Density and Specific 

Gravity Metrics in 

Biomass Research

Michael C. Wiemann

G. Bruce Williamson

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical

Report

FPL–GTR–208
March 2012

Wiemann, Michael C.; Williamson, G. Bruce. 2012. Density and specific 

gravity metrics in biomass research. General Technical Report FPLGTR-208. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory. 6 p.

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Abstract

Following the 2010 publication of Measuring Wood Specific 

Gravity... Correctly in the American Journal of Botany, 

readers contacted us to inquire about application of wood 

density and specific gravity to biomass research. Here we 

recommend methods for sample collection, volume measurement, and determination of wood density and specific 

gravity for use in biomass studies. We include discussion of 

the effects of wood deterioration and reliance on published 

density metrics. In addition, we note pitfalls associated with 

standard methods, limitations of using densities in biomass 

estimates, and interpretation of international databases 

giving wood densities.

Whether it is standing biomass, coarse woody debris, reliance on published density metrics, or interpreting international wood standards, the application should drive the 

methodology, and the methodology is critical to the results 

and interpretation. Wood-specific gravity, which is unitless, 

refers to oven-dry mass per volume at a specified moisture 

content, relative to the density of water. It is defined differently from wood density, which is mass per unit volume 

under any moisture condition. The moisture content of 

wood affects mass directly and volume indirectly through 

shrinkage or swelling, so in research “wood” is ambiguous 

without a specified moisture condition. Therefore, for all 

applications of density or specific gravity, the measurement 

conditions should be determined and published along with 

the density metrics.

Keywords: biomass, coarse woody debris, density, 

international standards, specific gravity

Acknowledgments

Many people contributed over many years to the research 

that prompted this report. Among them are Sayra Navas 

(cover photo), Toni Soto (Fig. 1a), and Lou Ann Karter 

(Fig. 2). The authors themselves are presented in Figure 1b 

(GBW and MCW), Figure 4 (GBW), Figure 5 (MCW), and 

Figure 6 (MCW). Roger E. Hernández of Université Laval, 

Québec, David Kretschmann and Samuel Glass of the Forest 

Products Laboratory, and Mary Beth Adams of the USDA 

Northern Research Station gave critical reviews that improved the manuscript. GBW was supported by the 

U.S. National Science Foundation (DEB‐0639114).

Contents

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

Density and SG in Biomass Estimation............................. 1

Relying on Published SG Values in Biomass Research..... 2

Standard Methods in Biomass Research............................ 3

Summary............................................................................ 4

Literature Cited .................................................................. 4
Density and Specific Gravity Metrics 

in Biomass Research

Michael C. Wiemann, Botanist

Forest Products Laboratory, Madison, Wisconsin

G. Bruce Williamson, Professor, Department of Biological Sciences

Louisiana State University, Baton Rouge, Louisiana

Introduction

In a recent American Journal of Botany article, we reiterated the distinction between wood density, which is wood 

mass/wood volume, and wood specific gravity (SG), which 

is oven-dry wood mass/wood volume/density of water (Williamson and Wiemann 2010). Because wood shrinks as it 

dries, the SG of wood can be determined at different wood 

volumes depending on its moisture content (MC): green volume when saturated with water, oven-dry volume when the 

MC is zero, and volume at a specified MC for intermediate 

values. These are typically referred to as basic SG, oven-dry 

SG, and SG at a specified MC, respectively. Wood mass for 

SG is always determined oven-dry, i.e., at a MC of zero. 

For wood density, the mass and volume are determined at 

the same MC, although the hybrid term “basic density” is 

defined as oven-dry mass divided by green volume. For any 

wood sample, the numerical values of SG and density 

differ from one another as MC varies above the oven-dry 

condition.

Clarifying these definitions and identifying errors in recently 

published ecological studies (Williamson and Wiemann 

2010) triggered queries about some applications of SG metrics. Many of the queries were focused on biomass research 

and international conventions. This is no surprise, given the 

importance of the global carbon cycle.

Density and SG in Biomass 

Estimation

Woody biomass of standing trees can be determined from 

estimates of tree volume, based on diameter and height measurements, and basic density or SG (Wenger 1984). Many 

biomass studies state that wood volumes are multiplied by 

“basic SG” when they mean “basic density.” The product 

of volume and density is a mass, whereas the product of 

volume and SG is a volume because SG is unitless. Biomass 

studies can “weight” wood volumes by basic SG and then 

convert volume into mass by multiplying by the density of 

water (1g/cm3). Or they can use the hybrid metric of basic 

density but not referenced as SG. SG of wood and bark in 

standing trees can be estimated from samples taken with increment borers (Fig. 1a). Best estimates of wood SG or density will include all of the wood, from pith to bark (Fig. 1b) 

and along the height (Rueda and Williamson 1992) and 

will be weighted to represent the whole tree (Williamson 

and Wiemann 2010). Likewise, if bark is to be considered, 

it should include both inner bark (phloem) and outer bark 

(periderm). Biomass can be determined more precisely by 

harvesting trees and determining species‐specific regression 

equations for a site or region (Nelson et al. 1999, Nogueira 

Figure 1. (a) Extraction of a core from Ochroma 

pyramidale using an increment borer in Costa 

Rica. (b) Verifying a bark-to-pith core.
General Technical Report FPL–GTR–208

2

et al. 2008). If a tree is felled, an unbiased measure of its 

density or SG can be obtained from wedges or complete 

disks.

Density of decomposing wood such as coarse woody debris 

(CWD) can be determined most accurately by measuring 

mass and volume directly, rather than borrowing density 

or SG values from the literature. In practice, the volumes 

of CWD are estimated from linear measurements in the 

field (Fig. 2) and a sub-sample taken to determine density. 

The measured volume of CWD should be multiplied by the 

sub-sample’s oven-dry mass divided by the sub-sample’s 

volume at field moisture conditions. Volume of a decayed 

sub-sample should be determined the same way that volume 

of the CWD was determined, usually from linear measurements, not from a different method such as water displacement, for several reasons: the methods may give different 

estimates because wood that is not saturated will absorb 

water during water displacement unless it is wax coated, and 

wood volume changes as wood decays and voids are created 

in the wood. Accurate linear measurement of a sub-sample 

requires that it be machined to a regular shape with right-angle corners (ASTM Standards 2010; D2395-07a, Test Method A). Mass should be determined after drying at 103 °C 

to constant weight. (Also, mass should be measured before 

drying to be able to calculate MC of the debris.) Some SG 

measurements of CWD are based on drying at only 65 °C 

to reduce loss of volatile compounds at high temperature. A 

study by Adams and Owens (2001) of CWD in a temperate 

hardwood forest showed that oven-dry mass was within 1% 

of the mass of wood dried at 65 °C.

Wood from moist forest floors will often be saturated, but 

debris at many sites is not saturated (Fig. 3). Decaying 

wood, even if apparently preserved by submersion, may 

exhibit exaggerated shrinkage with concomitant SG changes 

(Borgin et al. 1979). Given that CWD is always in some 

stage of decay, mass and volume determined from subsamples will provide a more accurate measure of density 

than the published values for species (Harmon et al.1986). 

Adams and Owens (2001) partitioned CWD of 21 species 

found on the Fernow Experimental Forest into three decay 

classes: 1) wood firm, 2) wood soft in some places, and 

3) wood soft throughout. Across all species, mean SG decreased by nearly 50% from class 1 to class 3. 

Measurement problems caused by decomposing wood are 

not confined to debris. In addition to the obvious problem of 

extracting an increment borer from a hollow or rotten tree, 

biomass of standing timber can be overestimated because 

of internal decay. 

Unlike decay, wetwood is the result of anaerobic bacterial 

action in a living tree. The presence of wetwood in and of 

itself does not require a density adjustment. Wetwood frequently accumulates methane gas under pressure. In such 

cases, coring wetwood converts the end of the increment 

borer into a gas valve, as in Figure 4 showing the ignited gas 

from a bored Bursera simaruba. Unexposed wetwood does 

not decay rapidly, nor does it lower the wood SG, because 

the bacteria feed on the cell contents, not cell walls. However, when a tree containing wetwood dies, decay follows 

rapidly (Shigo 1986) and may affect the wood density. 

Relying on Published SG Values in 

Biomass Research

Sources of published SG values vary from individual studies 

to large databases. Local sources often provide detailed data 

of methodology, specifying exact moisture conditions and 

drying temperatures associated with measurements. Compilations and databases are handier but do not ensure that 

standard or even equivalent methods have been employed 

among species in the same database. The Global Wood 

Density Database is currently the most comprehensive compilation of wood basic density (oven-dry mass/green volume) with over 16,000 values for more than 8,000 species. 

Initially published by Chave et al. (2009), it is now publicly 

accessible through Dryad Digital (www.datadryad.org/repo/

handle/10255/dryad.235) and provides density values with 

citations (Chave et al. 2009, Zanne et al. 2009). Values for 

Figure 2. Measuring volume of coarse woody debris with 

a reel tape measure and calipers in Wisconsin.

Figure 3. Small coarse woody debris in the Fernow 

Experimental Forest, West Virginia.
Density and Specific Gravity Metrics in Biomass Research

3

bark are harder to find, but Miles and Smith (2009) have 

compiled SG values for 156 North American tree species. 

Their range for bark (0.25–0.72) is almost the same as their 

range for temperate wood (0.29–0.78), so using a single value for bark can be as problematic as a single value for wood. 

When searching for SG or density data, it is often easy to 

start with a global or regional database and then to proceed 

to original data sources if verification of measurement conditions is needed.

Published SG values often provide an average from various 

sources, although the trees averaged may not be a random 

sample. The above-mentioned Dryad Digital database lists 

14 basic density values for Ochroma pyramidale (Cav. ex 

Lam.) Urb., with a range of 0.100 to 0.220 and a mean of 

0.158 g/cm3. The SG of wood of this species increases from 

about 0.04 to 0.40 from the pith to the bark in large trees 

(Whitmore 1973), so the values in the database would seem 

to represent sampling of small-diameter trees. Unfortunately, the actual wood samples for many published values have 

been discarded, making verification impossible. Even where 

wood samples are available, as in wood collections, they 

often lack accompanying herbarium vouchers. Nevertheless, 

thanks to numerous wood samples, outliers have little effect 

on the “mean” SG values of common species, and variation 

in SG among species is much greater than variation within 

a species. 

Wood SG for a given species can vary geographically, 

partially a result of climatic variation (e.g., Whitmore 1973, 

Wiemann and Williamson 1989b), and such variation is 

unlikely to be recorded in large compilations of specific 

gravities for a species, although a species SG may be listed 

as “variable.” Therefore, use of a single SG value to characterize each species globally may be appropriate for comparison of large datasets (e.g., Wiemann and Williamson 2002, 

Chave et al. 2006, 2009), but for single site studies, local SG 

values are likely to provide more meaningful analyses (e.g., 

Poorter et al. 2006, Wright et al. 2010). 

Finally, SG variation within trees is large, especially in 

tropical pioneers (Wiemann and Williamson 1988, 1989a,b, 

Omolodun et al. 1991, Rueda and Williamson 1992, Parolin 

2002, Nock et al. 2009). The SG for the tree depicted in 

Figure 5 varied from 0.07 (near the pith) to 0.45 (near the 

bark). The diameter of this tree was 146 cm, and the length 

of the increment borer from tip to handle was 81 cm. Surprisingly, it took only three attempts to reach the center of 

this tree. When SG varies linearly from pith to bark, as it 

does in Ceiba pentandra, an estimate of the cross-sectional 

SG can be found at one-sixth of the diameter inside the bark 

(dib) (Williamson and Wiemann 2010). For the tree shown, 

a weighted basic SG average of all 1-cm bark-to-pith segments gave a value of 0.384, whereas the basic SG of the 

wood at one-sixth of the estimated dib (23–25 cm from the 

cambium) was 0.381 (Wiemann and Williamson, unpublished data). An analysis of the accuracy of single-value 

estimations of SG is discussed in Wiemann and Williamson 

(2012).

Use of a single SG value may be appropriate for estimating 

standing biomass, although species-specific regressions of 

biomass on diameter are more accurate (Nelson et al. 1999). 

In wood decomposition studies, it may prove useful to determine initial densities for various components and sizes 

of tree debris because wood density of living trees varies by 

tree age and by location along trunks and limbs (Koch 1985, 

Nock et al. 2009, Williamson and Wiemann 2011).

Standard Methods in Biomass 

Research

U.S. foresters traditionally rely on ASTM Standards and 

the Forest Products Laboratory for guidance in determining 

wood properties, but standard sources become outdated. For 

example, measuring the volume of small wood samples has 

been problematic historically in density determinations. 

The Forest Products Laboratory published an endorsement 

of the maximum moisture content (MMC) method for 

Figue 4. Adjusting the flame from ignited methane 

streaming from the wetwood zone of a large Bursera 

simaruba.

Figure 5. Extraction of a core, using a large-diameter 

increment borer, for pith-to-bark SG measurements from 

a large Ceiba pentandra in Sarapiquí, Costa Rica.
General Technical Report FPL–GTR–208

4

estimating SG, a method that avoids volume determination 

(Smith 1954). Although still in use today, the method as 

published does not account for the high density of bound 

water versus free water (MacLean 1952). Bound water is 

denser than free water because of compaction of the adsorbed water (Skaar 1972). In the MMC method, void space 

in green wood must be filled completely by water. Although 

the air content of green wood is highly variable across species, it averaged about 20% in one study (Gartner et al. 

2004), so a vacuum would have to be drawn on a submerged 

sample that is small enough to permit removal of all residual

air. It is often necessary to oscillate between vacuum and 

pressure under water to remove all the air (Skaar 1972). The 

MMC method requires a value for the specific gravity of 

wood substance, which is not constant among species and 

may be affected by extractive content; Smith (1954) gives a 

range of 1.50 to 1.56, with an average of 1.53.

Yet another volumetric misguidance comes from the ASTM 

Standards (2010). Standard D2395-07a, Test Method B, 

describes wood volume determination by water immersion. In Modes I and IV, wood volume is observed directly 

as displacement in a water‐filled container or volumetric 

cylinder, whereas in Modes II and III, volume is obtained 

from the weight of water displaced. Although the standard 

includes the final caveat that “The precision and bias of 

these test methods for determining specific gravity are being established,” it is our experience that measurement of 

volume directly is less precise than measuring the weight of 

water displaced. Given that a balance is needed anyway to 

determine mass for the SG determinations of the standard, it 

is difficult to explain the inclusion of the direct volume measurement procedures. Figure 6 shows the accurate measurement of the volume of a small sample using water displacement (Mode II). 

SG and density standards outside the United States vary 

across continents, so results should be interpreted carefully 

from the methods. For example, French standards historically have characterized tropical woods by density at 12% 

or 15% moisture content, not by basic SG (Sallenave 1955). 

In practice this is often accomplished by drying samples at 

20 °C and 65% relative humidity.

International nomenclature is often confusing, especially 

under the metric system (CGS units), where basic density 

(in g/cm3) and basic specific gravity have the same numerical value. Therefore, it is wise to ignore cognates and 

carefully read the methods of each study. For example, francophone countries use “densité,” “densité basale,” “masse 

volumique,” and “poids spécifique” somewhat interchangeably. Failure to carefully read the methods led Williamson 

and Wiemann (2010) to misinterpret the drying temperature 

in Ruelle et al. (2007) as 65 °C when it was 20 °C and 65% 

relative humidity. Analogous cognates appear in Neotropical 

studies published in Portuguese and Spanish. It should also 

come as no surprise that scientists abroad have difficulty 

interpreting U.S. nomenclature, especially where standard 

practices differ. Some of the inconsistencies we enumerated earlier (Williamson and Wiemann 2010) were made by 

scientists trained outside the United States and may have 

resulted from different standards across continents.

Summary

SG of wood or bark is defined as oven-dry mass per unit 

volume at specified moisture content, relative to the density 

of water. This is different from density, which is mass, including moisture, per unit volume at any moisture content. 

Moisture in wood affects mass directly and volume indirectly through shrinkage or swelling, so in research it is essential to specify moisture conditions and methods employed 

to determine mass and volume. Likewise, careful reading 

of methods is critical to avoid misinterpretation of studies 

that involve standing trees, wet or dry coarse woody debris, 

partially decayed biomass, previously published or tabulated 

data, or international standards.

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Figure 6. Measurement of the green volume of an increment core segment by water displacement.
Density and Specific Gravity Metrics in Biomass Research

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