1st 2d 3d 1st 2d 3d 1st 2d 3d
1.0
(al Thinned from above
088 088
7] thinned from below
~ 0.8b (7
~ 4
o Yi, /
rn OS 47
- 47 “44
bere 061|77 - 4
= wy by 07
= 0.67 [7 Zs y
fo) 4 A 4/
_ o 4% a,
o 77 77 oe
= ; 3 Y
a a ig Le
o 0.4 y Ye iy
= y Z ig
i) fe Ye Z
=| 47 7
= os
c 77
iy] 4
a] 0.2 Z 016 016
a] 44
° y
= J
a y
es 6
0

50 90 130 170

Basal area (square feet per acre)

Figure 2—Periodic annual height growth by density level, thin-
ning method, and growth period.

Basal Area Growth

Volume Growth

10

Periodic gross annual basal area increment showed a linear upward trend for both
thinning methods and all periods as stand density increased (table 2, fig. 3). Trees
in plots thinned from above to the lowest density level in the first period grew 0.78
Square foot per acre compared with a maximum growth rate of about 3.5 square
feet in high density plots in the third period.

Significant differences (P <0.01) in basal area growth were found among density
levels and periods (table 3). The growth rate at the highest level was significantly
greater than that at the lowest level (3.03 vs. 1.78 square feet per acre), but all
other comparisons were nonsignificant. Average gross basal area growth increasd
from 1.92 to 3.20 square feet per acre from the first to the third period, and all dif-
ferences were significant.

Because of considerable mortality which tended to minimize differences in net
growth (table 3), no significant differences were found in net basal area growth
among density levels, periods, or thinning methods. For example, mortality offset
54 percent of the gross basal area growth during the third period compared with
only 15 to 20 percent during the first two periods. This resulted in a net growth rate
during the third period about equal to that of the first period. Mortality in plots
thinned from above amounted to 55 percent of the gross growth compared with
only a 15-percent loss in plots thinned from below.

Total gross cubic volume increment showed a positive linear relationship to stand
density similar to the basal area stand density relationship (table 2, fig. 4). Gross
annual increment varied greatly, ranging from a low of 27 cubic feet per acre dur-
ing the first period to a high of 204 cubic feet during the third period. The relation-
ship of volume growth to stand density was similar during the first two periods, but
during the third period the slope of the curves increased, which suggested greater
volume production per square foot of basal area (fig. 4).

Gross cubic volume growth increased significantly (P <0.01) with increasing stand
density from 66 cubic feet per acre annually at the lowest level to 114 cubic feet at
the highest (table 3). The growth rate at the lowest level was significantly less than
that at the other three levels, and level 2 was also significantly less than level 4.
Net cubic volume growth was considerably less than gross growth because of the
mortality, which ranged from 21 to 34 percent among density levels, and all dif-
ferences in net growth were nonsignificant.

Gross cubic volume growth increased from the first through the third period, but
differences between the first and second period were not significant (table 3). The
greatest increase occurred during the third period when the average annual growth
rate rose to 141 cubic feet per acre. A 40-percent mortality rate, however, reduced
net growth to 84 cubic feet per acre, which was not significantly different from the
first two periods. Gross growth in plots thinned from above was not significantly dif-
ferent from growth in plots thinned from below, but the much greater mortality in
the plots thinned from above resulted in a significant net growth advantage for the
plots thinned from below.

—— Thinned from above (3)
= - (3)
2 ---- Thinned from below Bas
fo} ZOO
5 4 ie
o o Om
® 6 ie
= 8 a (2)
—_ bk Tag -— 1
S ~ 3 pecs Wey ( )
FS o Bee aerial (1)
ro] 2 pra ane Sean)
zo = paso
ee es

Seas 2 = sae s
no W cae Bee =
(o) sa
-_
D
2 1
so]
2
o
a

10)

40 60 80 100 120 140 160 180 200

Basal area (square feet per acre)

Figure 3—Periodic annual gross basal area growth by thinning
method and growth period as a function of stand density;
numbers in parentheses are growth periods.

Table 3—Average annual net and gross basal area, total cubic volume and board-foot volume growth and
mortality per acre, by density level, growth period, and thinning method-’

Basa! area growth Total volume growth Merchantable volume growth

Item
Net Gross Mortal ity Net Gross Mortal ity Net Gross Mortality
- - - Square feet - - - Percent - - Cuble feet - - Percent - - Board feet - - - Percent
By density level averaged over all perlods and thIinnIng methods:
1 (low) 1.40a 1.78a 0.38a Zi 52a 66a 14a 21 429a 448a 19a 4
Z 1.41a 2.34ab -93a 40 69a 94b 25a 27 622b 627ab 5a 1
3 1.98a 2.73ab -15a 27 77a 107be 30a 28 625b 690b 65a 9
4 (high) 1.75a 3.03b 1.28a 42 75a 114c 39a 34 883c 928c 45a 5
By perlods averaged over all density levels and thinning methods:
1970-74 1.50a 1.92a 42a 22 5la 66a 15a 23 482a 502a 20ab 4
1975-79 1.95a 2.30b .35a 15 69a 80a lla 14 430a 4444 14a 3
1980-84 1.47a 3.20c 1.73b 54 84a =141b 57b 40 1,008b 1,073b 65b 6
By thinning methods averaged over all density levels and periods:
Above 1.27a 2.38a 1.31a 55 54a 96a 42a 44 579a 618a 39a 6
Be low 2.01a 2.36a -35b 15 83b 94a 11b 12 701b 729b 28a 4

1/ Based on plots without clearcut strips.

Means followed by the same letter are not significantly different.

11

240

—— Thinned from above

= ---- Thinned from below (3)
s 200 e
2 ae
wo ad (3)
o ita
: =
BO 160 uee
iano pie ais
ape bee
eo eee
T= 120 wee (2)
oO |
” ® “7
(2) “2 pa ae (1)
en) Eo on”
m5 Or 22s
23 80 Sai ie eee G)
en Bor (2) Sooo eS ee cient
o rune fi ee are
rot ae
a vii eres
° 40
EK

O

40 60 80 100 120 140 160 180 200

Basal area (square feet per acre)

Figure 4—Total periodic gross annual cubic volume growth by
thinning method and growth period as a function of stand densi-
ty; numbers in parentheses are growth periods.

At high stand densities, volume increment is generally greater but is distributed
over a large number of trees, many of which are smaller and slow growing. Thin-
ning transfers growth to fewer but faster growing trees in addition to utilizing poten-
tial mortality. For example, during the second period, in plots thinned from below,
58 trees per acre at the low density produced 78 percent of the cubic volume
grown by 219 trees per acre at the high level.

Gross board-foot volume increment increased linearly with greater stand density
during all periods (fig. 5). Annual growth at the lowest level averaged 448 board
feet per acre over all periods and thinning methods and increased to 928 board
feet at the highest density; significant differences existed among levels (table 3).
Board-foot mortality was less than 10 percent of gross growth, so significant dif-
ferences in net growth are similar to those of gross growth. Both gross and net
growth decreased nonsignificantly from the first to the second period but more
than doubled during the third period to about 1,000 board feet per acre annually
(table 3). The board-foot growth rate was significantly greater in plots thinned from
below for both gross and net increment. Ingrowth accounted for a considerable
amount (up to 75 percent) of the volume during the first two periods but decreased
during the third period as fewer trees remained to enter board-foot size (table 2).

en Be) ee)

Thinned from above Pie

Casa) a
~ ---- Thinned from below ok
oO ¢
. 1200 oie
® 2
a in
7 a
o Z
“ Zz
v 1000
° Z
2 aan
= 800 ao cae
~ -
5 $< Re (1)
o ea ene (2)
o ra Ce = zal (2)
E e00b oa
= a=
To) aC ei 5 ——
> Jiven See
3 bo
=|] a ae
= J hee
~ 400 Laem eeeck.
rH eens,
3 a
ro.)
o 200
se)
2
o
a

O DR ThE Te we SE Ee ee) ee

40 60 80 100 120 140 160 180 200

Basal area (square feet per acre)

Figure 5—Perioic annual gross board-foot (International 1/4-inch
rule) volume growth by thinning method and growth period as a
function of stand density; numbers in parentheses are growth
periods.

Culmination of mean annual cubic increment appears to have occurred at about
age 55; as expected, board-foot increment is still increasing (table 4). Cubic-and
board-foot volume growth rates measured in this study from age 55 to 70 agree
well with data from yield tables developed for larch in Montana (Schmidt and
others 1976). Based on the Montana data, mean annual cubic increment in this
study could be expected to decline slowly to about 100 cubic feet per acre at age
140 while mean annual board-foot increment increases to about 650 board feet per
acre at the same age.

Thinning with the feller-buncher caused about 29 percent of the total area to be
occupied by clearcut strips, with a corresponding 29-percent reduction in volume
growth compared with a thinned area completely occupied by trees. The growth
and mortality data on an area that includes the clearcut strips are shown in
table 2.

13

Total Yield and Tree Size

14

Table 4—Net mean annual increment of western larch per acre
Age Age
Basa] area per acre 55 60 65 70 55 60 65 70

Square feet = cileny CUT Cet Cet utr e 77) >) Board’ feet aye hans
AREA WITHOUT CLEARCUT STRIPS

Thinned from above:

50 130 120 115 110 488 472 452 468
90 110 104 99 96 337 335 334 376
130 125 119 120 125 491 482 494 568
170 114 108 102 100 328 354 358 409

Average 120 VS 109 108 411 411 410 455

Thinned from below:

50 125 119 115 113 446 437 429 450
90 122 lie, 115 116 380 392 363 453
130 131 125 121 120 354 374 372 473
170 135 131 128 134 481 507 527 605

Average 128 123 120 121 415 428 423 495

AREA WITH CLEARCUT STRIPS

Thinned from above:

50 93 85 82 78 347 335 321 332
90 78 74 70 68 239 238 237 267
130 89 84 85 89 349 342 351 403
170 81 UT TZ, 71 233 251 254 285

Average 85 80 77 77 292 292 291 S22

ThInned from below:

50 89 84 82 80 317 310 305 320
90 87 83 82 82 270 278 258 372
130 93 89 86 85 251 266 264 336
170 96 93 91 95 342 360 374 430

Average 91 87 85 86 295 304 300 350

Total net yield in plots thinned from below increased as stand density became
greater. Cubic-foot yield ranged from about 7,900 cubic feet per acre at the lowest
density level to about 9,400 cubic feet at the highest level in 1985 (table 5). Board-
foot yield was greatest at the highest level (42,000 feet per acre) in plots thinned
from below, but only small differences were found between the other three levels
(31,000 to 33,000 board feet per acre). Total yield in plots thinned from above did
not show a consistent relationship to stand density but varied from about 6,700 to
8,600 cubic feet per acre. Averaged over all density levels, total net cubic-foot yield
15 years after the initial thinning was about 12 percent greater in plots thinned from
below than in those thinned from above. In terms of board feet, plots thinned from
below have a 9-percent advantage in yield.

Table 5—Total net growth and yield of western larch per acre, by density level

and thinning method-/

Residual basal area level (square feet) and thinning method

50 90 130 170
Item Above Be low Above Be low Above Be low Above Be low
Number of trees
Total trees, 1970 420 490 495 557 408 596 513 525
Cut, 1970 287 432 238 442 177 400 113 306
Left, 1970 133 58 257 115 231 196 400 219
Cut, 1980 35 12 43 13 22 21 53 29
Left, 1980 98 46 214 102 209 175 347 190
Total trees, 1985 82 46 168 98 189 149 278 182
15-year mortality 37 -- 82 3 31 26 121 7
inches
Quadratic mean dlameter, 1985 11.4 16.3 10.4 14.6 12.5 UBIR~4 10.1 14.4
Percent
Trees, 10 Inches In d.b.h.
and larger, 1985 57.0 100 45.7 100 72.2 88.8 49.0 100
Cubic feet
Total volume:
Total stand, 1970 Upluel 6,892 6,032 6,688 6,883 7,192 6,285 7,402
Cut, 1970 5,530 5,206 3,126 3,714 2,792 2,744 1,615 1,601
Left, 1970 1,647 1,686 2,906 2,974 4,091 4,448 4,670 5,801
Cut, 1980 193 333 283 320 399 354 44 413
Left, 1980 1,726 1,955 3,050 3,409 4,623 4,782 4,958 6,327
Net 15-year growth 545 1,015 710 1,360 A223) 1,208 715 1,978
Total net yleld, 1985 Uae 7,907 6,742 8,048 8,608 8,400 7,000 9,380

Board feet (International 1/4-Inch rule)

Merchantable volume:

Total stand, 1970 26,816 24,555 18,540 20,898 27,020 19,479 18,045 26,446
Cut, 1970 24,000 16,224 12,853 7,395 13,030 2,689 7,991 2,006
Left, 1970 2,816 8,331 5,687 13,503 13,990 16,790 10,054 24,440
Cut, 1980 461 1,465 1,507 1,444 1,985 1,076 212 234
Left, 1980 4,921 10,171 7,361 17,425 17,118 20,402 15,082 31,991
Net 15-year growth 5,960 6,915 7,810 10,835 12,770 13,631 10,595 15,900
Total net yleld, 1985 32,776 31,470 26 , 350 31,733 39,790 33,110 28,640 42,346

1/ Based on plots without clearcut strips.

In plots thinned from below, quadratic mean diameter in 1985 ranged from 13.2 to

16.3 inches (table 5). Although stand diameter was not greatly different among den-

sity levels, plots at the highest density contained more unmerchantable trees,
which resulted in less merchantable volume when the second thinning was made.
For example, at the 90-square-foot density level, 13 trees per acre removed in the

second thinning from below yielded 1,444 board feet. In contrast, 29 trees per acre

cut from the 170-square-foot level yielded only 234 board feet.

15

Discussion

It is evident that western larch stands have considerable ability to maintain
diameter and volume growth despite attacks by the larch casebearer. In spite of
heavy infestation of the casebearer during part of the second period, growth in-
creased. Prolonged attacks, however, may have a more negative effect on growth.
It is also encouraging to observe the excellent growth response during the third
period after the second thinning when the casebearer was no longer present.
Although the casebearer did not impact diameter growth at breast height’ or
volume growth, it did have an adverse effect on height growth. In young stands a
sustained reduction in height growth caused by insect damage can make the
shade-intolerant larch lose its competitive advantage to its more shade-tolerant
associates and eventually be eliminated from the stand.

Gross cubic volume growth in plots thinned from above was about equal to growth
in plots thinned from below, but because of much greater mortality in plots thinned
from above net growth was considerably less in these plots. The smaller trees re-
maining are not able to withstand wind, snow, and ice after the protection of the
larger dominant and codominant trees is removed. Therefore, thinning from below
is recommended in previously unmanaged larch stands.

Selecting a suitable stocking level after thinning is a compromise between high
stand densities that produce the most wood volume and low densities that result in
greater diameter growth. In this study, net periodic cubic volume increment and
yield did not differ greatly among density levels; and diameter growth was also
relatively uniform, except at the lowest level. Therefore, the land manager has con-
siderable latitude in selecting an appropriate growing stock level to attain desirable
product characteristics. In terms of timber management, stands should be thinned
from below to a level that will maintain acceptable tree vigor and diameter growth
and will minimize mortality without sustaining unacceptable volume losses.

Comparisons of the stand density levels in this study with stocking-level curves for
larch prepared by Cochran (1985) show that plots initially thinned to 130 square
feet of basal area per acre (level 3) fall between the lower (minimum) and upper
(maximum) curves for site index 80 stands. The 50- and 90-square-foot levels are
below the minimum stocking curve, whereas the 170 level is near the maximum
curve. To reduce stand density to correspond to the minimum curve, a residual
basal area of about 110 square feet per acre is indicated if the average stand
diameter is about 12 inches.-‘/

4/Although diameter growth at breast height was not affected by
defoliation, the effects of defoliation are usually greatest in the
live crown and therefore diameter growth may have been reduc-
ed in the upper portions of the bole (Kulman 1971).

*/The lower and upper stocking-level curves given by Cochran
(1985) are based on 45 and 75 percent of normal stocking,
respectively.

Metric Conversions

In a shade-intolerant species such as western larch, early thinning is necessary.
Although thinning in older stands can increase diameter and volume growth of
residual trees, the greatest gain from thinning seral species (such as larch) is ob-
tained when thinning is begun much earlier. Schmidt (1966) suggests that the
ideal time for precommercial thinning of larch is when trees are about 10 years old
and 10 to 15 feet tall. Such early thinning maintains vigorous, full-crowned trees
and concentrates the rapid growth during the sapling and pole stages on crop
trees. In addition, trees in stands thinned early develop greater resistance to wind,
snow, and ice damage than trees growing in dense stands.

To summarize, early precommercial thinnings when trees are about 10 years old
and 10 to 15 feet tall are recommended in overstocked young larch stands. The
spacing selected should result in a diameter growth rate that will allow merchan-
table trees to be cut in the next (commercial) thinning. A range in spacings of 13 to
19 feet after the precommercial thinning should meet this objective. Using a closer
spacing assumes that smaller trees will be salable at the time of the commercial
thinning, whereas a wider spacing implies that larger trees are needed for the
commercial thinning. After the stand reaches merchantable size, thinnings from
below to reduce basal area to the minimum curve (Cochran 1985) are
recommended.

mile = 1.61 kilometers

foot = 0.3048 meter

inch 2.54 centimeters

acre = 0.4047 hectare

square foot/acre = 0.2296 square meter/hectare
cubic foot/acre = 0.0700 cubic meter/hectare
tree/acre = 2.47 trees/hectare

ee ee ee ee

Literature Cited

Cochran, P.H. Site index, height growth, normal yields, and stocking levels for
larch in Oregon and Washington. Res. Note PNW-424. Portland, OR: U.S. Depart-
ment of Agriculture, Forest Service, Pacific Northwest Forest and Range Experi-
ment Station; 1985. 23 p.

Kulman, H.M. Effects of insect defoliation on growth and mortality of trees. Annual
Review of Entomology. 16: 289-324; 1971.

Schmidt, Wyman C. Growth opportunties for western larch. Res. Note INT-50.
Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest
and Range Experiment Station; 1966. 4 p.

Schmidt, Wyman C.; Shearer, Raymond C.; Roe, Arthur L. Ecology and
silviculture of western larch forests. Tech. Bull. 1520. Washington, DC: U.S.
Department of Agriculture, Forest Service; 1976. 96 p.

Seidel, K.W. Response of western larch to changes in stand density and structure.
Res. Note PNW-258. Portland, OR: U.S. Department of Agriculture, Forest Ser-
vice, Pacific Northwest Forest and Range Experiment Station; 1975. 11 p.

Seidel, K.W. Growth of western larch after thinning from above and below to
several density levels: 10-year results. Res. Note PNW-366. Portland, OR: U.S.
Department of Agriculture, Forest Service, Pacific Northwest Forest and Range
Experiment Station; 1980. 20 p.

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