Growth and yield of western larch : 15-year results of a levels-of-growing-stock study

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Y Forest Service of Western Larch: 15-Year
(/ Pacific Northwest Results of a Levels-of-

Forest and Range

Experiment Station >Growing-Stock Study

Research Notes
1,31% PNW-398
July 1982 Sead

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

even-aged western larch (Larix occidentalis Nutt.) stand in northeastern
Oregon, first thinned at age 33, showed that trees growing at lower stand
densities grew more rapidly in diameter but did not grow faster in height
than trees in high density plots. Both basal area and total cubic volume
increment increased as stand density increased. Despite the large

. reduction in volurne increment at the lower densities, however, most of

the wood is concentrated on fewer, faster growing trees that can reach
usable size sooner.

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

. Introduction Levels-of-growing-stock and spacing studies provide information on
long-term growth and yield for managed stands that is needed to verify
simulation models and design thinning schedules to meet timber production
and multiple use objectives. In 1966, a levels-of-growing-stock study was
begun in young, even-aged western larch (Larix occidentalis Nutt.) in
northeastern Oregon. The study was done to provide information on the
growth response of western larch to a wide range of stocking levels. This
paper reports results 15 years after the study was begun. Results for the
first 5 ana 10 years were reported by Seidel (1971, 1977).

Study Area The study is located on the Union District of the Wallowa-Whitman

and Methods National Forest about 15 miles southeast of Union, Oregon, at an elevation
of about 4,000 feet. The stand was 33 years old in 1966 and has a site
index of about 80 feet at age 50.

1/sSite index based on curves in "Ecology and
Silviculture of Western Larch Forests" (Schmidt
et al. 1976).

K. W. SEIDEL is a silviculturist at the
Silviculture Laboratory, Pacific Northwest
Forest and Range Experiment Station,

1027 N. W. Trenton Avenue, Bend, Oregon 97701.

All plots were well stocked before the initial thinning, each containing at
least 25,000 square feet of bole area per acre (table 1). There were about
1,300 trees per acre, averaging 4.5 inches in diameter at breast height
(d.b.h.) and 45 feet tall. All trees are larch except for one plot at the
highest density level and one plot at the second highest level where about
40 percent of the bole area and basal area left after the initial thinning
was lodgepole pine (Pinus contorta Dougl. ex Loud.).

The soil is a Tolo silt loam, which is a well-drained Regosol developed
from dacite pumicite originating from the eruption of Mount Mazama
(Crater Lake) 6,500 years ago. It is underlain at a depth of about 3 feet by
a buried soil developed from basalt.

Ground vegetation on the study area is typical of the Abies grandis/
Calamagrostis rubescens plant community (Franklin and Dyrness 1973).
Genera of shrubs and herbs such as Arnica, Hieraciurm, and Ribes are
common.

The experiment is a levels-of-growing-stock study designed for thinning at
10-year intervals. It consists of a completely randomized design with two
replicates of five levels of growing stock installed on ten 0.4-acre plots
(each surrounded by a 30-foot-wide buffer strip). The growing-stock levels
selected for testing are 5,000, 10,000, 15,000, 20,000, and 25,000 square
feet of bole area per acre.£/ Actual stand densities after thinning in
terms of bole area and basal area are given in table |. The two plots

assigned to each density level were thinned to the same bole area level in
1966 and 1976.

In general, plots were thinned from below to leave the required number of
the largest and most vigorous trees as evenly spaced as possible (fig. |).
None of the slash from the thinnings was removed from the plots.

Diameters of all plot trees were measured to the nearest 0.1 inch after the
1965, 1970, 1975, and 1980 growing seasons. On each plot, about 15 trees,
proportionately distributed over the range of diameters, were measured
with an optical dendrometer in 1966, 1970, 1975, and 1980. The
measurements were used to calculate an equation expressing volume and
bole area of the entire stem inside bark as a function of diameter. New
equations were calculated after each measurement and used to compute
plot volumes (cubic feet and board feet, International |/4-inch rule) at the
beginning and end of the three 5-year growth periods (1966-70, 1971-75,
1976-80). Height growth of trees chosen for volume equation
measurements was measured by dendrometer.

Split-plot-in-time analyses of variance were used to test significance of
treatment effects; and nonlinear regression analyses related diameter,
basal area, and volume growth to residual bole area and basal area.

2/Bole area is a close approximation of the
cambial area of the main stem. See Lexen
(1943) and Smith (1962, p. 102) for a discussion
of the advantages of bole area as a measure of
stand density.

Table |--Stand characteristics per acre of western larch before and after the 1966 and 1976 thinnings and in 1971 and 1981

Density Volume2/
Number Quadratic
Bole Basal of Average mean Average Merchantable
Level!/ area area trees spacing diameter height4/ Total (including ingrowth)
- Square feet - Feet Inches Feet - - Cubic feet - -- Board feet
Before initia! (1966) thinning:
1 25,800 118.6 924 6.9 4.9 48.4 1,995 1,180 48
2 31,125 132.7 1,161 6.1 4.6 46.2 2,287 1,088 --
3 34,180 139.2 1,406 5.6 4.3 46.5 2,367 855 193
4 32,880 143.7 1,377 5.6 4.4 42.9 2,322 N25 --
5 32,700 135.6 1,459 by) 4.1 42.0 2,200 964 --
Average 31,337 134.0 1,265 D9 4.5 45.2 2,234 1,048 48
After 1966 thinning:
] 4,708 26.0 96 21.3 7.0 43.4 474 389 48
2 9,524 49.6 215 14.2 6.5 46.2 902 648 --
3 14,242 70.9 355 11.1 6.1 46.5 15272 782 193
4 19,313 96.4 546 8.9 2i/ 42.9 1,616 1,039 a
5 24,203 109.8 745 7.6 5.2 42.0 1,847 961 oe
1971:
| 6,374 40.3 96 253 8.8 55.4 794 678 948
2 12,069 68.2 215 14.2 7.6 BIIEZ/ 15333 1,060 294
3 17,797 93.4 354 Piel 7.0 53.3 1,780 1,261 532
4 23,810 120.5 539 9.0 6.4 49.1 2,250 1,562 345
5 29,121 134.3 740 7.7 5.8 48.0 2,510 1,435 102
Before 1976 thinning:
| 8,730 56.3 96 21.3 10.4 62.7 1,222 1,164 3,654
2 15,207 86.1 215 14.2 8.6 56.6 1,870 1,716 2,366
3 21,716 114.8 354 11.1 etl 58.2 2,471 2 sli73 1,464
+ 29,244 143.9 534 9.0 7.0 55.5 3,103 2,584 1,168
5 33,917 155.7 734 7.7 6.2 53.6 3,317 2,445 706
After 1976 thinning:
1 5,078 34.2 51 29.2 11.1 64.9 760 73) 2,876
2 10,006 59.3 129 18.4 9.2 62.8 1,301 1,216 2,368
3 15,012 82.7 225 13.9 8.2 62.7 1,808 1,627 1,464
4 20,029 104.0 333 11.4 7.6 60.9 2,248 1,957 1,168
5 24,779 121.0 464 9.7 6.9 61.7 2,621 2,138 706
1981:
1 6,592 44.6 51 2932 12.6 72.8 1,146 1,116 5,110
2 12,505 72.9 129 18.5 10.3 68.9 1,862 1,770 4,949
3 18,737 9953 224 13.9 9.0 67.8 2,412 2,264 3,583
8 24,433 121.2 329 11.6 8.3 66.4 2,986 2,740 2,797
5 29,960 137.6 462 9.8 7.4 66.0 3,398 2,959 1,357

Two plots for each density level.
2/ Average height of trees measured with dendrometer (about 15 per plot).

3/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 4inch top d.i.b.; board-foot (International 1/4-inch rule) volume—-trees 10.0-inch d.b.h. and larger to a 6-inch top d.i.b.

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Figure |.--One of the 10,000-square-foot-
bole-area density plots after initial thinning in
1966, with an average spacing of 14 feet. Basal
area is about 50 square feet per acre.

Results Diameter growth showed a consistent relationship to stand density during

Diameter Growth all three 5-year growth periods. Growth was greatest on the most heavily
thinned plots and decreased as bole area increased (table 2), partly because
the number of smaller, slower growing trees in the high density plots was
larger. During the second period, the diameter growth rate (0.32 inch per
year) at the lowest density was four times the growth at the highest
density (0.08 inch per year). All differences in diameter growth rate
between growing-stock levels were significant (P< 0.01).

Diameter growth was significantly greater (P<0.01) in the first period at |
each density level than in the second and third periods, but there was no
significant difference in growth between the second and third periods. No

significant interaction existed between growth periods and growing-stock
levels.

The effect of the second thinning (1976) was to prevent the normal decline
in diameter growth associated with age and greater stand density, rather
than increasing growth above that of the second period.

Table 2--Periodic annual increment and mortality per acre of western larch by age and density level after thinning at age 33 and 43

a A A RE RR RR A ARE ESET

All trees 75 largest trees
TRATES a SR oe
Age Merchantable Gross
and Residual Basal area growth Total volume growth volume growth total
density density Diameter (including ingrowth) Diameter volume
level growth! Ingrowth growth!/ growth
Bole Basal
area area Net Mortality Gross Net Mortality Gross Net Mortality Gross
A SS SS I SS SSS SN RP SS SS SS SRS SER SE SS Se RE SR ASE
Board Cubic
- Square feet - Inch - - Square feet - - - - Cubic feet - - - - Board feet - - - feet Percent Inches feet
Age, 33-38 years:
1 4,700 26.0 0.36 2.86 -- 2.86 64 - 64 180 -- 180 170 94.4 0.36 54
2 9,500 50.0 23 3.72 -- 3.72 86 -- 86 59 -- 59 59 100.0 +27 40
| 14,250 71.0 18 4.50 0 03 4.53 102 | 103 68 _ 68 43 63.2 «24 31
4 19,300 96.0 14 4.82 19 5.01 127 4 131 69 -- 69 69 100.0 «22 30
5 24,200 110.0 11 4.90 17 5.07 133 3 136 22 - 22 22 100.0 19 28
Age, 38-43 years:
1 6,400 40.0 032 3.20 - 3.20 85 - 85 541 -- 541 403 74.5 33 73
2 12,100 68.0 19 3.58 -- 3.58 107 - 107 414 -- 414 374 90.5 -20 48
3 17,800 93.0 NB) 4.28 - 4.28 138 - 138 186 _- 186 130 70.2 18 48
4 23,800 120.1 li2 4.69 16 4.85 170 3 173 164 -- 164 125 76.2 16 44
5 29,100 134.0 -08 4.29 li 4.40 161 2 163 120 - 120 96 80.0 14 32
Age, 43-48 years:
| 5,078 34.0 31 2.07 -- 2.07 78 -- 78 447 -- 447 129 28.1 31 78
2 10,006 59.0 -20 2.73 -- 2.73 113 - 113 517 -- 517 333 66.4 .22 79
3 15,012 83.0 “15 3.32 -- 3.32 121 -- 121 424 -- 424 323 76.6 18 55
4 20,029 104.0 13 3.44 -05 3.49 147 | 148 326 -- 326 227 73.1 15 51
5 24,779 121.0 -10 Sha5 V2 -06 3.38 156 | 157 131 _ 131 101 77.4 13 44

!/Arithmetic diameter growth of trees living through three 5-year periods (1966-70, 1971-75, 1976-80).

A significant (P< 0.01) curvilinear relationship existed between periodic
annual diameter increment and bole area or basal area of the stand at the
beginning of each growth period (figs. 2 and 3). Bole area and basal area
each accounted for about 98 percent of the variation in diameter growth
between plots. Because of the excellent correlation between diameter
growth and stand density, land managers can, with a high degree of
confidence, predict diameter growth rates after thinning larch stands of
this age and site index (80 at age 50).

During the 15 years of this study, the mean stand diameter increased by 7.7
inches in the lowest density plots compared with a 3.3-inch increase in the
highest density plots (table 3). About one-half of this increase in diameter
during the 15-year period is the result of removing the smaller trees in the
two thinnings. Because of the cutting of these smaller trees and faster
growth on the residual trees in the lowest density plots, the mean diameter
in these plots was 70 percent greater in 1981 than in the highest density
plots (12.6 vs. 7.4 inches) (table 1). Diameter growth of larch and
lodgepole pine was similar during all periods.

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Bole area at beginning of period
(thousand square feet per acre)

Periodic annual diameter growth (inches)

Figure 2.--Periodic annual diameter increment
by density level (bole area) and growth period.

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@ 1966 — 70 —0.022x,0.9

0.5 Y = 0.5926 — 0.5161(1— © )
« r2=0.977
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r2= 0.992

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“OO 14 28 42 56 70 84 98 112 126 140
Basal area at beginning of period
(square feet per acre)

Periodic annual diameter growth (inches)

Figure 3.--Periodic annual diameter increment
by density level (basal area) and growth period.

—_—__—

Table 3—Increase in quadratic mean diameter of a western larch stand from 1966
to 1980 as a result of growth and 2 thinnings

Increase in Increase attributed to--
Period and quadratic mean
bole area level diameter Thinning Growth
Thousand square
feet per acre Inches Inches Inches Percent
1966-70:
5 3.9 2.1 1.8 46
10 3.0 1.9 1.1 37
15 Del, 1.8 9 33
20 2.0 1.3 7 35
25 1.7 1.1 6 35
1971-75:
5 2.3 o/ 1.6 70
10 1.6 6 1.0 63
15 1.2 oP) / 58
20 1.2 6 6 50
25 1.1 of 4 36
1976-80:
5 1.5 -- 1.5 100
10 1.1 ne Vel 100
15 8 -- 8 100
20 7 -- Y/ 100
25 a) -- 5 100
1966-80:
5 7.7 2.8 4.9 64
10 5.7 2.5 3.2 56
15 4.7 2.3 2.4 51
20 3.9 1.9 2.0 51
25 3.3 1.8 1.5 45
Height Growth Height growth was relatively uniform among density levels, average

increment ranging from 0.9 foot to 1.6 feet annually (fig. 4). A significant
difference (P< 0.05) in height growth among density levels was found
because of the increased growth at the lowest level. Differences in

growth between periods were not significant, but there was a tendency for
height growth to decline during the third period at the two highest stocking
levels. Larch and lodgepole pine both grew in height at about the same rate.

Mortality Mortality was light during the 15 years of this study. Only 12 of the 1,567
study trees died during the first period, 7 during the second period, and 3
during the third period. All mortality occurred in the two highest density
levels, except for one tree that died in one of the middle density plots.
During the third period (1976-80), a light to moderate infestation of the
larch casebearer (Coleophora laricella Hbn.) was present on al! plots.

Basal Area Growth

Volume Growth

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1976-80, age 43-48

Md 1966-70, age 33-37
| 1971-75, age 38-42

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Periodic annual height growth (feet)
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0 5% “10 15 "20 5
Bole area level after thinning (thousand square feet per acre)

Figure 4.--Periodic annual height increment by

density level (bole area) and growth period.

Basal area increment increased during all periods with increasing stand
density, although there was a slight decline at the highest level during the
second and third periods (table 2). Differences among density levels were
significant (P<0.01). Growth slowed significantly (P< 0.01) from the first
to second period and from the second to third period (figs. 5 and 6). Bole
area and basal area were about equal as predictors of basal area
increment. The interaction between density and growth period was also
significant (P< 0.01), primarily because of the increase in growth at the
lowest density level from the first to the second period in contrast to a
decrease at the other four levels.

Total gross cubic volume increment was excellent during the 15-year study
period, reaching a high of 173 cubic feet per acre annually during the
second period (table 2). Volume growth increased with rising growing stock
level during all periods, and difference among density levels was significant
(P<0.01). Gross cubic increment increased significantly (P< 0.01) from an
average of 104 cubic feet per acre per year during the first period to 134
the second period and then declined slightly to 123 cubic feet per acre
annually during the third period. Net volume growth was essentially the
same as gross volume growth because of the small amount of mortality
during the study.

Volume increment was about twice as great on the highest density plots as
on the lowest during all periods; but much of the growth at high densities is
distributed on a large number of smaller, slower growing trees.

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< Bole area at beginning of period
a (thousand square feet per acre)

Figure 5.--Periodic gross annual basal area
increment by density level (bole area) and
growth period.

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(square feet per acre)
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tk

14 28 42 56 70 84 98 112 126 140

Basal area at beginning of period
(square feet per acre)

Figure 6.--Periodic gross annual basal area
increment by density level (basal area) and
growtn period.

Periodic gross annual basal area growth

EN

The regressions of volume increment on bole area (fig. 7) and on basal area
(fig. 8) accounted for about 80 to 92 percent of the variation in volume
increment during the three periods. Although these regressions show
increased growth with rising stand density, increment was only slightly
more at the highest density than at the second highest level during the
first period and decreased somewhat during the second and third periods
(table 2). This suggests that full site utilization occurs as stocking
approaches 25,000 square feet of bole area per acre and that increasing
stocking beyond this level may not result in an increase in total volume
increment.

Gross volume growth of the 75 largest trees per acre responded to stand
density in the same manner as diameter growth during all three
periods--growing faster at the lowest density level (table 2). Growth of
these larger trees was about twice as great at the lowest density as at the

highest, showing how growth is transferred to the larger trees more rapidly
in the low density plots.

Os 6 9 T2 15 18 21 24°°277330
Bole area at beginning of period
(thousand square feet per acre)

Figure 7.--Periodic total gross annual cubic
volume increment by density level (bole area)
and growth period.

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180

150

120

1966 — 70

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[744 28 42 56 70 84 98 112 126 140
Basal area at beginning of period

(square feet per acre)

Figure 8.--Periodic total gross annual cubic
volume increment by density level (basal area)
and growth period.

Periodic total gross annual volume growth

Board-foot volume increment increased significantly (P< 0.01) during the
second and third periods compared with the first period. Growth in the
15,000- and 20,000-square-foot-bole-area plots during the third period
showed significant (P< 0.01) increases over the first two periods because
trees were now merchantable size or reaching that size (ingrowth).
Ingrowth still accounts for the largest portion of the board-foot growth on
most plots during the third period, except for the lowest density plots
where all trees are now merchantable.

Mean cubic annual increment continues to increase at all density levels
(table 4). Based on data from yield tables developed for larch in Montana
(Schmidt et al. 1976), culmination of mean annual increment will probably
occur at 60 to 70 years of age.

Discussion

Table 4--Net mean annual increment per acre of western larch

Age (years)

Bole area level 33 38 . 43 4g

Thousand square

feetipenacres: )) Wa ee Cubic feet - - - - - -=---
5 60 6] 64 65

10 69 U7 76 80

15 72 76 83 87

20 70 78 89 )5)

25 67 75 85 93

Growth response to changes in stand density was similar during all three
5-year periods, with diameter growth decreasing and cubic volume and
basal area growth increasing as stocking increased. Thinning from below
has effectively concentrated growth on a small number of crop trees in the
low density plots where the average diameter in 1981 was 12.6 inches
compared with 7.4 inches in the high density plots (table 5).

Because the greatest diameter growth occurs at low stand densities,
whereas high densities result in the most cubic volume growth, it obviously
is not possible to maximize both diameter growth per tree and volume
growth per acre. Therefore, if markets for small trees exist and frequent
commercial thinnings are possible to utilize mortality, a high residual
stand density is indicated to more fully use the productive capacity of the
site and to maximize wood production. If, on the other hand, no pulpwood
market exists and the management objective is to shorten the rotation and
increase water and forage yields, a heavier precommercial thinning is
necessary--with a sacrifice of some volume growth.

In a shade intolerant species such as western larch, early thinning should
have a high priority. Competition in young, overstocked larch stands
results in reduction of the crown with subsequent decreases in diameter
and height growth. In addition, small, low-vigor trees are not as resistant
to damage from wind, snow, insects, and diseases as trees having adequate
growing space. Schmidt (1966) suggests that the ideal time for
precommercial thinning of larch is when trees are about 10 years old and
from 10 to 15 feet tall.

Although the greatest benefits from precommercial thinning occur in
young, 10- to 15-year-old stands before crowns begin to shorten,
considerable gains are still possible from precommercial thinning in older
stands as demonstrated in this study. The younger stands, however, should
be given preference for precommerial thinning.

Table 5--Total net growth and yield of western larch by stocking level (per acre)
a eS SIE SSR ESS SS SS SS ST SS St SES

Residual! bole area level (thousand square feet)

Item
> 10 15 20 2D
Number of trees

Total trees, 1966 924 1,161 1,406 Uo77, 1,459

Cut, 1966 828 946 1,051 831 714

Left, 1966 96 215 B55 546 745

Cut, 1976 45 86 130 213 281

Left, 1976 51 129 225 B35 464

15-year mortality -- -- | 4 2

Total trees, 1981 51 129 224 329 462

Inches
Quadratic mean diameter, 1981 12.6 10.3 9.0 8.3 7.4
Percent
Trees 10 inches in d.b.h.
and larger, 1981 100 51.7 24.3 14.1 4.7
Cubic feet

Total volume:
Total stand, 1966 1,995 2,287 2,367 ZeSZ2 2,200
Cut, 1966 1,521 1,385 1,095 706 353
Left, 1966 474 902 1,272 1,616 1,847
Cut, 1976 462 569 663 855 696
Left, 1976 760 1,301 1,808 2,248 2,621
Net 15-year growth 1,134 1,529 1,803 2,225 2,247
Total net yield, 1981 3,129 3,816 4,170 4,547 4,447

Board feet

Merchantable volume:
Total stand, 1966 48 -- 193 -- -_-
Cut, 1966 = =: — oe oe
Left, 1966 48 -- 193 -- _
Cut, 1976 778 -- -- -- =
Left, 1976 2,876 2,366 1,464 1,168 706
Net 15-year growth 5,840 4,949 3,390 2,797 1,365
Total net yield, 1981 5,888 4,949 3,583 2,797 1,365

Metric Conversions 1 mile = 1.6! kilometer
1 foot = 0.3048 meter
| inch = 2.54 centimeter
1 acre = 0.4047 hectare
| square foot per acre = 0.2296 square meter per hectare
| cubic foot per acre = 0.0700 cubic meter per hectare

l 13

Literature Cited

14

Franklin, Jerry F.; Dyrness, C. T. Natural vegetation of Oregon
and Washington. Gen. Tech. Rep. PNW-8. Portland, OR: U.S.
Department of Agriculture, Forest Service, Pacific Northwest Forest
and Range Experiment Station; 1973. 417 p.

Lexen, Bert. Bole area as an expression of BROWNS stock. J. For.
41(12): 883-835; 1943.

Schmidt, Wyman C. Growth opportunites for young 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; 1976. 96 p.

Seidel, K. W. Growth of young even-aged western larch stands after
thinning in eastern Oregon. Res. Note PNW-165. Portland, OR: U.S.
Department of Agriculture, Forest Service, Pacific Northwest Forest
and Range Experiment Station; 1971. 12p.

Seidel, K. W. Levels-of-growing-stock study in thinned western
larch pole stands in eastern Oregon. Res. Pap. PNW-221. Portland, OR:
U.S. Department of Agriculture, Forest Service, Pacific Northwest
Forest and Range Experiment Station; 1977. 14 p.

Smith, David Martyn. The practice of silviculture. 7th ed. New
York: John Wiley & Sons, Inc.; 1962. 578 p.

The Forest Service of the U.S. Department of
Agriculture is dedicated to the principle of multiple
use management of the Nation’s forest resources
for sustained yields of wood, water, forage, wildlife,
and recreation. Through forestry research,
cooperation with the States and private forest
owners, and management of the National Forests
and National Grasslands, it strives — as directed by
Congress — to provide increasingly greater service
to a growing Nation.

The U.S. Department of Agriculture is an Equal
Opportunity Employer. Applicants for all Department
programs will be given equal consideration without
regard to age, race, color, sex, religion, or national
Origin.

Pacific Northwest Forest and Range
Experiment Station

809 NE Sixth Avenue

Portland, Oregon 97232