Wood Quality Attributes and Their Impacts on Wood Utilization


S.Y. Zhang[1]


With the worldwide move to intensive forest management to meet wood demand and preserve the environment, the wood supply has been undergoing a significant change in terms of wood quality. Increasing amounts of wood come from managed short-rotation forests, and the proportion of this resource will continue to grow until it dominates the world’s wood supply in the first half of this century. The changing quality characteristics of the wood supply have a profound impact on wood processing and utilization throughout the value recovery chain. They will eventually impact on the forest products industry and affect customer satisfaction. They also raise new issues for forest management to deal with.

How can the forest resource be managed to allow for quality products, maximum value, and minimum manufacturing costs, or so-called value-added forest management? How can the resource be processed and utilized more efficiently to produce product attributes of value to customers? Before addressing these issues, we need to define: 1) how foresters, manufacturers and customers perceive wood quality (in terms acceptable to all), and 2) what wood quality attributes manufacturers and customers require. This paper focuses on these two questions.

Wood quality is defined as the combination of all wood characteristics that affect the value recovery chain and the serviceability of end products. This paper examines the various wood characteristics considered critical wood quality attributes by foresters, manufacturers and customers. It also discusses how these properties affect the value recovery chain and the serviceability of solid products.


Over the past decades the world’s managed fast-growing forests have been increasing steadily. The managed resource is expected to dominate the world wood supply in the first half of this century. Worldwide, the transition from total dependence on depleting inherited stocks to reliance on a managed resource has been associated with a significant decline in wood quality (Zobel 1984; Kellogg 1989). For example, a wood quality index reported by Constantino and Haley (1988) for the British Columbia Coast showed that log quality had declined by almost 25% between 1925 and 1980. The managed resource is usually characterized by younger age, smaller stem diameter, larger taper, larger knots, higher juvenile wood content and different wood characteristics and processing properties.

What do these changes mean to the forest industry? Changes in wood quality have triggered a chain reaction (Zobel 1984). Wood quality has a profound impact on wood processing, end-product quality and marketing (Zobel 1984; Kellison et al. 1984; Kellogg 1989). In the B.C. Coast, for example, declining log quality has reduced productivity and product quality while increasing production costs (Costantino and Haley 1988).

Can forests be managed efficiently to produce the wood quality desired by manufacturers? And how can a different resource be used efficiently to manufacture the quality products desired by customers? Before addressing these questions, we need to define: 1) how foresters, manufacturers and customers perceive wood quality (in terms acceptable to all), and 2) what wood quality attributes manufacturers and customers require.


1. Why Define Wood Quality?

Wood "quality" means different things to different people. While foresters think of tree size and form, and lumber manufacturers see large, straight and clear logs, customers associate wood quality with other attributes. The building industry, for example, is mainly interested in strength, stiffness and dimensional stability, but it cares very little about basic wood properties (Kliger et al. 1994). A survey of literature from the Commonwealth Agricultural Bureau database produced over one thousand references to "wood quality". In many cases, the term was used as a synonym to wood density. Many forest scientists appear to consider wood density and fibre length as the key wood quality attributes regardless of end uses. Kliger at al. (1994) ascribed this confusion to poor communication among forest management, wood manufacturing and wood-using sectors. If this situation continues, neither the industry nor the end users will be getting what they need. A common definition of wood quality and a better understanding of its impacts on wood manufacturing and customer satisfaction would facilitate communication among all parties concerned.

2. A Definition of Wood Quality

Many attempts have been made to define wood quality (Keith 1985), but the definition proposed by Mitchell (1961) appears to be the most widely cited: "Wood quality is the resultant of physical and chemical characteristics possessed by a tree or a part of a tree that enable it to meet the property requirements for different end products". Almost everybody agrees that wood quality must consider specific end uses, and this definition duly emphasizes their significance, but it fails to consider other aspects that are of importance to manufacturers, foresters or customers. It cannot, therefore, be applied to the forest management/manufacturing/customer chain.

As wood properties affect various aspects of the manufacturing process, wood quality must be defined in terms of the value recovery chain. In addition, the definition needs to include serviceability, and cover attributes of interest to end-users, which may or may not have a direct impact on manufacturing, but will continue to matter long after the product has been sold and installed. This paper accordingly defines wood quality as "all wood characteristics that affect the value recovery chain and the serviceability of end products".


Wood Quality Attributes and Value Recovery Chain

As stated by Johansson et al. (1990), each end product has a unique set of requirements. This paper focuses on solid wood products, especially lumber. In line with the proposed definition, this paper examines characteristics that affect the value recovery chain and serviceability. The importance of examining the whole value recovery chain from trees to products has been stressed in the literature. Wood quality attributes affect the manufacturing and product value at almost every step of the chain.:

1. Harvesting: Tree size is the most important attribute affecting logging costs. Logging costs per unit of wood decrease significantly with increasing tree diameter and height (Holtzscher and Lanford 1997). Log recovery also increases with tree height and diameter. When purchased on a volume basis, denser wood is more costly to log. Decay and other defects also increase logging costs.

2. Transportation is more costly with increasing wood density and moisture content.

3. Storage of logs or lumber over extended periods may cause biodegradation. If no prevention measures are taken, sapstain, decay or discoloration may occur (Johansson et al. 1990; Zabel and Morrel 1992) and reduce wood value. Such defects are directly related to specific wood properties (e.g., extractives content, heartwood/sapwood, moisture content, pH).

4. Lumber conversion involves a series of operations where wood properties can affect value recovery:

4a) Bucking is affected by tree size, stem deformations and wood defects.

4b) Debarking: Moisture content and bark characteristics affect debarking costs. Bark content also affects lumber yield.

4c) Lumber volume recovery is closely related to log diameter (Steel 1984; Oberg 1989; Zhang et al. 1998). Stem taper is a key factor in volume recovery (Steel 1984). Stem shapes can affect lumber volume recovery, dimension and grade yield (Steel 1984; Oberg 1989).

4d) Lumber dimensions: Larger trees yield larger-size lumber, for which customers usually pay higher prices.

4e) Manufacturing costs: Many wood properties can affect manufacturing efficiency. It is less costly to process larger diameter logs (Jennings 1989). Bark characteristics, knots and other defects increase manufacturing costs.

5. Lumber drying: The basic characteristics of wood determine its relationship to water and its response to drying. Anatomical characteristics related to moisture diffusion are closely linked to wood drying rate. Anatomical differences may affect wood drying. It takes longer, for instance, to dry high-density wood with restricted moisture flow or wood with wet pockets. Drying costs are also directly related to initial moisture content.

Drying defects represent a major loss in product value. Some drying defects (e.g., warping, collapse, chemical stains, check) are closely related to basic wood characteristics. For example, lumber warping was related to specific wood characteristics (Perstorper et al. 1995). Surface checking in oak is mainly due to wide wood rays that create spots of surface weakness.

6. Lumber grading determines not only lumber value but also possible end uses. Most lumber in Canada is still graded visually, but machine-stress rated (MSR) systems have become increasingly important.

6a) Visual grading consists in observing surface defects and other wood characteristics, and predicting strength properties according to predetermined rules. Visual grading under Canada's National Lumber Grades Authority (NLGA 1996) rules takes into consideration many wood characteristics (e.g., knots). Wane is another major cause of lumber downgrade. Reducing log taper and improving log shapes will not only increase lumber volume recovery, but also reduce the occurrence of wane. Wood defects (e.g., sweep, decay) and other characteristics may also affect lumber grade.

6b). Mechanical grading: Lumber has highly variable mechanical properties. As a result, design strength values allocated to visual grades are significantly lower than average values, which represents a significant loss. The MSR (machine-stress-rated) system measures the stiffness of individual lumber pieces and estimates their strength, then assigns them to various stress categories. The MSR system consequently leads to more efficient lumber use.

Wood characteristics (e.g., knottiness, wood density, defects) affect MSR yield (Barrett and Kellogg 1991; Zhang et al. 1998; Zhang et al. 2002). Any improvement to these quality attributes through appropriate forest management (e.g., longer-rotation, appropriate spacing, pruning) can increase MSR yield, adding value to the resource.

Wood Quality Attributes and Serviceability

The serviceability of lumber depends upon its end uses. For structural applications, its strength and stiffness, dimensional stability, and durability are important considerations for customers. For furniture manufacturing, other characteristics (e.g., machinability, dimensional stability, appearance) become important. Wood treaters prefer jack pine to balsam fir because it is more permeable. These important quality attributes for customers and manufacturers are determined, to a varying degree, by basic wood characteristics. For example, durability is affected by extractives; appearance is determined by anatomical and chemical properties. Dimensional stability, strength and stiffness similarly depend on basic wood characteristics. These service-related characteristics therefore need to be considered with other wood quality attributes to reflect customers’ and manufacturers’ concerns.

Wood Quality Attributes and Their Impact on Wood Utilization

For solid wood products, the following characteristics are considered to be important wood quality attributes:

1. Stem diameter is a major wood quality attribute for the sawmilling industry. With increasing stem diameter, logging and manufacturing costs decrease, whereas lumber volume recovery and grade yield increase significantly.

2. Stem form or defects may considerably reduce lumber volume recovery and lumber quality.

3. Stem taper has the greatest influence on the value of a tree of a particular diameter. Lumber volume recovery decreases significantly with increasing stem taper (Steel 1984). Wane results mainly from stem taper. Large taper also results in grain deviation, hence reduced mechanical properties.

4. Tree age affects wood formation and basic wood characteristics. Juvenile wood content increases with tree age.

5. Bark is usually removed from logs. During the debarking process, logs are often damaged, which causes wood fibre loss. Bark contamination may reduce chip quality and value.

6. Stem decay makes it more costly to harvest and process trees. Decay also reduces product quality..

7. Knottiness reduces lumber mechanical properties, and it is one of the most common causes for visual lumber downgrade. Knots also affect machinability and may cause downgrade in appearance products.

8. Reaction wood possesses inferior wood properties. It may lead to processing and quality problems.

9. Growth stresses may cause defects in standing trees and felled logs, thus reduce product yield and quality.

10. Grain deviation can reduce lumber strength and cause warping.

11. Juvenile wood is usually associated with poor dimensional stability, and decreased strength and stiffness.

12. Heartwood/sapwood ratio affects major end uses differently. Heartwood is usually more durable, but it tends to be less permeable and more difficult to dry, treat and paint. In the panel industry, higher extractives content in heartwood is often associated with reduced adhesive efficiency.

13. Anatomical characteristics: Basic wood characteristics determine wood physical and mechanical properties. Wood stiffness and shrinkage are closely related to microfibrillar angle. Density and permeability are both related to wood anatomical features.

14. Chemical composition is directly related to wood physical and mechanical properties. It affects pulp yield and quality.

15. Ring characteristics are directly related to aesthetic properties (e.g., grain, figure, colour, surface). They also affect machinability.

16. Moisture content in logs affects many operations throughout the value recovery chain. Moisture content in logs and boards is also related to the formation of mould, decay and sapstain.

17. Wood density has long been considered the most important wood quality attribute. To a large extent, wood density determines the suitability of a species for a specific end use. High-density wood is usually associated with high lumber strength and stiffness. Some panel producers (e.g., OSB, particleboard) prefer low-density wood. High density wood is usually associated with high pulp yield.

18. Dimensional stability influences construction efficiency and structural serviceability. Warping may cause serious problems for both structural applications and appearance products.

19. Mechanical properties are highly important attributes for the building industry.

20. Durability (commonly used to mean decay resistance) is critical for exterior applications and wood construction.

21. Wood permeability is closely related to wood drying rate and treatability. It may also affect finishing properties.

22. Aesthetic characteristics are critical for appearance products.

23. Machinability includes a number of operations considered critical for secondary processing.

24. Fire resistance can be an important attribute in wood construction.

25. Finishing characteristics are of importance to secondary manufacturing and housing construction.

26. Engineering properties include fastening and gluing characteristics, which are valued by manufacturers of engineered wood products.

27. Other wood characteristics include acoustic, thermal, electrical and other properties that may be of importance to specific end uses.

Interrelations Among Wood Quality Attributes

Some wood quality attributes appeared to share a unique link. Stem diameter, stem shape, stem taper and tree age are related to stem and log characteristics; wood anatomical and chemical characteristics constitute basic wood characteristics; gross wood characteristics (e.g., juvenile/mature wood, sapwood/heartwood) refer to woods that are distinctly different in the basic wood characteristics; others are either wood physico-mechanical properties or service-related attributes that are virtually determined by the basic wood characteristics. On the other hand, most wood quality attributes are somehow interrelated, and thus any attempt to group them into categories appears somewhat arbitrary. In general, gross wood characteristics are related to stem and log characteristics (e.g., age, diameter). For example, juvenile wood and heartwood content usually increase with increasing age or diameter. Reaction wood is often associated with stem shapes or straightness. Since gross wood characteristics are related to stem characteristics, the basic wood characteristics of a stem depend, to some extent, on the stem characteristics. This implies that, for a given species, stems and logs of different ages or diameter classes have different wood characteristics at the gross, anatomical and chemical levels, which suggests that these stems or logs may have different physico-mechanical properties and service-related attributes.


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[1] Senior Scientist and Group Leader, Forintek Canada Corp. Sainte-Foy, Québec, Canada G1P 4R4.
Tel: (418)-659-2647; Fax: (418)-659-2922; Email: [email protected]