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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.
@s @ 1966 — 70 —0.022x,0.9
0.5 Y = 0.5926 — 0.5161(1— © ) « r2=0.977 \ \ \ S----5 1971-75 anon 0.4 se Y = 0.8087 — 0.7354(1-& ~ yee . S r2=0.977 N A ae —-—-- 1976-80 @ — 0.023x, 0.89
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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
= te)
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) [@) w
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.
£ hee! > 6 ° he 5) o— 5 2's ag $3‘ 08 2.3 = 2 ® 3+ =I) 8 7 ir a 1971-75 4, O65 ye ¥=4.60(1-¢7 71%) os We aaa (a oh C86 iG * ¥=3.55(1-27 21% o . r2 = 0.885 =. 6 i 0 3 6 S912 15 18 21 #24 27 30 < 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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—)om = «amo ame = * . a ~_—*
Nh
a — 0.037x, 1.67
)
(square feet per acre) )
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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F
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» 180
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21-90 e 1966 — 70
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oo 60 au hen 1971-75 sane
_o Y = 182.00(1 ~© )
by r= 0.841
iS) 30 21976 80 een
on V=1G8 (ic mame
Oo r2 = 0.800
a)
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a.
210
180
150
120
1966 — 70
(cubic feet per acre)
90 Y= 2100002" 5) r2 — 0.886 == 1971-75
a = a x
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A
30 ya iogt—e OP
r2 — 0.828
[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.
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