Biomass Determination 

Using Wood Specific 

Gravity from Increment 

Cores

Michael C. Wiemann

G. Bruce Williamson

Forest

Products

Laboratory

General

Technical 

Report

FPL–GTR–225

United States Department of Agriculture

Forest Service
July 2013

Wiemann, Michael C.; Williamson, Bruce G. 2013. Biomass determina‐

tion using wood specific gravity from increment cores. General Technical 

Report FPL-GTR-225. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 7 p.

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Abstract

Wood specific gravity (SG) is one of the most important 

variables used to determine biomass. Measurement of SG 

is problematic because it requires tedious, and often dif‐

ficult, sampling of wood from standing trees. Sampling is 

complicated because the SG usually varies nonrandomly 

within trees, resulting in systematic errors. Off-center pith 

and hollow or decayed stems pose further problems in 

biomass estimation. When the pattern of variation within a 

tree is known, as it often is for a given species, the sampling 

procedure can be greatly simplified by sampling wood that 

approximates a mean value for the tree. Even when a pattern 

is not known, a representative approximation can produce 

reasonable estimates of SG. If whole above-ground woody 

biomass is required, corrections may be necessary to ac‐

count for differences with height or in branches.

Keywords: biomass, increment borer, radial variation, 

specific gravity, tree cores

Acknowledgments

Mary Beth Adams, Scott Bowe, and Steven Hubbard gave 

critical reviews that improved the manuscript. GBW was 

supported by grants from the U.S. National Science 

Foundation (DEB 0639114 and DEB-1147434).

Cover photo, by GBW, is of a large balsa tree (Ochroma 

pyramidale) on the Osa Peninsula, Costa Rica, with 

increment borer and extracted core.

Contents

Introduction.........................................................................1

Approximation Method.......................................................2

 Variation with Height......................................................3

 Branchwood versus Trunkwood .....................................4

Concluding Remarks...........................................................5

Literature Cited ...................................................................5
Biomass Determination Using Wood 

Specific Gravity from Increment Cores

Michael C. Wiemann, Botanist

Forest Products Laboratory, Madison, Wisconsin

G. Bruce Williamson, Professor

Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 

Introduction

Traditionally an index of wood properties, wood specific 

gravity (SG) of standing trees has become a fundamental 

component of biomass determinations in ecosystem studies 

and a cornerstone of functional trait analysis in community 

ecology (Fearnside 1997; Chave et al. 2009; Baraloto et 

al. 2010; Zanne et al. 2010; Wright et al. 2010). Many eco‐

logical studies use a single SG estimate for an individual 

tree or a given species. An extensive database compiled by 

Zanne et al. (2009) has gone a long way to fulfill the need 

for wood SG data. However, the Zanne database, like many 

others, is a metafile compiled from many contributors whose 

methodologies for SG determinations are likely to differ 

(Williamson and Wiemann 2010a) and whose trees may not 

be representative of the local flora. Furthermore, compiled 

databases lack SG values for hundreds of species, especially 

those from the tropics.

Field determinations of SG are the logical alternative to pre‐

viously published values from other sites. On‐site measure‐

ments provide true values, eliminating potential biases from 

geographic variation and nonrandom samples in databases. 

However, determining SG in the field is time consuming and 

difficult because wood samples must be obtained by fell‐

ing trees or extracting cores. Historically, felling trees was 

widely employed by foresters, but in the mid‐1800s Max 

Pressler invented the increment borer (Pressler 1866). To‐

day, extracting cores is generally preferable to felling trees 

as conservation and ongoing research demand a nondestruc‐

tive sampling methodology. 

The best estimates of tree SG are derived from cores that 

extend from bark to pith because they reveal the full extent 

of radial variation. Sampling a tree’s outer few centimeters 

of wood with an increment borer or an increment hammer 

is relatively simple. However, such a small sample is useful 

only if it is representative of wood across the tree bole. For 

some species this may be the case, but for many others SG 

varies dramatically across the radius (Whitmore 1973; Wie‐

mann and Williamson 1988, 1989a, 2012; Williamson et al. 

2012). Radial variation in SG is sufficiently extensive that 

it merits consideration in the prediction of physical or me‐

chanical properties as well as in the estimation of biomass. 

Following is a summary of reported trends in radial varia‐

tion in wood SG:

I. SG increases from pith to bark. 

A. Panshin and de Zeeuw (1980) for 26 gymno‐

sperms, 12 diffuse‐porous temperate 

angiosperms, three ring‐porous temperate 

angiosperms, and 15 tropical angiosperms.

B. Extreme linear increases in many tropical pio‐

neers (Fimbel and Sjaastad 1994; Wiemann and 

Williamson 1988, 1989a, 2012).

C. Extreme nonlinear (convex up) increases in the 

tropical pioneer Schizolobium parahyba (Vell.) 

S.F. Blake (Williamson et al. 2012).

II. SG decreases from pith, then increases to bark. 

A. Panshin and de Zeeuw (1980) for14 gymno‐

sperms and one diffuse‐porous temperate angio‐

sperm.

B. Schüller et al. (2013) reported only two such spe‐

cies (both late successional) in their study 

of 338 Mexican rainforest trees representing 

45 species.

III. SG decreases from pith to bark. 

Panshin and de Zeeuw (1980) for seven gym‐

nosperms, nine diffuse‐porous temperate angio‐

sperms, five ring-porous temperate angiosperms, 

and one tropical angiosperm.

IV. SG increases from pith, then decreases to bark.

Tropical angiosperm Astronium graveolens Jacq. 

(Wiemann and Williamson 2012). This pattern 

may result from the combined effects of juvenile 

wood and the bulking of heartwood by extrane‐

ous materials.

V. SG is constant from pith to bark. 

Temperate angiosperms Magnolia grandiflora

L. (Wiemann and Williamson 2012) and Alnus 

rubra Bong. (Harrington and DeBell 1980; Gart‐

ner et al. 1997) and seven tropical angiosperms 

(Wiemann and Williamson 1989b, 2012).

VI. SG is erratic.

In tropical Apeiba aspera Aubl. due to irregu‐

larly spaced wide parenchyma bands (Wiemann 

and Williamson 2012).
General Technical Report FPL–GTR–225

2

The prevalence of within-tree variability requires that es‐

timation of SG be more carefully considered. Ideally, each 

species could be characterized, not simply by an SG value, 

but by a function between SG and radial distance. However, 

radial increases in SG appear to be determined by age, not 

tree diameter (de Castro et al. 1993; Nock et al. 2009; Wil‐

liamson and Wiemann 2010b, 2011; Williamson et al. 2012). 

Consequently, determining radial variation may 

require boring many individuals from bark to pith. 

Unfortunately, boring trees from bark to pith is often dif‐

ficult and replete with problems, as previously encountered 

by dendrochronologists and foresters (Grissino‐Mayer 2003; 

Jozsa 1988; Maeglin 1979; Phipps 1985):

1. Trees are too large for the borer to reach the pith 

(Figs. 1, 2).

2. Trees have hollow or rotten centers (Fig. 3).

3. Borers miss the pith, passing to the side of it. 

4. Borers are difficult to insert in trees with dense wood.

5. Borers are difficult to extract, for multiple reasons.

Given the problems of bark-to-pith boring, we developed 

an alternative technique for estimating tree SG even when 

radial variation is substantial (Williamson and Wiemann 

2010a). The method has been tested and found to perform 

well for the majority of trees studied (Wiemann and 

Williamson 2012).

Approximation Method

Our method presumes that a sample of wood can be extract‐

ed at a point in the tree cross section that is representative of 

all the wood in that cross section. For the majority of trees 

that we tested (Wiemann and Williamson 2012), this sample 

is located at 2/3 the distance from the pith to the bark, so 

that its extraction requires that a tree be bored only to 1/3 of 

its radius inside of the bark. This method has the additional 

advantage that it avoids the problems enumerated above be‐

cause only a short outer section of the tree is bored. For ex‐

ample, in very large trees, such as the canopy emergent Ceiba pentandra (L.) Gaertn., shown in Figure 1, boring to the 

pith is problematic, but boring to 1/3 the radius is relatively 

easy. Because the location of the pith is usually unknown, 

especially in a large tree, we tested the accuracy of cores 

taken from 1/6 the diameter inside of the bark (1/6 DIB); 

in a symmetrical tree, this point coincides with 1/3 of the 

radius inside the bark (Fig. 2). Errors may be introduced to 

the extent that a tree cross section is not symmetrical or its 

pattern of SG with distance from the pith is not linear.

The method we recommend to determine the mean SG of 

a tree, at the height at which it is bored, is as follows:

1. Determine if the species has a straight line trend in SG 

from pith to bark. This can be increasing, decreasing, 

or flat, and can be ascertained by reference to species 

descriptions or by complete bark to pith sampling of a 

few individuals. 

2. Measure the diameter outside the bark (DOB) in the 

boring direction with tree calipers.

3. Measure the bark thickness using a bark gauge or an 

increment borer.

4. Calculate the diameter inside the bark (DIB) by subtract‐

ing twice the bark thickness from the DOB.

5. Determine the point on the radius at which measured 

SG is equal to mean SG for the whole diameter.

6. For symmetrical trees in which the SG is a straight-line 

function of distance from pith, this value is 1/6 DIB. 

This is the depth to which the tree must be bored to 

determine its mean SG at that height.

7. For trees in which SG is not a straight-line function of 

distance from pith, the value must be determined, either 

from prior knowledge of the radial trend or by com‐

pletely boring a number of representative individuals to 

determine the trend. 

8. Bore the tree about 2 cm beyond the calculated depth 

to extract a segment whose SG is representative of the 

SG of the cross section as a whole. The depth to which 

the increment borer should be inserted into the tree will 

depend on the distance to 1/6 DIB and the length of the 

representative sample to be extracted. Shorter samples 

will more accurately represent the distance from the 

bark, but longer samples will give smaller weight and 

volume measurement errors. We recommend a seg‐

ment length of 3 cm centered over the point of 1/6 DIB. 

The depth to which the borer should be inserted can be 

marked on the increment borer shaft with a marking 

crayon or with a small piece of tape or modeling clay.

Figure 1. Extraction of a core using a large-diameter 

(12-mm) extra long increment borer. The auger measures 

81 cm from handle to the tip, and the diameter of this Ceiba pentandra from Costa Rica, measured above the buttresses with a diameter tape, was 146 cm. The auger was 

inserted until the handle was stopped by the buttresses, 

producing a core that we estimated was about 2 cm short 

of reaching the pith.
Biomass Determination Using Wood Specific Gravity from Increment Cores

3

9. Remove the wood core and the borer immediately; bor‐

ers left in the tree are much more difficult to remove due 

to “springback” of the compressed wood surrounding 

the shaft. Measure and mark the distance along the core 

from the bark to the indicated midpoint of the represen‐

tative sample; in the case of trees that have a linear pith 

to bark SG trend and no eccentricity, the midpoint will 

be 1/6 DIB.

10.Cut the 3-cm segment from the core so that 1/6 DIB is 

centered in the segment, leaving 1.5 cm on each side of 

the mark.

11. Label the segment with a pencil or indelible ink, and 

store it so that it will not lose moisture below its fiber 

saturation point. Storage in sealed tubes, in water, or in 

cellophane film will prevent shrinkage due to drying, 

such that green volume can be measured at a later time 

by water immersion.

12.In the lab, green volume can be determined by water im‐

mersion on a precision balance. If the sample is a perfect 

cylinder, volume can be calculated from the diameter 

and length of the segment. Low-density woods can be 

cut cleanly using a razor blade, but harder species slice 

irregularly, yielding less accurate volume estimates by 

calculations. Cores can also be air dried if an air-dry vol‐

ume is required. Finally, they must be ovendried at 101–

105 °C to determine ovendry weight for SG calculation.

13.If there is reason to believe that the pith is not in the cen‐

ter of a tree, it might be advisable to take a second core 

on the opposite side. Indicators of off‐center pith are 

leaning stems or stems growing on slopes, although this 

is by no means foolproof. For angiosperms, the shorter 

pith-to-bark radius will usually be on the underside of 

leaning stems; the reverse is true for gymnosperms. 

Cores taken parallel to hill slope, or perpendicular to 

the direction of tree lean, are more likely to have equal 

length radii and representative wood; the problem then 

becomes where to aim the increment borer in order to 

aim directly at the pith. Also, special care must be taken 

to preserve the integrity of these cores because they 

sometimes fragment more easily than usual. If the tree 

has branches fairly close to the point of insertion of the 

increment borer, aiming the borer in the same direction 

as a branch will also aim it at the pith.

If the assumption of a linear trend from pith to bark is 

violated, the method may still give adequate estimates for 

some purposes. Sajdak (1968) presented SG versus distance 

from pith data for Acer saccharum Marsh. His species aver‐

age values for 61 trees from Michigan showed a parabolic 

relationship, with the highest value, 0.60, adjacent to both 

the pith and the bark (at 23 cm from pith), and the lowest 

value, 0.58, at 10–15 cm from the pith. Even though the 

trend for these trees was not linear, the maximum difference 

between the highest and lowest average SG values was only 

3% because the radial variation was low, so any core seg‐

ment would give an adequate value. Sajdak also presented 

data for four individual trees, and the most extreme of them 

showed only a 7% difference between the highest and 

lowest SG values. Our study of bark-to-bark samples from 

35 trees (23 species) included seven species that did not 

have linear pith-to-bark trends; of these, the SG of a seg‐

ment taken at 1/6 DIB was within 10% of the weighted 

SG in all seven (Wiemann and Williamson 2012).

The more serious problem of possible off-center pith in a 

pioneer that shows a steep radial trend in SG can be handled 

by taking two cores extracted from opposite sides of the 

tree. For example, 1/6 DIB segments from an eccentric stem 

of Heliocarpus appendiculatus Turcz. from Costa Rica 

gave a 36% SG underestimate on the short (11-cm) radius 

and a 14% SG overestimate on the long (18-cm) radius 

(M.C. Wiemann and G.B. Williamson, unpublished data). 

Averaging the SG values reduced the error to 11%. Boring 

both sides to 1/6 DIB still entails less work than boring to 

the pith, which can be difficult in such an eccentric tree. 

Variation with Height

These procedures give SG estimates that are valid only at 

the sampling height. If estimates are required at heights 

that cannot be sampled easily (such as near ground level or 

high on the bole), knowledge of the SG trends with height 

are required. For whole‐stem woody biomass estimates, the 

relationships of stem diameter and SG with height must be 

known. Figure 3 shows the changes in shape and diameter, 

from ground level to the point of stem bifurcation, in a 

Figure 2. Bombacopsis quinata (Jacq.) Dugand log from 

Panama showing diameter (180 cm), distance from bark 

to pith (105 cm), 1/3 of the “radius” (35 cm) and 1/6 of the 

DIB (30 cm). Because the growth of this tree was slightly 

eccentric, the distance from bark to pith in the direction 

indicated is somewhat greater than half the diameter, and 

1/3 of this “radius” does not equal 1/6 of the DIB. Although 

the oversized (81-cm) increment borer of Figure 1 would 

not be able reach the pith of this stem, even a standard 

(45-cm) auger could be used to bore to 1/6 of the DIB. The 

smaller log to the right is a 140-cm-diameter Anacardium 

excelsum (Bertero and Balb. ex Kunth) Skeels; it also has 

a slightly off-center pith.
General Technical Report FPL–GTR–225

4

46-cm DBH Acer saccharum from Wisconsin. Note the hol‐

low, fluted butt disk, the off‐center pith in disk 15B, and the 

triple pith in disk 15F. Diameter with height can be inferred 

if the pattern of stem taper is known, or stem diameters at 

any height can be measured from the ground using a den‐

drometer (Wenger 1984). In either case, SG variation with 

height must be measured or estimated.

Although SG also varies from the base to the top of a tree, 

Zobel and van Buijtenen (1989) asserted that whole tree SG 

could be reliably estimated from breast-height SG for both 

gymnosperms and angiosperms. However, their notes on SG 

variation with height in some 40 species are heavily biased 

toward temperate trees, so studies on individual species 

might be necessary to make whole tree estimates for tropical 

species.

In temperate gymnosperms, SG as a function of height with‐

in a tree exhibits many species‐specific patterns, such as de‐

creasing linearly or curvilinearly, increasing, decreasing fol‐

lowed by an increase, or remaining relatively constant (Pan‐

shin and de Zeeuw 1980; Wahlgren and Fassnacht 1959; 

Wahlgren et al. 1966). Okkonen et al. (1972) related these 

trends to percentage of latewood. Some authors developed 

models to calculate SG from relative height, for example, 

Pong et al. (1986) for Pseudotsuga menziesii (Mirb.) Franco 

and Tsuga heterophylla (Raf.) Sarg., Antony et al. (2010) for 

Pinus taeda L., and Markstrom and Yerkes (1972) for Pinus 

ponderosa Laws.

Angiosperms show no consistent pattern. The most common 

trend in temperate angiosperms is an increase in SG with 

height, but other patterns have been reported (Taylor 1968, 

1969a,b, 1971, 1979; Taylor and Wooten 1973; Wooten 

1968; Wooten and Taylor 1968). Manwiller (1979) found 

that for small stems (<17 cm DBH) of 22 species of the 

southeastern United States, SG tended to remain constant 

or to decrease with height, but with substantial variation 

depending on species. Data on trends of SG with height 

are very limited for subtropical and tropical species. Taylor 

(1973) reported an increase in SG with height in Eucalyptus 

grandis Hill ex Maiden planted in South Africa, and Skol‐

men (1972) found the same trend in Eucalyptus robusta Sm. 

planted in Hawaii. Whitmore (1973) and Rueda and Wil‐

liamson (1992) found a decrease in SG with height in Costa 

Rican Ochroma pyramidale (Cav. ex Lam.) Urb., Velásquez 

et al. (2009) found a decrease with height in Venezuelan 

Erisma uncinatum Warm., and M.C. Wiemann and G.B. 

Williamson (unpublished data for one tree of each species) 

found lower SG with height in the pioneer species Ochroma 

pyramidale and Trema micrantha (L.) Blume, higher SG 

with height in the pioneer species Cecropia peltata L. and 

Cecropia obtusifolia Bertol., and higher SG with height in 

the nonpioneer species Rollinia microsepala Standl. Bhat 

et al. (1990) found that the SG of stems and branches in‐

creased with height in five species, decreased in two, and 

was irregular in four others. More data are clearly needed 

to define prevalent patterns.

Branchwood versus Trunkwood

Fegel (1941) reported that branchwood SGs of four ring‐

porous angiosperms, eight diffuse‐porous angiosperms, and 

eight gymnosperms of New York State were 10%, 6%, and 

34% higher, respectively, than trunk wood SGs. Sarmiento 

(2011), averaging SG values from 1909 individual trees 

representing 565 French Guiana species, found that outer‐

most trunk wood SG (0.201–0.960) was 9% greater than 

the branchwood SG (0.237–0.949) among all trees, with the 

greatest differences in species with high SG values. Because 

inner wood typically has lower SG than outer wood, this dif‐

ference might diminish or disappear if the whole trunk cross 

section were considered. Swenson and Enquist (2008) com‐

pared stem and branchwood from 33 trees and 14 shrubs 

representing 27 species from Puerto Rico. Their samples 

Figure 3. Cross sections from a 46-cm-DBH Acer saccharum from Wisconsin, showing change in shape, size, and stem integrity with height. Heights above ground are 15 Butt, <10cm; 15A, 3 m; 15B, 7 m; 15C, 10 m; 15D, 13 m; 15E, 16 m; 15F, 20 m. 

DIBs are Butt, 56 cm; A, 40 cm; B, 38 cm; C, 35 cm; D, 32 cm; E, 31 cm; F, 30 cm. The 12-mm-diameter increment borer has a 

45-cm shaft. Although it is long enough to bore to the center of the tree, the hollow stem might make its extraction difficult. 

Boring to 1/6 DIB would be a solution to this sampling problem.
Biomass Determination Using Wood Specific Gravity from Increment Cores

5

included the outer wood from the tree stems, complete 

wood cross sections from 1- to 2-cm-diameter tree branches, 

complete wood cross sections from 1- to 2-cm-diameter 

shrub stems, and complete wood cross sections from 0.4- to 

0.6-cm-diameter shrub branches. They found that stemwood 

had higher SG than branchwood in the same plant. By con‐

trast, Okai et al. (2003) found that branchwood SG was 12% 

and 6% greater than stemwood SG in two Ghanaian species, 

but their sampling included inner, middle, and outer wood 

from large trees (2.5 m DBH) and large branches (≥10 cm) 

and included both sapwood and heartwood. Of 11 species 

grown in India, Bhat et al. (1990) found branchwood had 

the same SG as stemwood in all but two species.

Concluding Remarks

Estimating woody biomass in standing trees depends on the 

accurate measurement of SG. As SG is known to vary across 

the tree diameter, estimating whole tree biomass requires de‐

termination of tree mean SG. Where complete tree harvest is 

impractical or impossible, a method to estimate whole tree 

SG is proposed here. When the pattern of radial variation is 

known, a point along the radius has wood that is representa‐

tive of the whole cross section. For trees with linear or no 

changes in SG across the radius, that point is 1/6 of the tree 

diameter inside the bark. With few exceptions, sampling to 

a depth of 1/6 of the DIB gives a good estimate of the SG of 

a cross section. Depending on how much of the tree is under 

consideration, corrections may be required for variation of 

SG with height and in branchwood.

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