ESTIMATING TREE

SPECIFIC GRAVITY OF MAJOR

PULPWOOD SPECIES OF WISCONSIN

U.S.D.A. FOREST SERVICE RESEARCH PAPER FPL161 1971

SUMMARY Specific gravities were determined for increment

cores and merchantable boles of nine major pulpwood species of Wisconsin as a part of a Wisconsin Wood Density Survey. The best estimator of mean specific gravity of the standing conifer and aspen species was specific gravity of the increment cores adjusted to simple regression equations. For red pine and black spruce, a multiple regression with two variables in addition to specific gravity of the core improved the simple regressions reducing the standard deviation of regression. For two species, white spruce and eastern hemlock, a slight but significant reduction in the stan­dard deviation resulted from introducing a second var­iable for each species. The results of a covariance analysis of six of the species native to both Wisconsin and Maine indicated the possibility of using a common regression with one variable--specific gravity--for red pine and white spruce, but indicated a need to use separate regressions for the other four species.

ESTIMATING TREE SPECIFIC GRAVITY

MAJOR PULPWOOD SPECIES OF WISCONSIN

by DlMlTRl PRONlNl Forest Products Technologist

Forest Products Laboratory2

Forest Service, U.S. Department of Agriculture

Wisconsin, an industrial and agricultural state, has abundant forest resources. Forty-five per­cent of the state is forested; of the forest land 61 percent is in the northern part of the state (10).3

Of every 100 acres of forest, 44 acres are com­mercial forest. About 200,000 acres of the forest lands are used for parks and reserves.

A reinventory of Wisconsin’s forest resources was begun in 1967 and is being continued by the Forest Service in cooperation with the State of Wisconsin. Included in the reinventory is a survey of wood density to determine the variation in spe­cific gravity among and within commercially valu­able species. Wood density is a useful index to the suitability of wood for many purposes (2,6,7). The Wisconsin Wood Density Survey is similar to den­sity surveys made in 12 southern states (8,13,17), 11 western states (12), and in the State of Maine (16).

In the work reported in this paper, estimates are made of the specific gravity of the merchantable bole of nine wood species of commercial impor­tance for pulpwood. Increment cores were taken from stems at breast height of trees over the range of the species and from trees of a variety of diameter sizes. The trees were felled, and the specific gravity determined for both the increment core and the merchantable bole. The relationship between the specific gravity of the core and that of the bole is expressed as a regression equa­tion (11,14).

Trees of the following nine species were sampled: Red pine (Pinus resinosa Ait.), jack pine (Pinus banksiana Lamb.), white spruce (Picea glauca (Moench) Voss.), black spruce (Picea mariana (Mill.) B.S.P.), balsam fir (Abies bal­samea (L.) Mill.), tamarack (Larix laricina (Du Roi) K. Koch), eastern hemlock (Tsuga canadensis (L.) Carr), quaking aspen (Populus tremuloides Michx.), and bigtooth aspen (Populus grandiden­tata Michx.).

The intensity of sampling for each species was roughly proportional to the volume of standing timber of the species within the Forest Survey units of Wisconsin.

For quaking aspen, 20 trees were sought at each of six sampling locations, The 20 trees were selected to provide four trees in each of the following diameter classes:

Inches 4.6 to 6.5 6.6 to 8.5 8.6 to 10.5

10.6 to 12.5 12.6+

For bigtooth aspen, sampling procedures were changed slightly. The same diameter classes were used, but fewer trees were sampled at 10 loca­tions. Thus, two trees in each diameter class (10 trees at each location) were sampled at each of the locations.

1The author gratefully acknowledges the assistance of A. N. Foulger, L. E. Lassen, E. A. Okkonen, J. Ward, H. E. Wahlgren, and other members of the Forest Products Laboratory staff for collecting the samples and analyzing the data. Also grate­fully acknowledged is the assistance of foresters of State and National Forests in Wisconsin.

2Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3Numbers in parentheses refer to Literature Cited at the end of the paper-.

For the conifers, sampling procedures were again varied. The following diameter classes were used:

Inches

4.6 to 5.9 6.0 to 7.5 7.6 to 8.9 9.0 to 10.5

10.6+ Only one tree was sampled in each diameter class (five per location) for each species. How­ever, the number of locations varied by species according to the relative volumes of standing timber.

Only natural stands were sampled. However, because of the probable future importance of plantation-grown red pine in Wisconsin, Maeglin’s equation (5) for plantation-grown red pine is in­cluded for reference in table 1.

The locations of the sampled trees are shown within the four survey units of Wisconsin (fig. 1). Locations of sampled trees according to counties are shown in the Appendix.

FIELD PROCEDURE

One increment core (to the pith) was extracted at breast height from each selected tree. Knots, scars, and other irregularities were avoided by boring above, below, or to the side of them. The lengths of the cores were measured to 0.02 inch, and the core diameters were taken as the borer diameter calibrated to 0.001 inch with a taper gage. Core measurements were checked in the laboratory before they were used in calculating specific gravities. The diameter of the tree out­side bark was measured to 0.1 inch.

After the cores were collected, the trees were felled, and cross-sectional disks about 1.5 to 2 inches thick were cut at breast height and at the end of each 100-inch pulpwood stick thereafter to a merchantable diameter of 3 inches, Disks were marked with an identification number, and the inside bark diameter of each disk recorded. Disks were collected to provide samples from which the specific gravity could be calculated for the merchantable portion of the bole.

After felling, the height of the tree was measured to the nearest 0.5 foot, and the age at stump height was determined by counting the annual rings.

LABORATORY PROCEDURES

Specific gravity of the increment cores and disks from all of the sample trees was determined at the Laboratory. The green volume of each core was computed from the field measurement of the core length and the calibrated diameter of the borer. The cores were then ovendried to a con­stant weight.

The green volume of disks was determined by the water-immersion method (4); the disks were then ovendried to a constant weight. The specific gravity of each cross section was calculated on an ovendry weight to green volume basis. The average specific gravity of each 100-inch pulp­wood stick was computed as the mean specific gravity of the disks at either end of each stick. A single disk cut at breast height was used to estimate the specific gravity of the ,butt stick. The average specific gravity of the tree was cal­culated from the specific gravity of each pulpwood stick, weighted by the volume of each stick.

ANALYTICAL PROCEDURE

The relationship of tree specific gravity (Y) to core specific gravity (X1 ) and other characteris­

tics were examined by multiple regression analy­sis, using the following independent variables:

X1 = specific gravity of increment core

X2 = age

X3 = diameter at breast height

X4 = rate of growth, (X /X )32

X5

= total height

For simple linear regressions, the only variable significant for all species was core specific gra­vity (table 1). These simple linear relations for all of the conifers and both of the aspen species are shown graphically in figure 2.

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Table I.--Regression equations used in Wisconsin Wood Density Survey (Phase I I I )

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Figure 1.--Location of four survey units and of species used in Phase I l l of the Wisconsin Wood Density Survey. M 138 180

Figure 2.--Linear regressions of tree specific gravity and increment core specific gravity . M 138 184

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ANALYSIS OF DATA

The regression equations developed to predict tree specific gravity from the specific gravities of increment cores are shown in table 1.

Table 2 presents by species the total number of trees, the average range of age (years), diam­eter at breast height (d.b.h.), total height (feet), and specific gravity (merchantable bole).

Specific gravity values for each species grouped by diameter class, giving number of trees in each class and ranges in specific gravity are shown in table 3. Examination of these data suggests spe­cific gravity may decrease with increase of diam­eter for all conifers studied except for red pine. The reverse occurs for both quaking and bigtooth aspen. However, the regression equation showing the relationship of specific gravity to diameter was significant only for eastern hemlock.

COMPARISON OF SIX SPECIES NATIVE TO BOTH WISCONSIN AND MAINE

Because balsam fir, tamarack, black and white spruce, red pine, and eastern hemlock grow under similar climatic conditions, they were sampled in both Wisconsin and Maine, and regression equa­tions predicting tree specific gravity were cal­culated for both the Maine Wood Dens it y Sur­vey (15) and the Wisconsin Wood Density Survey. Wisconsin and Maine regression equations were compared by using a covariance analysis of simple regressions with one independent variable, single core specific gravity (3).

For black spruce and balsam fir, the covari­ance analysis showed a significant difference in regression slopes between Maine and Wisconsin. For eastern hemlock and tamarack, the two areas did not differ in regression slope, but did differ in levels. Therefore, for these four species a common regression should not be used for the two states (fig. 3).

For white spruce and red pine, the covariance analysis showed no significant differences be­tween states in either slope or level. Therefore, a single common regression with one variable (X

1--single core gravity) can be used. It is

graphically shown in figure 4.

The equations common to both states are the following for which is tree specific: gravity; X1, specific gravity of increment core at d.b.h.;

n, number of samples; r 2 , coefficient of determi­nation; and S standard deviation:

yx White spruce

Y = 0.163613 + 0.534057 X1

where n = 149 2

r = 0.5342 S = 0.018 yx

Red pine

Y = 0.243262 + 0.349366 X1 where n = 115

2 r = 0.3222 s = 0.022 xy

Multiple regressions of the next “best” inde­pendent var iables were also computed. For Wisconsin red pine, the inclusion of rate of growth (X4) and the total tree height (X

5 ) improved the

precision of the estimate. For red pine the equa­tion was the following:

Y = 0.23015 + 0.41451 X1 - 0.2144 X

4 + 0.000536 X

5 where n = 50

2 r = 0.5904 S xy = 0.011

For Maine red pine, the predicting equation was also significantly improved by the same variables (X1, X4, X5), The resulting equation was:

Y = 0.35537 + 0.15563 X1 - 0.25122 X

4 + 0.000297 X

5 where n = 65

2 r = 0.7048 S = 0.016yx

However, covariance analysis showed a signi­ficant difference between these two multiple re­gressions; therefore, no single equation can be fitted for use in both states.

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- -T a b l e 2 . S p e c i f i cg r a v i t y o f Wiscons in c o n i f e r s and aspen spec ies by age, d iameter , and h e i g h t

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Table 3.--Specitic g rav i t y o f core and t r e e by diameter c lass f o r combined sample p l o t s

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Figure 3.--Regressions for four species showing significant variance between the Maine and the Wisconsin species. M 138 186

Figure 4.--Regressions for red pine of Maine and Wisconsin show that a single regres­sion with one variable, core specific gravity, can be used. M 138 183

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VARIATION OF SPECIFIC GRAVITY WITH HEIGHT

The trend of variation in specific gravity within a tree is important because it has been shown that different species of coniferous trees show oppo­site trends (1,9,15).

All curves for highest height classes in both Maine and Wisconsin show the same trends for the same species (figs. 5 to 11).

Calculation of the averages by using a large number of trees in a class, as with jack pine (19 or 20 trees), made it possible to show smooth curves (fig. 10), whereas few trees in a class ( tamarack with five trees) gave sharp fluctua­tions (fig. 6).

Figure 5.--Relationship of specific gravity of red pine (disks) to tree height.

M 138 182

Trees sampled during the wood density survey in Wisconsin were segregated into height classes with an approximately equal number of trees in each class. Figures 5 to 11 show the relationship of average specific gravity (by disk section) to tree height for both Wisconsin and Maine trees; also shown for Wisconsin trees only are the spe­cific gravities of disks at a particular height. Jack pine, red pine, and tamarack showed a clear decrease in specific gravity with height. White and black spruce, balsam fir, and eastern hemlock showed a reverse trend; specific gravity de­creased only to a height of 16 to 24 feet, which usually coincided with the start of the crown, and the decrease was followed by a steady increase to the top.

Figure 6.--Relationship of specific gravity of tamarack (disks) to tree height.

M 138 185

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Figure 7.--Relationship of specific gravity of Figure 8.--Relationship of specific gravity of white spruce (disks) to tree height. black spruce (disks) to tree height.

M 138 181 M 138 178

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Figure 9.--ReIationship of specific gravity Figure IO.--Relationship of specific gravity of balsam fir (disks) to tree height. of jack pine (disks) to tree height.

M 138 177 M 138 176

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Figure 11.--Relationship of specific gravity of eastern hemlock (disks) to tree height.

M 138 179

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CONCLUSIONS

The best estimator of mean specific gravity of standing coniferous and aspen species in Wiscon­sin was the specific gravity of sampled incre­ment cores adjusted to simple r e g r e s s i o n equations.

For red pine and black spruce it was worth­while to use a multiple regression with specific gravity of the core plus two additional variables,

diameterrate of growth (expressed as ) and total age

height. This improved the simple regressions reducing the standard deviation of regression

from 0.020 to 0.011 for red pine and from 0.017 to 0.011 for black spruce.

For white spruce, the use of a secondvariable, rate of growth, resulted in a slight but significant reduction in the standard deviation, 0.002. The standard deviation of the regression for eastern hemlock also was reduced by 0.002 by introducing a second variable, diameter at breast height.

The covariance analysis of data from the Wisconsin Wood Density Survey with the data from Maine demonstrated both the possibility of using a common regression with one variable (single core specific gravity) for the red pine and the white spruce of both states and a need to use sep­arate regressions for the other species.

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LITERATURE CITED

1. Baker, Gregory 1967. Estimating specific gravity of

plantation-grown red pine, Forest Prod. J. 17(8): 21-24.

2. Christopher, J. F., and Wahlgren, H. E. 1964. Estimating specific gravity of south

Arkansas pine. U.S. Forest Serv. Res, Pap, SO-14, So. Forest Exp. Sta., New Orleans, La.

3. Freese, Frank 1964. Linear regression methods for for­

est research. U.S. Forest Serv. Res. Pap. FPL 17, Forest Prod. Lab., Madison, Wis.

4. Heinrichs, J. Frank, and Lassen, L. E. 1970. Improved technique for determining

the volume of irregularly shaped wood blocks. Forest Prod, J. 20(4): 24.

5. Maeglin, R. R. 1966. Predicting specific gravity of

plantation-grown red pine. U.S. Forest Serv. Res. Note FPL-0149, Forest Prod. Lab., Madison, Wis.

6. Mitchell, Harold L. 1958. Wood quality evaluation from incre­

ment cores. Tappi 41(4): 150-156.

7. 1964. Patterns of variation in specific

gravity of southernpines and other coniferous species. Tappi 47(5): 276-283.

8. , and Wheeler, P. R. 1959. Wood quality of Mississippi’s pine

resources. U.S. Forest Prod. Lab. Rep. No. 2143, Madison, Wis.

9. Spurr, Stephen H., and Hsiung, Wen-yeu 1954. Growth rate and specific gravity

in conifers. J. Forest. 52(3): 191­200.

10.

11.

12.

13.

14.

15.

16.

17.

Stone, R. N., and Thorne, H. W. 1961. Wisconsin’s forest resources. U.S.

Forest Serv. Lake States Forest Exp. Sta. Pap. No. 90, St. Paul, Minn.

Taras, M. A., and Wahlgren, H. E. 1963. A comparison of increment core

sampling methods for estimating tree specific gravity. U.S. Forest Serv. Res. Pap. SE-7, SE Forest Exp. Sta., Asheville, N. C.

U.S. Forest Service 1965. Western Wood Density Survey

Report Number 1. U.S. Forest Serv. Res. Pap. FPL-27, Forest Prod. Lab., Madison, Wis.

1965. 1965 Status Report. Southern Wood Density Survey. U.S. Forest Serv. Res. Pap. FPL-26, Forest Prod. Lab., Madison, Wis.

Wahlgren, H. E., and Fassnacht, D. L. 1959. Estimating tree specific gravity

from a single increment core. U.S. Forest Prod. Lab. Rep. No. 2146, Madison, Wis.

, Hart, Arthur C., and Maeglin, Robert R. 1966. Estimating tree specific gravity of

Maine conifers. U.S. Forest Serv. Res. Pap. 61, Forest Prod. Lab., Madison, Wis.

, Baker, G., Maeglin, R. R., and Hart, A. C. 1968. Survey of specific gravity of eight

Maine conifers. U.S. Forest Serv. Res. Pap. FPL 95, Forest Prod, Lab., Madison, Wis.

Wheeler, P. R., and Mitchell, H. L. 1962. Specific gravity variation in Miss­

issippi pines. U.S. Forest Prod. Lab. Rep. No. 2250, Madison, Wis.

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APPENDIX

Phase I l l o f Wisconsin Dens i ty Survey

Locat ion of Sampled Trees

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Locat ion o f Sampled Trees--continued

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