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Although tree dimensions and external quality characteristics (such as branch size, sweep, and scarring) may have traditionally been sufficient to specify a log-sort, consideration is now being given to specifying such wood properties as density, stiffness, micro fibril angle, spiral grain, extractives content, and consumption of energy for processing. More frequently, these internal wood "attributes" are being taken into consideration as important influences on the estimation of timber value. Additional specifications required by wood buyers add extra complexity to the already complex task of log producing and sorting. It has been shown that, without any premium prices and incentives, such requirements for log grades can reduce the total value for the forest owner. Seven second-growth Douglas-fir stands of similar age class in Western Oregon were sampled, totaling 1,400 trees and more than 3,000 logs. Various measurements were taken and several parameters calculated, including acoustically estimated stiffness and mill veneer recovery, revenues, and costs. A general methodology for estimating relative breakeven prices of Douglas-fir peeler logs that a mill or any other log purchaser could afford to pay based on acoustic assessment of veneer stiffness differences is presented.
Green veneer was the largest source of revenue averaging about 80 percent as compared to that from chippable material and unpeeled cores combined. Smaller trees incurred higher manufacturing costs, up to a 40 percent difference between the largest and smallest delivered average-size log. The sample with the greatest net revenue ($1,145 per thousand board feet) was 3 percent higher than the next one and more than 16 percent higher than the lowest one. These results show that stand stiffness grading based on acoustic velocity measurements of Douglas-fir peeler logs could be used as a surrogate measure for potential net returns and hence a premium price could be requested on logs from such stands.
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Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) is one of the most important raw material resources for the forest products industries of the United States, Canada, New Zealand, and parts of Europe (Gartner et al. 2002). The unique attributes (appearance, strength, and machinability) of its wood have established and maintained the Pacific Northwest as a major factor in domestic and international markets for forest products. International and U.S. wood product markets, especially high-quality structural lumber and veneer markets, are likely to continue to demand Douglas-fir logs (Schuler and Craig 2003).
Over the last several decades, however, as demand for high-quality timber has been rapidly increasing, the availability of old-growth Douglas-fir and other softwoods has been diminishing across North America and timber resources have gradually shifted to intensively managed young growth stands (Adams et al. 2002, Zhang et al. 2004). Due to the higher proportion of juvenile wood, younger stands usually yield lower quality timber (Gartner 2005) with greater variability in product performance (Carter et al. 2006). As global forest products markets are becoming increasingly competitive and complex, the successful transformation of managed second-growth stands into quality products is crucial for the existence of a robust forest industry (Kellogg 1989, Barbour and Kellogg 1990, Eastin 2005). Good measurements and predictions of both the external and internal properties of the wood in each stem are essential for optimally matching logs to markets (Clarke et al. 2002). Assessing a forest stand's quality (Acuna and Murphy 2006), determining its most appropriate use, time of harvest, and the processing technique to be used, and consequently distributing the products to the right location are all important management decisions for achieving reduced costs and increased product values (Murphy et al. 2005).
Wood modulus of elasticity (MOE), also known as stiffness, is one of the most important mechanical properties and is the most frequently used indicator of the ability of wood to resist deflection and distribute loads in a structure. Despite its high variability which is dependent upon site, genetics, silviculture, and location within the tree and stand, MOE has long been recognized as a critical product variable in both solid wood and pulp and paper processing (Eastin 2005). It is a particularly important parameter in the conversion of raw timber material into veneer and plywood products requiring high stiffness wood. With the ever growing use of engineered wood products, including roof trusses and laminated veneer lumber (LVL), the demand for lumber and veneer with high MOE values has increased.
For many years, the sawmilling industry has utilized acoustic technology for lumber assessment. Stress wave nondestructive testing (NDT) methods are currently used for veneer grading programs, and strong correlations have been reported between stress wave velocity and wave attenuation and the corresponding mechanical properties of LVL (Brashaw et al. 2004). Commercially, longitudinal stress wave techniques have allowed LVL manufacturers to translate mechanical veneer characteristics, such as stiffness and strength, into LVL material with low variability and predictable strength properties (Kunesh 1978). NDT instruments that are compact and easy to operate and are based on acoustic principles have been developed for measuring stiffness of logs and standing trees (Dickson et al. 2004). Acoustic NDT methods have been successfully used for evaluation of mechanical properties of various wood products (structural lumber, poles, and pulp logs) and species as well as in tree selection and breeding based on stiffness (Huang et al. 2003). Past research has indicated a high correlation between yield of structural grades of lumber and acoustic velocity of standing trees (Wang et al. 2001, Lindstrom et al. 2002, Grabianowski et al. 2006, Lasserre et al. 2007, Wang et al. 2007a) and processed logs (Ross et al. 1997, Joe et al. 2004, Wang et al. 2007b, Waghorn et al. 2007, Amishev and Murphy 2008c). But, it is important to note that acoustically evaluated stiffness is influenced by other wood characteristics such as density and microfibril angle (Huang et al. 2003), knots and distorted grain (Briggs et al. 2008), moisture content (MC) (Amishev and Murphy 2008b), and temperature (Carter et al. 2005).
Worldwide forest harvesting has become increasingly mechanized over the last several decades. This is especially true in areas where the harvested tree size is decreasing and the capability of one or two machines to fell, delimb, buck, and sort a tree or a group of trees is an appealing advantage. Drivers for this shift from manual to mechanical harvesting systems generally include productivity/cost improvement goals or labor-related issues. Additionally, mechanization also provides a platform for innovative measurement systems which could lead to improved log segregation based on a wider range of wood properties (Murphy 2003). In recent years, mills and markets have begun to include additional characteristics to specify the logs they require with consideration now being given to such wood properties as stiffness, strength, density, spiral grain, extractives content, and consumption of energy for processing (Andrews 2002, So et al. 2002, Young 2002).
Readily available tree (e.g., diameter at breast height [DBH]) and stand growing (age, spatial location) conditions were found to have limited or no predictive capability regarding the quality of the resulting veneer (Amishev and Murphy 2008b, 2008c). New technologies have been evaluated and implemented for measuring internal wood properties. Segregation of logs, based on hand-held acoustic tools that measure stiffness, is already being used by some forest companies to improve the value of lumber recovery (Green and Ross 1997, Matheson et al. 2002). Internal wood properties of logs are likely to be more commonly measured and specified by markets in the near future. Amishev and Murphy (2008c) have demonstrated that acoustic technology could be a promising and valuable tool for in-forest assessment of veneer-grade Douglas-fir log stiffness early in the supply chain even on a whole-tree basis. Moreover, Amishev and Murphy (2008a) have determined and investigated some issues and opportunities associated with installing resonance-based acoustic technology on a processor/harvester head and evaluated suggested working procedures based on feasibility and productivity considerations. Two forest products companies in the Pacific Northwest have expressed to the authors of this paper their inclination to impose stiffness requirements if such technology was commercially available.
Although wood producers are already sorting logs according to both external and internal properties (Jappinen 2000, Matheson et al. 2002), there is scarce evidence of markets paying premium prices for logs with superior internal characteristics, such as high stiffness. The economic importance of different wood properties varies with the products recovered from a tree or log, the grading methods applied, and the price structure used (Aubry et al. 1998). Only a few comprehensive veneer recovery reports on Douglas-fir have been published in the literature (Lane et al. 1973, Fahey 1974, Fahey and Willits 1991, Fahey et al. 1991), and even fewer studies were found that link wood characteristics and their effect on product value (Green and Ross 1997, Willits et al. 1997). Such studies are crucial sources of information in estimating the effect of different wood characteristics on economic value. Although product standards, mill equipment, and the size and quality of the resource have changed since these comprehensive studies were performed, relationships reported in them are still useful in financial analyses.
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