| Wood quality can be considered a concept that emphasizes particular wood properties (anatomical, chemical and physical) that individually, or in combination, have a positive influence on a specific wood product. It can be difficult to define wood quality as perceptions of what constitutes quality differ among divisions of the forestry and wood using industries and ultimately depend on the end product.
Wood Quality and Uses
The importance of wood quality has been recognized throughout recorded history, as native people globally recognized the unique properties of wood from different tree species and used particular species where best suited for specific applications, a well known example is the use of the wood of yew (Taxus species) for the manufacture of archery bows. In more recent times numerous interesting examples exist, including the utilization of lignumvitae (Guaiacum officinale) for the manufacture of bearings for the propeller shafts of ships (the hard, dense wood is self-lubricating and can withstand high working pressures). Pernambuco or pau Brasil (Caesalpinia echinata) is used almost exclusively for the manufacture of bows used to play stringed instruments such as the cello, viola and violin (the wood is strong and has unique resonance properties).
The widespread use of the heartwood of any durable species (the heartwood of many species can be high in extractives that are repellant to fungi and insects), such as osage orange (Maclura pomifera), black locust (Robinia pseudoacacia), bald cypress (Taxodium distichum) and redwood (Sequoia sempervirens), for applications where the wood is in contact in the ground. Finally, another interesting example includes the use of woods known as sanitary woods, e.g. black and white ash (Fraxinus nigra and Fraxinus americana) and kaihikatea (Darycarpus dacryioides) from New Zealand, as the wood from such species does not stain or taint food products by contact, and were used to make boxes and tubs for storing butter and other products whose quality would otherwise be compromised by the extractives present in wood of most species.
Solid Wood Products Properties
Wood properties of importance generally differ depending on the end product but specific gravity or wood density i.e. the weight or mass of wood per unit volume, is possibly the most important wood quality indicator as it influences both the yield and quality of fibrous and solid wood products. For example, wood density in softwoods has a strong correlation with pulp yield, pulp quality, and the strength and stiffness of wood (Saranpää 2003; Clark and Daniels 2002; Stamm and Sanders 1966).
Another important property determining the stiffness of wood is the microfibril angle (MFA) of the S2 layer of the secondary cell wall. The S2 layer is much thicker than the other cell wall layers (S1 and S3) and consequently largely determine the properties of the cell wall (Megraw 1985), and the subsequent properties of the wood. At a given density wood having low MFA (10 to 15 degrees) will be considerably stiffer than wood having high MFA (30 to 45 degrees). MFA is also important in determining shrinkage properties and is a useful indicator of compression wood. Such wood is undesirable for solid wood products because it becomes brittle and cracks and warps easily when dried.
Despite the importance of density and MFA as wood quality indicators, in the wood products industry stiffness (modulus of elasticity, MOE) and strength (modulus of rupture (MOR) are most commonly used to indicate wood quality. MOE is used to describe the stiffness of wood and is the ratio between stress and strain, while MOR is defined as the load carrying capacity of a material. For small clear wood samples wood density and MFA are important determinants of stiffness, while density largely determines strength. For lumber knot frequency, size and location, slope of grain and load duration are all important in determining product performance.
Pulp and Paper Properties
For paper products fiber (or tracheid) dimensions are important, in addition to microfibril angle and density. Fiber length, fiber diameter (in both the radial and tangential direction), cell wall thickness and coarseness are all important wood quality indicators for paper manufacture. A comparison of the properties of paper made from a softwood pulp (for example Pinus taeda) and a hardwood pulp (Eucalyptus species) illustrates the importance of fiber dimensions on product properties. Eucalyptus pulps are renowned for their ability to produce high quality printing and writing papers and tissue products compared to P. taeda pulps that are used for the manufacture of products such as paper grocery bags and corrugated linerboard.
Eucalypt fibers are relatively short, slender and thin-walled and these properties lead to excellent sheet formation and a sheet that has high bulk, excellent surface properties and good density, stiffness and optical properties. These properties can be contrasted with paper or paperboard manufactured from P. taeda pulps that have inferior surface properties but superior strength properties. Inferior surface properties of P. taeda pulps can be directly related to the relatively large fiber dimensions of softwood tracheids, particularly those of the latewood (thick walled and do not collapse well), while improved strength properties, particularly tear and tensile strength, arise from the relatively long tracheids of softwoods that have greater potential for many more interfiber bonds than the short fibers of Eucalyptus.
Chemical Pulps
For chemical pulps, wood chemistry as well as wood density, is important. The relative proportions of cellulose, hemicellulose and lignin are all important in determining the desirability of a species for the manufacture of chemical pulps and collectively determine pulp yield (defined as the oven dry mass of pulp expressed as a percentage of the original oven dry wood). Predominately young plantation woods are used for the manufacture of chemical pulps but if the wood of older trees is used extractive content (non-structural compounds that can be extracted by solvents) and composition can become very important as extractives will lower pulp yield and increase chemical demand.
The monomer composition of hemicellulose and lignin in hardwoods may also considered, for example, hardwood lignin is comprised of guaiacyl and syringyl monomers and guaiacyl lignin is easier to remove during pulping, hence in advanced breeding programs trees having higher guaiacyl contents are preferred.
For mechanical pulps different criteria are important and include the absence of color as the pulps are only partially bleached, a minimum resin content and low density wood as it reduces energy consumption.
Wood Quality Management
Wood quality can be improved by tree breeding with properties such as density and pulp yield being included with growth characteristics in many programs, however, the inclusion of a range of properties on a large-scale has been inhibited by the time and cost involved in measuring wood properties on a large-scale. The development of rapid, non-destructive techniques for the measurement of wood properties including SilviScan (physical properties), near infrared spectroscopy (chemical and physical properties) and acoustics (stiffness) provide technologies that make the measurement of wood properties on a scale that until recently was not possible and increases the potential for gains in wood quality in the future.
Silvicultural methods are also important in determining wood quality with spacing at the time of planting, fertilization (timing, frequency and dose) and thinning (timing and stocking post thinning) all being important determinants of wood quality. For example plantation grown trees having a low initial stocking, i.e. widely spaced, and heavily fertilized when young to promote rapid early growth, will have a large juvenile core and while such trees will reach harvestable size quickly their wood quality, assuming they will be cut for sawlogs, will be low. For plantation trees grown for veneer production pruning at the appropriate time and to an appropriate height are important as both determine the volume of clear wood produced.
References
Clark, A., and Daniels R.F. 2002. Modeling the effect of physiographic region on wood properties of planted loblolly pine in Southeastern United States. Forth workshop, IUFRO working group, S5.01.04, Harrison Hot Springs, B.C., Cananda, Sept. 8 – 14.
Megraw, R.A. 1985. Wood quality factors in loblolly pine. The influence of tree age, position in tree, and cultural practice on wood specific gravity, fiber length, and fiber angle. Tappi Press, Atlanta.
Saranpää, P. 2003. Wood density and growth. In: Wood Quality and its Biological Basis. Edited by J.R. Barnett and G. Jeronimidis. Blackwell Publishing Ltd. Oxford. UK: 87-117.
Stamm, A.J., and Sanders H.T. 1966. Specific gravity of the wood substance of loblolly pine as affected by chemical composition. TAPPI J. 49: 397-400.
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Laurie Schimleck is Associate Professor, Warnell School of Forestry and Natural Resources, The University of Georgia, Athens, GA 30602, U.S.A.
Alex Clark is Wood Scientist, USDA Forest Service, Southern Research Station, Retired, Athens, GA, 30602, U.S.A.
Posted 27 February 2008
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