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Stem Characteristics and Wood Properties: Essential Considerations in Sustainable Multipurpose Forestry Regimes

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Stem Characteristics and Wood Properties: Essential Considerations in Sustainable Multipurpose Forestry Regimes Proceedings from the Wood Compatibility Initiative Workshop, number 15. STEM CHARACTERISTICS AND WOOD PROPERTIES: ESSENTIAL CONSIDERATIONS IN SUSTAINABLE MULTIPURPOSE FORESTRY REGIMES David D. Marshall1 and Dean S. DeBell2 ABSTRACT The management of forest resources in the Pacific Northwest has changed greatly since the first harvests of large- diameter old-growth trees to the more recent harvests of younger, small diameter trees grown in plantations managed for rapid growth and wood production. This change continues with an increased emphasis on managing for both wood production and a range of other objectives, including aesthetics, wildlife habitat and other nontimber values. Much of our accumulated silvicultural knowledge can help in developing regimes to meet these new goals, however, these alternative regimes may produce trees and wood with very different characteristics than in the past. This could have large impacts on value. To date, only minor attention is placed on the impacts of silvicultural practices on wood quality, especially for species other than Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco). We provide four examples using research on western hemlock (Tsuga heterophylla (Raf.) Sarg.), red alder (Alnus rubra Bong.), black cottonwood (Populus trichocarpa Torr. & Gray) (and hybrid poplar) and western redcedar (Thuja plicata Donn.) to demonstrate how silvicultural practices can alter stem characteristics and wood properties. KEY WORDS: Wood quality, Douglas-fir, (Pseudotsuga menziesii), western hemlock, (Tsuga heterophylla), red alder, (Alnus rubra), black cottonwood, (Populus trichocarpa), western redcedar, (Thuja plicata). INTRODUCTION derived values other than wood production via multi-pur­ pose forest management regimes. The vast forests that early settlers found in the Pacific Northwest provided the basis for a large forest products Although most of the technology of forest management industry. For the first century of its existence, raw materials was developed with the goal of enhanced wood production to supply this wood processing industry came primarily (Curtis and Carey 1996), the various silvicultural practices from the harvest of these existing older forests. Over the can be readily modified to meet a broad range of multiple past several decades, however, the forest industry in the objectives (Curtis et al. 1998). The values that society seeks region has been in transition. Beginning in the 1950s, a from such multi-purpose forestry may include commodity shift began from reliance on Douglas-fir from naturally or non-commodity uses that come directly or indirectly grown, primarily old-growth stands, to younger natural from trees in the form of wood products, wildlife habitat, forests, and then to managed Douglas-fir plantations. These water, other plants, scenery and recreation. changes are accompanied and influenced by research and development of plantation establishment practices, spacing Management planning and decisionmaking must guidelines, fertilization and improved genetic stock. More consider the value of forest outputs (whether commodity recently, numerous events in the region have prompted fur­ or non-commodity); such consideration requires realistic ther changes to occur. These changes include an increased assessment of quality and not just quantity of the outputs. importance of more species and a greater emphasis on forest Although wood outputs are probably the most easily valued, 1 Research Forester and 2 Retired team leader, Pacific Northwest Research Station, Olympia Forestry Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512-9193. 145 Table 1—Some potentially important stem characteristics EVALUATING WOOD QUALITY AND and wood properties for assessing wood quality POTENTIAL SILVICULTURAL IMPACTS Stem Characteristics Wood Properties Wood quality—as a term or concept—is not easy to define nor simple to measure or apply. There are many Diameter Ring characteristics possible products and quality measures that will vary with (e.g. width) product or end use. Therefore, no single measure is neces­ sarily appropriate for all uses. General definitions of wood Straightness Density—specific gravity quality are similar to the one given by Briggs and Smith (average and variability) (1986), “Wood quality is a measure of the aptness of wood for a specific use.” Central to this definition is the notion Grain Fiber dimensions of wood’s suitability for an end product. This suitability is (length and angle) dependent on various stem characteristics and wood prop­ erties that affect the yield of end products, cost of process­ Age Reaction wood ing raw material from forest to end products and the serviceability value of end products (product quality). Branch/knot Growth stresses (e.g. Some potentially important stem and wood property char­ (size and distribution) reaction/compression wood) acteristics are given in Table 1. Juvenile wood / mature Of the species that are found in the northwest, most of wood distribution the information about wood quality and potential impacts from silvicultural practices is for Douglas-fir. There are Chemical content several studies which used this information to predict future trends in value. Ernst and Fahey’s (1986) analysis Bark Moisture content suggested that the major impact of the change from old- growth to young-growth logs would be a decrease in log Decay Heartwood / sapwood size and a decrease in appearance grade lumber. They sug­ proportion gested pruning to increase the production of clear wood for appearance grades and thinning to increase average size and average recovery. They also pointed out that thinning past evaluations of management regimes and benefits were might also produce larger limbs and a greater proportion of quantified in terms of improved volume yields or decreased juvenile wood resulting in a decrease in quality. Juvenile time to achieve a desired tree size. Yet the change from wood is the column of wood that forms around the pith of harvesting of old-growth stands with trees of large size or a tree; it has inferior mechanical properties and dimensional younger managed stands to harvests in stands based on stability (Senft et al. 1985). The transition from juvenile to regimes designed to meet multiple objectives, not only mature wood is not well understood and probably refers to produces other valued benefits, but trees grown under these a zone, but this transition is thought to occur about 20 years new regimes can have different stem characteristics and from the pith in Douglas-fir (Megraw 1986). Maguire et al. wood properties which may result in different values. It is (1991) developed models to predict maximum branch size critical to accurately assess the value of wood produced and juvenile wood core for Douglas-fir. Projecting stands because the economic returns from harvesting wood often with different thinning strategies (both residual density and finance, directly or indirectly, the costs of forest ownership structure) they found that stand density management could and management for other forest derived values. Still, little impact branch size, whorl frequency and the amount of attention is paid to wood quality considerations when juvenile wood produced. Briggs and Fight (1992) used developing silvicultural regimes, particularly for species these methods and the TREEVAL model to predict the other than Douglas-fir. quality and value of products produced from two stands with widely differing initial spacing (370 and 1,876 trees per hectare). They found that if they ignored tree character­ istics other than tree size, the larger trees produced by the wider initial spacing had greater value (1.06 times greater 146 at age 70). However these wider spaced trees also produced two other examples that demonstrate how wood properties larger limbs, had a greater proportion of juvenile wood and impact wood quality for black cottonwood (and hybrid lower density wood. Considering the associated tree char­ poplar) and western redcedar. acteristics and wood properties of the trees produced by silvicultural regimes, the resulting value at age 70 of the Western hemlock has increased in importance in the denser stand was 1.31 times greater than the widely spaced region because of: (1) expanding emphasis on ecosystem stand. Barbour et al. (1997) also used these methods to management and alternative management regimes that analyze the potential impact on wood quality from ecosys­ favor more shade-tolerant species, (2) growing concern tem management on federal lands. They found that man­ over Swiss needle cast and its negative impacts on the agement strategies could be designed to accelerate the growth of Douglas-fir on many coastal sites were western development of important ecosystem structures while pro­ hemlock grows well, and (3) western hemlock’s machin­ ducing wood comparable to that grown in commercial ability and color that make it an excellent candidate for plantations. However, they also concluded that early heavy value-added uses. However, fluting (longitudinal depres­ thinnings would produce greater proportions of lower grade sions in the bole from inconsistent radial growth) is common lumber. Use of lighter and later thinning treatments could stem characteristic of western hemlock growing in Alaska reduce this impact. To date, little research has been done (Julin et al. 1993) and it can cause difficulties in debarking, for Douglas-fir on how fertilization and genetics impact sawing and peeling logs. In addition, out of roundness
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