Enhancing Forest Value Productivity through Fiber Quality

will be easier to identify fiber quality research gaps and their linkage to the issue of enhancing forest productivity. Defining Fiber Quality Quality is generally defined as “suitability for a use” (Briggs and Smith 1986). Further precision requires statement of a specific product and the key performance requirements of that product in service. Given these product performance requirements, the physical, mechanical, and chemical properties of wood that contribute to the requirements can be identified (Gartner 2005). These wood properties, which arise from the biology of tree growth, have a hierarchical structure ranging in scale from meters to nanometers (Figure 1; Moon 2008). At the “macroscale” are knots, growth rings, juvenile wood (JW), and mature wood (MW) [1] etc., and at the “microscale” are fibers and chemical structures. Given the product, performance requirements, and linkage to the governing properties at the appropriate scale, one can assess how silvicultural practices may alter wood properties and subsequent product performance. Table 1 consolidates information, adapted from Gartner’s (2005) approach, with respect to the performance requirements of a floor joist. For each performance requirement, Table 1 lists governing wood properties and indicates how intensive silviculture that reduces rotation age, with corresponding increases in JW and sapwood content, may change the wood properties with subsequent changes in joist performance. In practice, logs can be converted into a spectrum of “wood elements” (Figure 2) reflecting points along the hierarchical scale of Figure 1. Wood elements can be tested and sorted and combined with adhesives and other materials in the design and manufacture of engineered wood products (EWP) that meet specific performance requirements with low variability between items. Depending on the wood element chosen for designing an EWP, the point along the hierarchical structure and the scale both change. For example, if a log is converted into fiber for paper or fiberboard, macroscale organizational patterns such as growth rings, sap/ heart and JW/MW zones, knots, and so on, are largely destroyed and comingled so they contribute relatively little to product variability, which then is mainly governed by the microscale aspects such as specific fiber types, fiber geometric structure, and chemistry. If a log is sawn into a lumber, many aspects of the macroscale remain and contribute to variability between the pieces. Wood properties are a result of the structure interactions within and between these scales. Consequently, to fully understand the response of a bulk wood product to a load or environment situation, the properties and response characteristics must be understood across all scales (Moon et al. 2006). Measuring Fiber Quality Forestry has well-developed sampling procedures, widely used technology for measuring trees, and software models for estimating stem volume and projecting growth, size, and yield. Development of counterparts for fiber quality has lagged far behind. Consequently, there has been little progress in developing fiber quality models. With the shift to intensive planning of growth and yield and marketing, a more diverse mix of products from forests, the need for technology to measure and monitor fiber quality, is becoming more important. Inability to measure, monitor, and predict quality has costly ramifications. First, it is very expensive for a manufacturer to purchase and process timber with low yields of products with the performance requirements expected by customers. Value is also lost when timber with potential to yield high value products is misallocated to a lower use. Second, wood variability increases the risk of product failure in service with subsequent replacement and possible litigation costs. Conservative design can reduce risk of failure but is inefficient and costly in terms of wood use. Alternatively, frustrated users may decide to choose a nonwood product instead. Third, inability to routinely monitor fiber quality characteristics and integrate it into forest planning may lead to choices of cultural practices that fail to maintain or improve quality or meet future needs. Fiber Quality Assessment Visual Fiber Quality Assessment Visual grading relies on the human eye or imaging observation of external quality characteristics on the surfaces and ends of trees, logs, and products. For trees, the number, diameter and pattern of branches or branch indicators, evidence of decay, to name a few, observed on the bark surface are common quality indicators. These observaFigure 1. Hierarchical structure and scales of wood. (Moon 2008) Journal of Forestry • June 2010 175 tions may be combined with tree diameter and age to estimate growth rate. Similar clues are used on logs but bark may be removed and exposure of log ends allows growth rate and latewood, sapwood, and JW/MW content to be taken into account. For products such as lumber and veneer, the specific placement of knots and other defects exposed by processing can also be considered. Visual grading presents a number of issues. First, to keep visual approaches manageable for the human eye and brain, only a small number of broad grades occur for trees, logs, and products. Second, although visual grades partition the overall variability into more homogeneous groups, much variability remains within a grade. Third, because of the difficulty and expense of training and certifying graders, grade definitions tend to be inflexible with respect to changing customer needs. Fourth, there are issues with misclassification due to human error and the poor correlation between visually assessed external characteristics and actual internal properties. Consequently, visual methods do not provide precise, accurate estimates of properties. For example, the peeler-sawlog log grading system used in the Pacific Northwest for Douglas-fir places 85% of logs in two grades: grade 2 and grade 3 sawlog (H & W Saunders, Ltd., 2001). These two log grades respectively allow knot diameters up to 2.5 or 3.0 in., have minimum small end diameter limits of 12 and 6 in., and have no growth rate restriction. Logs of vastly different quality occur in each of these grades and studies commonly show no relationship between recovered product quality and these grades (e.g., Fahey et al. 1991) and in one case the relationship was inverted (Sonne et al. 2004). Similar issues occur for visual grading of lumber and veneer (MacPeak et al. 1990, Biblis et al. 1993). Madsen (1992) noted for visual grading of dimension lumber that “we do have a lot of strong material in the sawmill production but that our present grading system does not enable us to economically identify the strong pieces.” A number of factors are challenging traditional visual grading rules as a reasonable method for assessing quality and matching the resource to product performance requirements. Domination by Construction. Construction of buildings and their repair and remodel activities constitute the primary use of wood in the United States. Much of the construction wood is lumber that must be long, deep, and straight as well as stiff and strong. As the timber resource shifted from large old-growth to second growth and then to intensively managed plantations (IMP), the “small-diameter tree” and the “short-rotation tree” problems emerged. Small Diameter. The small-diameter tree arises from various situations. First, Table 1. Floor joist—Performance requirements, influencing biological and chemical wood properties, and effects of shorter rotation. Performance property Influencing biochemical wood properties Effect of shorter rotation On juvenile wood On sapwood Net effect on performance Stiffness and strength Knots Increase ? Lower stiffness and strength Density Decrease ? Microfibril angle Increase ? Slope of grain Increase ? Dimensional stability (warp) Slope of grain Increase ? Poorer stability and more warp Microfibril angle Increase ? Extractives content ? Decrease Treatability Sapwood percent ? Increase Easier to treat a Juvenile wood refers to the growth rings at the center of a cross-section. b Sapwood refers to the growth rings just under the bark that are actively conducting water and nutrients from the soil to the crown. c Depends on the pattern of growth rate ?, Effect uncertain. Figure 2. Wood element progression of scales from large-scale solid wood to chemical components. 176 Journal of Forestry • June 2010 there was the shift from the legacy of largediameter old-growth to relatively unmanaged “second growth that was harvested when trees were younger and smaller. Second, second growth has been replaced by IMPs that commonly produce small, but harvestable, trees in less time; these trees also have the short-rotation problem discussed later in this article. Third, a large acreage of old, small-diameter timber occurs in the interior west. Small-diameter trees produce small logs from which it is difficult to obtain products with the required length and cross-