Structural lumber from suppressed-growth ponderosa pine from northern Arizona.

Abstract

Lumber was sawn from 150 suppressed-growth ponderosa pine trees, 6 to 16 inches in diameter, harvested near Flagstaff, Arizona. This paper presents grade recover and properties for dry 2 by 4's sawn from the logs and graded by a variety of structural grading systems. Flexural properties met or exceeded those listed in the National Design Specification. When graded as Light Framing 43 percent of the 2 by 4's made Standard and Better and as Structural Light Framing, 34 percent made No. 2 and better. Warp was the biggest factor limiting grade yield. About 7 percent of the lumber would make a machine stress-rated (MSR) lumber grade of 14501, but with no established market such production is not recommended. If graded as laminating stock, about 8 percent of the lumber qualified as L3 or better. A comparison of the results from this study with those from a companion study indicates that appearance grades offer the highest value alternative for lumber produced from this resource.

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Ponderosa pine (Pinus ponderosa, L.) is one of the most important softwood species in western North America. It is found in commercial quantities in every state west of the Great Plains. Wood from mature trees is relatively lightweight, nondurable, nearly white, and has straight grain and a medium texture. It is easy to work with hand tools, glues well, and is average in paint- and fastener-holding abilities. It is a principal millwork species, being used for window framing, sashes, doors, moulding, shelving, and paneling. In round-wood form, it is used for posts, poles, and house logs (Lowery 1984). As structural lumber, it is sold as part of the Western Woods species grouping (NDS 2005).

Natural regeneration of ponderosa pine is sporadic over much of its range, with successful germination thought to be the result of the chance combination of a heavy seed crop and favorable weather during the following growing season (Bums and Honkala 1990). Historically, the ponderosa pine forests were primarily open stands of mature trees interspersed with pockets of younger trees and grassland. Prior to the early 1900s, frequent low-intensity fires killed off competing vegetation, including ponderosa pine seedlings, and help maintained open stands of large, fire-resistant trees (Fiedler et al. 1997). Fire suppression, along with livestock grazing and timber harvest, has promoted the conversion of the historical forest to dense stands containing a preponderance of small-diameter trees. Under these conditions, tree growth is often suppressed. Because ponderosa pine is intolerant of shade, rapid growth of seedlings prior to crown closure will produce a relatively large core of juvenile wood, generally defined as the first 20-years of growth (Shuler et al. 1989, Voorhies and Groman 1982). Juvenile wood tends to have higher than normal longitudinal shrinkage and may warp excessively. This has been identified as a primary problem in the utilization of small-diameter ponderosa pine (Fahey et al. 1986).

Management goals for ponderosa pine forests include reducing stand density to increase resistance to insect and disease attack, reduce the risk of catastrophic wildfires, provide diverse mosaics of wildlife habitat, and provide economic benefit to local communities (Willits et al. 1997). Increasing product utilization, along with reducing harvesting and processing costs, is critical for restoring ecological processes in ponderosa pine forests (Fiedler et al. 1997, Larson and Mirth 1998, Rummer et al. 2003). Several studies have shown that old growth trees (generally defined as more than 150 years old) produce a larger proportion of high value "shop and select" grades of lumber than do younger ("blackjack") trees (Ernst and Pong 1985, Fahey and Sachet 1993, Fahey et al. 1986). Fahey and Sachet (1993) concluded that lumber from second growth ponderosa pine logs is primarily in the Dimension grades. These older studies, however, were not specific to suppressed stands and often were limited in the lumber grading options investigated, especially for engineered product applications. Recent studies have shown that small-diameter trees growing in dense stands may have higher annual ring density and smaller knots than more open-grown trees and can be used as an input raw material for higher value products ranging from visually and mechanically graded lumber to veneer for structural composite lumber (Willits et al. 1997, Erickson et al. 2000, Green et al. 2005).

In a previous paper we compared volume and value recovery from 6- to 16-inch (152- to 406-mm) diameter breast height (DBH) suppressed-growth ponderosa pine harvested near Flagstaff, Arizona (Lowell and Green 2001), which was manufactured into structural and nonstructural lumber. In that paper, the structural dimension lumber was limited to the Structural Light Framing grading system (WWPA 1998). The objective of this paper is to further investigate the yield of structural lumber graded under a wider range of structural grading systems.

Procedures

Log selection and processing

Trees were selected from the Fort Valley Research and Demonstration Forest, Flagstaff, Arizona, and harvested in summer 2006. The demonstration project contained three experimental blocks with four treatment plots each. The experimental blocks represented different initial stand conditions: blackjack (young growth), yellow pine (old growth), and a mixture of the two age groups. The treatments within blocks were different thinning prescriptions designed to return stands to presettlement conditions. This involved thinning from below in which the larger, older trees were retained. Trees to be left had been marked, but no thinning treatment had been applied prior to sample selection for this study. Sample trees came from three of the four treatment plots in the mixed age block.

A sample of 150 trees ranging in DBH from 6 to 16 inches (152 to 406 mm) was selected. A matrix of five 2-inch (51-mm) diameter classes was used, and the trees selected represented those that would have been removed under the silvicultural prescription. The trees had an average age of 88 years. The sample was randomly divided into two subsamples, one to be sawn for dimension lumber and the other for appearance-grade lumber. Only the 2 by 4's from the dimension lumber sample are discussed in this report. The logs were sawn and the lumber kiln-dried by the Fremont Lumber Company, Lakeview, Oregon, owned by the Collins Companies. A more detailed discussion on selection and processing procedures and the overall results for both subsamples are presented in Lowell and Green (2001). Simpson and Green (2001) give additional information on kiln-drying procedures in the Fremont Lumber Company sample.

Grading and testing

The dry and surfaced 2 by 4's were shipped from the sawmill to the University of Idaho where they were graded as structural products by a lumber inspector of the Western Wood Products Association (WWPA). Each 2 by 4 was graded under several structural grading systems including Structural Light Framing, Light Framing, and the visual requirements for machine stress-rated (MSR) lumber and laminating grades (AITC 1993, WWPA 1998). If the grade of the lumber could be increased by trimming 2 to 4 feet (0.6 to 1.2 m) from the end, the trimmed length and trimmed grade were recorded. The lumber was conditioned for several months at approximately 70 [degrees]F (21 [degrees]C) and 55 percent relative humidity.

Modulus of elasticity (MOE) was determined by transverse vibration (Etv) using a E-Computer (Metriguard, Inc., Pullman, Washington) with specimens supported at the ends and vibrated in the flatwise orientation. Specimens were then tested to failure on edge in static bending using third-point loading and a span-to-depth ratio of 21:1 following the procedures of ASTM D 198 (ASTM 2005). The rate of loading was approximately 2 inches (51 mm) per minute. In accordance with ASTM Standards D 2395 and D 4442-92, ovendry moisture content (MC) and specific gravity (SG) based on ovendry weight and ovendry volume were determined from sections taken near the failure region after testing (ASTM 2005).

MSR simulation

Simulations of MSR grades were conducted for a range of potential grades having static edgewise MOE values ranging from 1.0 to 2.4 x [10.sup.6] psi (6.9 to 16.5 GPa). Individual pieces in the simulation of MSR grades had to meet four criteria to qualify for a specified grade: (1) fifth percentile (minimum) MOE, (2) fifth percentile (minimum) modulus of rupture (MOR), (3) grade average MOE, and (4) visual grading requirements for edge knots. Traditionally, for mechanically graded lumber, the fifth percentile non-parametric point estimate must equal 82 percent of the target average MOE value (i.e., 0.82 x average grade MOE). This limits the variability of the lower half of the MOE distribution of the grade to a coefficient of variation (COV) of 11 percent. Thus, the minimum MOE for a 1.3E grade would be 0.82 x 1.3 1.07 x [10.sup.6] psi (7.4 GPa). The minimum MOR value would be 2.1 times the allowable bending strength (Fb) for the specified grade. In addition to the MOE and MOR requirements, knot size is limited by grade category. More information on MSR lumber may be found in Galligan and McDonald (2000) and the Summer 1997 issue of Wood Design Focus (FPS 1997). A more comprehensive discussion of our MSR simulation procedures, along with several grading alternatives, is given in Green et al. (2005).

Laminating grades

Traditional ponderosa pine lumber used in glulam is referenced in glulam standards as part of a species grouping called Western Species. As part of this grouping, only visual grading of the lumber was conducted. The rules for visual rating are based entirely on the characteristics that are readily apparent to the human eye, such as knot size, slope of grain, and wane. The following tabulation is an example of the knot size limitations for visual glulam grades: