Oyomoare Osazuwa-Peters1 and Amy E. Zanne1
1Department of Biology, University of Missouri – St. Louis
Wood density is defined as mass of wood per unit volume. It is an important trait for understanding the function and ecology of woody species, as well as estimating stored biomass and carbon content (Chave et al. 2009). It is used as an indicator of wood quality and tissue allocation patterns and a predictor of plant performance.
Wood density is an emergent property in the sense that it is determined by the chemical and structural organization of cells, proportion of space or void volume, and moisture content. Moisture content is an important factor that has to be accounted for when measuring wood density because mass and volume – the components of wood density – change with moisture content (MC) due to the hygroscopic nature of wood.
The forestry literature distinguishes wood density from specific gravity (Simpson 1993, Williamson & Wiemann 2010). Specific gravity (SG) is the density of a material relative to the density of water (= 1.0 g/cm3 at 4oC). SG is unitless because it is a ratio of wood to water density. Ecologists have typically not distinguished between the two measures, which can lead to confusion when comparing across studies. We use the typical application of wood density applied by ecologists here but define how mass and volume should be measured and annotated in hopes of increasing clarity.
Wood density is typically estimated using oven dry mass as the numerator. The denominator – volume – can be measured either when green (= basic density) or air or oven dry to a specified MC. Due to variation in the wood density values obtained with these different volumes as denominators, basic density is used as the standard to which other values are converted.
There are conversion equations and tables to move between SG and wood density at different moisture contents. Chave et al. (2009) used a multiplier of 0.861 to convert wood density in which volume was measured at 10-18% moisture content to basic density, while Simpson (1993) used various non-linear equations for conversions. These latter equations may work better for temperate species for which they were developed and tested than tropical species; in one tropical dataset, they progressively underestimated wood density especially at the dense end outside of the range of temperate species.
To measure wood density via the main protocol described below, the following equipment is needed:
- Drying oven (able to maintain temperatures between 100 – 105oC)
- Electronic weigh balance (precision will depend on size of wood)
- Flask or beaker (size depends on size of wood sample to be immersed), ideally with a relatively small neck to minimize evaporative water loss
- Water (ideally distilled or RO)
- Needle and thread (or alternative practical means of immersing wood sample into water without introducing additional volume)
- Modeling clay or blue tack placed on the thread just above the needle (can help make the wood hang straight). This is more important for small pieces of wood.
- Coin envelopes (to store wood while drying)
Figure 1 Fresh volume determination using the water displacement method; threaded needle used to immerse wood segment into water in a container placed on an electronic weighing balance, which gives mass of water displaced.
Units, terms, definitions
Wood density ( ) = mass/volume
Basic density = oven dry mass/green volume
Dry wood densityMC = air dry mass at specified moisture content/green volume
Basic specific gravity = (oven dry mass/green volume)/density of water
Wood density is usually expressed as g/cm3, kg/m3or lb/ft3.
The density of wood with no air spaces is ~1.5 g cm-3. Wood density is thus bounded between 0 and 1.5 g cm-3.
Obtaining wood samples
The part of the plant (branches, trunks, roots, etc.) harvested for wood density depends on the research question of interest. Evidence shows that wood density varies radially and axially within an individual tree. Heartwood density is typically higher than sapwood density, and branch wood density is typically lower than trunk wood density (Patino et al. 2008, Swenson & Enquist 2008, Sarmiento et al. 2011). Using trunk or branch wood density estimates alone can potentially lead to overestimates or underestimates of total plant wood density (e.g., when estimating whole plant biomass).
Samples from a woody species can be obtained as:
1. Stem discs from the trunk at a consistent height from the trunk. This is a destructive sampling technique, most suitable when felled trees are available.
2. Stem segments cut from branches. This is a less destructive sampling technique. Often branch samples of 2.5 cm in length are harvested at a given distance back from the branch tip or cross sectional diameter. Bark and pith are removed (Hacke et al. 2000).
3. Stem segments taken from the trunk using an increment borer. This is a less destructive sampling technique. Depending on the question, the core can be sliced into segments (e.g., at certain distances along the length or separating heartwood from sapwood).
Measuring wood density
There is no consensus on the best way to measure wood density. We describe a simple and direct method and briefly mention more sophisticated indirect methods.
Gravimetric/volumetric procedure: This involves determining the fresh volume of a piece of wood and its dry mass, preferably as oven-dry mass.
1. Fresh/green volume:Samples can be measured directly after harvesting (or after storing for brief periods while wood is kept cool (in a refrigerator or ice chest), wrapped in plastic with moist paper towels). In some studies, wood samples are placed in water to obtain maximum hydration for a given period of time (e.g., overnight).
A. When the wood segment has a regular shape, volume can be determined with vernier calipers. For example, in the case of a cylinder shaped wood sample, the length and diameter can be obtained using a vernier caliper, and the volume computed with the appropriate formula (ℼr2l, where r = radius and l = length).
B. When the wood segment is irregularly shaped, the volume displacement method is ideal.
i. Wood segments are attached to a needle on a thread (difficult for very dense woods as needles tend to bend or break). Modeling clay or blue tack can be added to the thread just above the needle to help small pieces of wood to hang straight.
ii. A flask or beaker with water is placed on an electronic balance and the balance is tared (or set to 0). If it is not possible to tare the balance, then the mass of the beaker can be measured (M1).
iii. The thread is then used to immerse the wood segment into water.
iv. The mass of water displaced (the mass determined while the wood is immersed in the water while on the balance) is measured (M2). The top of the wood should be just below the meniscus avoiding immersing the needle. The wood should also not touch the sides of the flask.
v. The mass of water displaced by the wood segment (=M2-M1) is equivalent to the fresh volume of the wood segment, assuming that the density of water = 1 g/cm3.
vi. Note, in the absence of an electronic balance, a less accurate method to obtain fresh volume involves carrying out the volume displacement in a graduated cylinder and the volume of water displaced equals fresh volume.
2. Dry mass: Values are obtained by drying wood samples between 100 – 105oC until constant mass is attained (typically 24 – 72 h), in a well-ventilated oven. The advantage of oven drying at >100oC is that bound water, including water in the cell wall, can only be completely dried off at these temperatures. Alternatively, if oven drying is not possible, wood is air-dried and moisture content of the wood is quantified (e.g., with a wood moisture meter) and specified for conversion to standard values.
Other more sophisticated methods: these provide indirect estimates of wood density, and can be ranked based on the kind of information provided ranging from one to three dimensions. The table below is based on De Ridder et al. (2011), which provides information on the various types of indirect methods available for determining wood density.
|Dimension of output||Method|
|Density is estimated from relationship between tree ring width and density variation from tree rings in series|
|Density estimated from resistance drilling|
|Density estimated from measurement of cell-wall thickness with a transmission light microscope|
|High-frequency densitometry uses the dielectric properties of wood to quantify relative density variation along wood surfaces|
|Density estimated from radiographies (e.g., X-ray and gamma-ray as ionizing radiation techniques, neutron imaging, color video camera imaging, magnetic resonance imaging and microwave polarimetry).|
|Density estimated from thermograms, more applicable to decay diagnosis|
|Density estimated using acoustic methods, more applicable to decay diagnosis|
|Density estimated from X-ray, gamma-ray and neutron tomography.|
Notes and troubleshooting tips
For fresh volume, the time involved depends on the nature of the samples; it is a relatively fast process for soft and moderate density woods, but may be protracted for high density woods because of the time and effort required to insert the threaded needle into the wood sample. On average, fresh volume can be determined in as quick as three minutes per wood sample. Dry mass determination requires at least 24 hours or longer at 100-105oC to reach constant mass.
Links to resources and suppliers
Several guides for coring trees:
- Grissino-Mayer, H. D. (2003) A manual and tutorial for the proper use of an increment borer
- Ferry Slick, J. W. et al. (2007) Wood Density as a Conservation Tool: Quantification of Disturbance and Identification of Conservation-Priority Areas in Tropical Forests
The latter also has a guide for measuring wood density.
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Bowyer, J.L., Shmulsky, R. & Haygreen, J.G. (2007). Forest products and wood science: an introduction. Wiley-Blackwell.
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Simpson, W. T. 1993. Specific gravity, moisture content, and density relationship for wood. (General technical report FPL, GTR-76): 13 p. : ill. ; 28 cm. (https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr76.pdf)
Swenson, N.G. & Enquist, B.J. (2008). The relationship between stem and branch wood. Am. J. Bot., 95, 516-519.
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