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Evaluating the bending properties of clear wood specimens produced from small-diameter ponderosa pine trees.

Abstract

The bending properties of clearwood test specimens produced from small-diameter pine trees were evaluated to provide a basis for determining how to better utilize this resource. Specimens (16 in by 1 in by 1 in) were processed from 57 small-diameter trees harvested from the Manitou Experimental Forest located near Woodland Park, Colorado. Trees were categorized into three groups, 20 "normal" disease-free trees harvested from dense stands typical of the region, 21 mistletoe-infested trees from similar stocking conditions, and 16 disease-free trees from more open growing conditions. Test specimens were tested in bending and data for 542 specimens were evaluated statistically using a nested design, considering both within- and between-tree variation. This analysis revealed that property variation was much greater transversely across the cross section than longitudinally along the length of the tree. Modulus of elasticity (MOE) and modulus of rupture (MOR) values were highest for test specimens from normal trees. Specimens produced from open-grown trees had significantly lower MOE and MOR values than specimens from both normal and mistletoe-infested trees. While mean MOE and MOR values for specimens from mistletoe-infested trees were lower in comparison to normal wood values, only the difference between mean MOE values was statistically significant. No significant differences existed in the mean specific gravity between the groups. There was not a linear relationship between specific gravity and either MOE or MOR; however, there was a weak linear relationship between sample growth increments per inch and both MOE and MOR, which may provide an opportunity for visually evaluating (grading) small-diameter trees.

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The Colorado State Forest Service has estimated that 2.4 million acres of forestland along the Colorado Front Range are at risk for catastrophic wildfire (CSFS 2001). In recent years this area has experienced numerous large high-intensity wildfires, including the Hayman Fire, which burned 137,759 acres of forestland from June 8 to July 18, 2002. In response to this threat, there is a need to treat forestland on a landscape scale and large volumes of small-diameter ponderosa pine trees could potentially be removed to reduce fire risks and improve ecological conditions.

Much of the wood biomass from current treatment efforts is left in the woods stacked in piles, lopped and scattered, or chipped and blown back onto the forest floor. This is primarily due to the high cost of removal relative to the value of this material. Products that this material is suited for tend to have relatively low value such as landscape mulch and fuel. To help offset the removal costs and better utilize the resource, there is interest in using small-diameter material for higher value products, including structural applications. To achieve this goal, a better understanding of this material must be developed.

Ponderosa pine is considered a yellow pine. The heartwood is yellowish to light reddish brown or orange and the sapwood is white to pale yellow. The sapwood is typically up to 2 inches or more wide in older trees. In young open-grown trees, sapwood can comprise more than half the volume. According to the Wood Handbook (USDA 1999), ponderosa pine wood is moderately light in weight and has an average green specific gravity (SG) of 0.38. It is moderately stiff in bending with an average green modulus of elasticity (MOE) of 1.0 X [10.sup.6] psi and moderately low in bending strength with an average green modulus of rupture (MOR) of 5,100 psi. In addition, ponderosa pine is moderately soft and moderately low in shock resistance. Uses for this material include lumber, piles, poles, posts, mine timbers, veneer, and railroad cross ties, while clearwood is suitable for millwork such as window frames, doors, shelving, mouldings, sash doors, paneling, and trim (Alden 1997). Sawmill and secondary mill residues are used to produce particleboard and paper, as well as for animal bedding and landscape mulch. However, high percentages (up to 90% or more) of trees removed to improve ecological conditions are less than 12 inches (Lynch and Mackes 2003) and most are less than sawlog size. These trees tend to have many limbs and as a result, wood produced from these trees has high knot frequencies. These trees tend to have relatively high percentages of juvenile wood and compression wood. In addition, many ponderosa pine trees along the Colorado Front Range are infested with dwarf mistletoe (Maffei 1989).

Juvenile wood is the secondary xylem formed early in the life of the vascular cambium. Juvenile wood has been described as the area of rapid changes in wood properties near the pith, before the formation of wood with more uniform properties (Zobel and Sprague 1998). Generally, physical and mechanical properties of juvenile wood are inferior to those of mature wood formed later in the life of the tree. In a study of plantation-grown loblolly pine, MOE showed a 5-fold increase between juvenile and mature wood, with one tree studied showing a 10-fold increase (Bendtsen and Senft 1986). Small clearwood samples of lodgepole pine that were taken 3.5 inches from the pith showed significantly higher MOE and MOR than samples taken just 2 inches from the pith due to the occurrence of juvenile wood (Pellerin et al. 1989). The same tests conducted on the juvenile wood (first 10 rings) and mature wood of loblolly pine yielded similar results. In general, the MOR of the juvenile wood was just 48 percent of the mature wood. It was concluded that the results were due to the lower SG of the juvenile wood (Pearson and Gilmore 1971).

Although consensus on the precise cause of juvenile wood has not been established, theories include proximity to apical meristems, presence of photosynthetic bark, mechanical support factors, and environmental factors. The duration of time over which juvenile wood is formed varies between species. Ponderosa pine may have over 20 rings of juvenile wood. The actual age of demarcation between juvenile wood and mature wood varies based on the feature being measured. For instance, in a study of plantation-grown loblolly pine, 13 years was considered the age of maturity for SG and mechanical properties, while 18 years was considered the age of maturity based on cell length (Bendtsen and Senft 1986).

When the vascular cambium is producing juvenile wood, it is susceptible to environmental forces that result in the formation of compression wood. Compression wood is a specialized form of secondary xylem occurring in softwoods, typically formed on the lower side of branches and leaning stems, and is thought to form in an effort to support the limbs and straighten the tree. On a macroscopic scale, compression wood can be identified from the eccentric growth rings that form around the pith of the stem (compression side of leaning tree). It tends to have a more reddish color and a more gradual transition from earlywood to latewood in species where the transition is typically more abrupt.

Bowyer et al. (2003) states that compression wood is typically higher in SG than normal wood, and exhibits longitudinal shrinkage 10 times greater than normal wood, and radial and tangential shrinkage is about half of normal, as a result of higher microfibril angles. Mechanical properties of compression wood also vary from normal wood. In general, when compression wood is in the green state, bending, compression, and toughness values are higher than is found in normal wood. However, as the material dries, the expected increases in mechanical properties are not as dramatic as is found in normal wood. If mechanical properties are considered on the basis of SG, compression wood is inferior. Generally, as the percentage of compression wood increases, MOE decreases. In bending tests of Sitka spruce containing compression wood, it was shown that as little as 10 percent of compression wood in the cross section of the sample resulted in greater deflection (Dhubhain et al. 1988). Although a similar relationship with MOR was not observed, it was noted that 70 percent of the samples containing compression wood failed in a brash manner.

Juvenile and compression wood share similar properties, including presence of short cells, high microfibril angles, and higher lignin contents. There is no way to separate the properties of juvenile wood from reaction wood, where they occur together near the pith (Zobel and Sprague 1998). A study of plantation-grown loblolly pine found approximately 35 percent compression wood fibers in early years, decreasing with age (Bendtsen and Senft 1986).

Dwarf mistletoes are parasitic flowering plants that depend on tree hosts for water and nutrient needs. Because of the damage they inflict upon the living tree, including branch distortion, growth reduction, and decreased longevity, they are classified as pathogens (Conklin 2000). A subspecies of dwarf mistletoe, Arceuthobium vaginatum subsp. Cryptopodium is considered to be one of the most serious pathogens of ponderosa pine in Colorado (Maffei 1989). Dwarf mistletoe has been linked to changes in wood quality, including pitchy, distorted grain, and lowered mechanical properties. Work by Piirto et al. (1974) showed that, compared to uninfected regions, lodgepole pine wood from infected regions exhibited changes in properties, including lower percentage of latewood, narrower growth rings, higher alcohol benzene extractives, decrease in tracheal length, and increase in microfibril angles (leading to increased longitudinal shrinkage).

Understanding the effects that high percentages of abnormal wood, pathogens, and forest growth conditions have on wood behavior is critical for improved utilization, especially for use in structural applications. The purpose of this research was to evaluate the bending behavior of small clearwood specimens from small-diameter ponderosa pine trees to provide a basis for improved utilization of this material. It was a cooperative effort of the Rocky Mountain Research Station, the Colorado State Forest Service, and the Department of Forest, Rangeland and Watershed Stewardship at Colorado State University.

Methodology

Collection of mechanical test data conformed to provisions of ASTM Standard Test Designation: D 143-94 (ASTM 1997). Small-diameter ponderosa pine trees were randomly selected for evaluation from a section of the Manitou Experimental Forest located near Woodland Park, Colorado. Selected trees had a diameter at breast height (DBH) of less than 12 inches and were classified into one of three groups:

1. Normal (N)

2. Open grown (O)

3. Mistletoe (M)

Normal trees included those growing in dense stands typically found in the region. These trees grow relatively slowly, with stems usually having more than 10 growth increments per inch on average. Open-grown trees were more solitary and were normally younger trees growing relatively fast, often having stems with less than 10 growth increments per inch. Mistletoe-infested trees were found on several sites in the test area and exhibited growth characteristics more similar to normal trees.

Core samples were taken from selected trees prior to harvesting to insure that a sufficient number of trees would be harvested in each group. In this study, trees that had more than 10 growth increments per inch on average were considered to have normal growth characteristics and trees that had less than 10 were considered to be open grown. Although core samples were taken of mistletoe-infested trees, they were all found to have more than 10 growth increments per inch and were classified as one group: mistletoe infested.

Even though the standard only required a minimum sample of five trees, larger sample sizes were taken because of small tree size and the high amount of property variation anticipated. A total of 57 trees were harvested, of which 20 were classified as normal, 16 as open grown, and 21 as mistletoe infested.

Trees were felled and skidded to a landing where they were tagged for identification. Tree-length logs were transported to Casey Lumber near Woodland Park. At the mill, trees were segmented into 6-foot sections beginning at the stump of the tree to a minimum diameter of approximately 5 inches on the small end. The remainder of the treetop was discarded. Logs were sawn with a circular saw, rough-sawing the logs on two sides. The tree number and bolt number (vertical position in the tree) were marked on the 1-inch-thick centerboard that included the pith. The boards were then covered with plastic and transported to Colorado State University where they were processed into 1-inch-wide by 1-inch-thick specimens using a table saw. The breakdown of 1-inch members is shown in Figure 1. Specimens (1 in by 1 in) were cut to a 16-inch length using a radial arm saw. Specimens were cut to length in such a way as to maximize clear material in the middle of the member. Specimens were clearly marked for identification and then submerged in a tank of water to maintain them in the green condition.

Prior to testing, cross sectional dimensions were measured for each specimen using hand-held calipers. Green specimens were then tested to failure in accordance with ASTM Standard D 143-94 using an ATS 900 test machine with a 1,000-pound load cell. A center load was applied to specimens and the test span was 14 inches. The rate of loading was 0.05 in/min. Load-deflection curves were recorded using a data-acquisition computer.

From load-deflection curves, MOE and MOR were calculated for each specimen using the following equations:

MOE = [[P.sub.pt]L]/[4[[DELTA].sub.pl]b[h.sup.3]]

[FIGURE 1 OMITTED]

where [P.sub.pt] = load at proportional limit (lb); L = test span (14 in); [[DELTA].sub.pl] = deflection at proportional limit (in); b = specimen width (in); h = specimen height (in).

MOR = [3[P.sub.u]L]/[2b[h.sup.2]]

where [P.sub.u] = ultimate load at failure; L = test span (14 in); b = specimen width (in); h = specimen height (in)

Average SG and moisture content were determined for each test specimen as specified in the standard. The number of annual rings in each specimen was counted and an average number per inch calculated. The experimental design selected as a framework for testing and statistically analyzing specimens was a nested design with individual trees as the primary sampling unit. As noted previously, trees were selected at random and processed to obtain specimens at predetermined locations along the length and through the cross section of the trees. Therefore, a nested design was appropriate even though wood specimens are normally treated independently, because specimens from the same tree were related (at least genetically) and likely had some similarities in properties due to the relationship. The mixed procedure in SAS (SAS Institute 1999) was used to compute the nested analysis of variance. Comparisons of positions within trees for individual groups were based on Dunnett's T3 procedure, which assumes heterogeneous variance among positions (Dunnett 1980).

Results and discussion

Data from 542 clearwood specimens were evaluated statistically. There were 234 specimens processed from the 20 normal trees, 121 specimens processed from 16 open-grown trees, and 187 specimens from 21 mistletoe-infested trees. Generally, fewer specimens were obtained from open-grown trees because they had considerably more taper and as a result, were often shorter in length than slower growing trees. Both within- and between-tree variations were considered.

Within-tree variation

Within-tree variation was considered both longitudinally along the length of the tree and transversely through the cross section relative to the pith. Generally, the variation was considerably greater transversely through the cross section than along the length of the tree. Based on nested design covariance parameter estimates, SG variation was 2.7 times greater through the cross section than along the length of the tree, MOE variation was 4.7 times greater, and MOR variation was 3.5 times greater.

Mean property data are summarized in Table 1 for normal, open-grown, and mistletoe-infested wood specimens taken along the length of trees. Although data for normal growth specimens indicated a slight decrease in SG with increasing height, the change was not statistically significant in this study. No clear trends were present for open-grown and mistletoe trees.

Decreased SG with increasing height longitudinally along the length of the tree was expected because it was generally thought that SG decreased with increasing height. SG was thought to be lower in tops due to higher proportion of juvenile wood. Zobel and Sprague (1998) found that the SG of juvenile wood in the butt logs of older pine was higher in SG than that for juvenile wood in the tops.

Data summarized in Table 1 reveal that the butt section typically had lower mean MOE and MOR compared to the second section higher up the tree. Although mean MOE differences were not statically significant for normal growth trees, they were for open-grown and mistletoe-infested trees. Only mean MOR values for open-grown trees were significantly different. Nonetheless, mean MOE and MOR had a tendency to decrease from the second section to the top of the tree. This pattern was observed in many of the trees evaluated and is contrary to findings from other studies. Bending tests conducted with lumber produced from plantation-grown loblolly pine lumber revealed that the MOE and MOR of 2 by 4's and 2 by 6's cut from butt logs were higher than for samples cut from higher in the tree (Biblis et al. 1995). A similar comparison of lumber cut from fast-grown and slow-grown slash pine showed that both the MOE and MOR were higher in samples cut from butt logs than samples cut from higher in the tree (MacPeak 1990).

One explanation for this phenomenon could be stresses that the tree must endure over time. Consider the tree a structure standing in the forest. Over time, the tree is subjected to repetitive loads such as wind and snow loads. These are similar to loads that houses and other manmade structures are exposed to over time. As an example, when exposed to wind loads, the tree behaves as a cantilever beam and the bending moment is most severe at the butt of the tree. Fortunately, trees are well suited for withstanding these loads because not only do trees normally have the greatest diameter at the butt, but the highest quality wood material tends to be found in the lower main stem of the tree as well. However, in areas such as the Colorado Front Range, where the wind frequently blows and wind loads are considerable, damage can occur to the tree. In extreme situations, loads can exceed the resistance of the tree, in which case it blows down.

The concept of cumulative damage has been applied to manmade structures (Gerhards 1979). This concept is based on the reality that a structure will be subjected to stresses that constantly change over its service life. Over time, resistance decreases according to the magnitude and duration of the stress. Similarly, a tree standing in a forest is subjected to changing levels of stress. Although the tree has the ability to grow new wood and heal over time, damage can still accumulate. Damage will be greatest where the stress is the highest. For wind loads, this will usually occur where the tree stem contacts the ground. When a tree sways in the wind, damage can occur that reduces the mechanical properties (MOE and MOR) of the wood under stress. When enough damage accumulates over the life of the tree, it will fail.

This phenomenon may also explain the variation in MOE and MOR observed transversely through the cross section. Mean property data are presented in Table 2 for the lowest (butt) section of the normal, open-grown, and mistletoe-infested trees evaluated. The initial decrease in SG from the pith outward 1 inch followed by a significant increase was expected. However, the variation patterns for both MOE and MOR were not typical of those reported in the literature. Data presented in Table 2 reveal that initially there was a significant increase in both MOE and MOR for specimens that were taken 1 inch on center from the pith, followed by a significant drop for normal growth specimens taken 2 inches away. Specimens taken 3 inches from the pith had even higher mean SG, although not statistically different due most likely to small sample size. This is contrary to findings from a study of plantation-grown loblolly pine where Bendtsen and Senft (1986) concluded that the combination of increases in SG and cell length could explain 40 to 85 percent of the improvements in mechanical properties as wood matures, depending on the mechanical property being examined. Therefore, MOE and MOR should increase with increased distance from the pith. However, subjected to wind loads, severe bending stresses can build through the cross section of the tree. These are normally greatest toward the outer perimeter of the tree. As damage accumulates over time, the decrease in properties would be greatest where stresses are the highest. This phenomenon could partially explain the patterns of variation observed in Table 2.

Variation between trees and groups

Mean SG, MOE, and MOR are summarized for each individual tree and group in Table 3. Box plots of SG, MOE, and MOR data for each group are presented in Figures 2, 3 and 4. These box plots reveal that variation between trees within the three groups is fairly comparable. Based on nested analysis least square means testing the statistical significance of differences between the groups, differences among means for SG, MOE, and MOR are summarized in Table 4.

The mean SG of the three groups ranged between 0.398 for normal trees to 0.408 for the mistletoe group. The open-grown group was in between with a mean value of 0.403. These means were all very comparable to the SG value of 0.40 listed for green ponderosa pine in the Wood Handbook (USDA 1999). There were no significant differences between mean SGs of the three groups. This was somewhat unexpected because SG is generally thought to be lower for open-grown conifers than comparable forest grown trees due primarily to the expanded juvenile core found in open-grown trees (Bowyer et al. 2003). Although the exact reason for lack of significance is not known, it could be due to relatively high percentages of compression wood being present in the open-grown trees.

Statistically, the normal trees had the highest mechanical properties, with a mean MOE of 0.817 X [10.sup.6] psi and MOR of 4,897.0 psi. Open-grown trees had the lowest mean values with a mean MOE of 0.458 X [10.sup.6] psi and MOR of 3,397.9 psi. Mistletoe-infested trees fell in between with a mean MOE of 0.716 X [10.sup.6] psi and MOR of 4,513.5 psi. However, even normal tree wood values were lower than values for MOE (1 X [10.sup.6] psi) and MOR (5,100) given in the Wood Handbook (USDA 1999). For open-grown trees, the mean MOE was just 45.8 percent and MOR 66.6 percent of Wood Handbook values.

Differences between slower growing trees (both normal and mistletoe infested) and open-grown trees were most pronounced. This is due to the relatively high percentage of juvenile wood (and possible compression wood) found in open-grown trees. These results were expected, based on other studies in the literature. Tension tests conducted on juvenile samples cut from the first eight rings, and mature wood of loblolly pine indicated that strength and stiffness decrease with increases in juvenile wood. The ultimate tensile strength (UTS) of the 100 percent juvenile wood samples yielded values just 45 to 59 percent of the mature wood samples. Likewise, the MOE of the juvenile wood samples yielded values of just 51 to 63 percent of the mature wood samples (Kretschmann and Bendtsen 1992). A comparison between 20-year-old fast-grown and 50-year-old slow-grown slash pine of similar DBH yielded similar relationships between mechanical properties. The MOE and MOR of the fast-grown material were all significantly lower than the slow-grown material (MacPeak 1990).

Differences between normal and mistletoe-infested trees were less pronounced, although statistically significant for MOE; differences for MOR were not significantly different. Lower MOE and MOR values were expected based on studies in the literature. Piirto et al. (1974) observed that mechanical properties of mistletoe-infested trees were adversely affected, with MOE. MOR, and work to proportional limit all lower in wood from infested trees (both infested and uninfested sections of the tree), than in control samples.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Relationships between SG and MOE or MOR

Plots of SG vs. MOE or MOR were generated. Regression lines fitted to data with specimens considered as independent observations revealed that adjusted [r.sup.2] values were very low, 0.003 for the line fitted to the relationship between SG and MOE and 0.055 for the line fitted to SG and MOR data. A similar analysis considering only data from normal specimens yielded more significant linear relationships between SG and both MOE and MOR. However, adjusted [r.sup.2] values for these analyses were still low (0.065 and 0.159, respectively).

Although the existence of a strong statistical relationship between SG and both MOE and MOR is well documented for normal wood (USDA 1999), the lack of significance for wood from small-diameter trees tested in this research was not unexpected. Similar results were encountered in a study involving the mechanical testing of white fir studs conducted by Gorman et al. (1999). Nonetheless, data collected as part of this research did not provide evidence of a strong relationship between SG and either MOE or MOR, suggesting that due to relatively high percentages of abnormal wood (both juvenile and compression wood), generally accepted relationships between SG and mechanical properties may not be as significant as those typically found with wood from large slower growing trees.

[FIGURE 5 OMITTED]

Relationships between growth rate of sample and MOE or MOR

The relationships between average annual rings per inch and both MOE and MOR were a evaluated for clear test specimens considering them as independent observations. Plots of average annual rings per inch vs. MOE and average annual rings per inch vs. MOR are shown in Figures 5 and 6. A regression analysis of this data revealed that there is a weak relationship between rings per inch of samples and both MOE and MOR. Adjusted [r.sup.2] values for these analyses are 0.327 and 0.261, respectively. This relationship is not generally acknowledged in literature. In a review of literature, Bendtsen (1978) concluded that low SG and "poor" fiber characteristics are related to the age of the wood, and not growth rate. Studies indicated that wood of the same age (number of rings from the pith) tends to have similar SG, despite growth rate, and differences could be explained by normal tree variation.

Bowyer et al. (2003) state that for softwoods with or without prominent latewood, there is little correlation of wood density to growth rate, and that the greatest problem with accelerated growth of trees used for the production of wood products is the enlarged juvenile wood core. Therefore, it is likely that the enlarged juvenile core is the factor that accounts for lower MOE and MOR values found in open-grown trees and not faster growth rates per se. This theory was supported by a study (Pearson and Gilmore 1980) of mechanical properties conducted using wood specimens processed from loblolly pine trees that were harvested from a naturally regenerated 41-year-old forest, a 25-year-old plantation, and a 15-year-old plantation of trees bred for fast growth and straightness. This study revealed a direct relationship between SG and mechanical properties. Latewood percentage, SG, MOR, MOE, maximum crushing strength, and toughness were higher for samples taken from the "outer" (near bark) portion of the log as opposed to those from the "inner" (near pith) portion. It should be noted that not all "inner" samples were juvenile wood, nor were all "outer" samples mature wood. The mechanical properties of the younger (all plantation) trees (as determined by small, clear samples, ASTM D-143) were lower than those of the naturally regenerated stand, as a result of the lower SG of the younger trees. Part of this lower density must be a result of the large percent of juvenile wood in the cross section of the log. Percentages equaled 43 percent in 15-year-old pine, 21 percent in 25-year-old pine, and just 6 percent in 41-year-old naturally regenerated trees. Based on the determination that SG and associated mechanical properties increased with age, Pearson and Gilmore (1980) concluded that samples cut from the "outer" portion of fast-grown trees allowed to grow long enough may be expected to develop properties similar to the "older" samples that were tested.

[FIGURE 6 OMITTED]

Even though studies in the literature suggested that the relationship between growth rate and mechanical properties may not be significant for material cut from larger trees, the relationship that exists between sample growth rings per inch and mechanical properties for the small-diameter trees evaluated in this research could be important because it may provide a way to visually identify trees that are likely to have inferior mechanical properties. In other words, for small-diameter ponderosa pine trees grown along the Colorado Front Range, a relatively fast growth rate may prove useful as an indicator of young open-grown trees that tend to have low MOE and MOR.

Conclusion

The purpose of this research was to evaluate the bending properties of wood from small-diameter ponderosa pine trees to better utilize this material. In this study, 542 small, clear test specimens cut from normal, open-grown, and misteltoe-infested ponderosa pine trees were evaluated. Generally, variation was greater through the cross section of trees than along their length. Based on results, there was only significant variation in SG laterally away from the pith for normal and mistletoe trees. No significant variation in SG occurred either laterally from the pith or longitudinally along the length of open-grown trees. Both MOE and MOR varied both laterally away from the pith and longitudinally along the length of the tree. Specimens from normal trees had the highest mechanical properties. Specimens from open-grown trees had significantly lower mechanical properties than those from both normal and misteltoe-infested trees. Specimens from misteltoe-infested trees had lower mean MOE and MOR than specimens from normal trees; however, differences were not statistically significant for MOR.

Although the relationships between SG and both MOE and MOR are not as significant as typically found for wood from larger trees, the relationship between average growth increments per inch and mechanical properties presents an opportunity for nondestructive visual evaluation of this material. Although more research is required on larger members, this relationship could provide a basis for visually grading small-diameter material. Based on findings from this research, wood from open-grown small-diameter ponderosa pine is inferior to material from trees with more normal growth characteristics and should not be used for structural application. Wood members from normal and mistletoe trees grown in relatively dense stands are more suitable for structural applications assuming that grading criteria can be established for removing faster grown material.

Literature cited

Alden, H.A. 1997. Softwoods of North America. Gen. Tech. Rep. FPL-GTR-102. USDA Forest Serv., Washington, DC. 151 pp.

American Society for Testing and Materials (ASTM). 1997. Standard methods of testing small clear specimens. ASTM Standards D 143-94. ASTM, West Conshohocken, PA.

Bendtsen, B.A. 1978. Properties of wood from improved and intensively managed trees. Forest Prod. J. 28(10):61-72.

_______ and J. Senft. 1986. Mechanical and anatomical properties in individual growth rings of plantation-grown eastern cottonwood and loblolly pine. Wood and Fiber Sci. 18(1):23-38.

Biblis, E.J., H. Carino, R. Brinker, and C.W. McKee. 1995. Effect of stand density and flexural properties of lumber from two 35-year-old loblolly pine plantations. Wood and Fiber Sci. 27:25-33.

Bowyer, J.L., R. Shmulsky, and J.G. Haygreen. 2003. Forest Products and Wood Science: An Introduction. 4th ed. Blackwell Publishing, Oxford, UK. 564 pp.

Colorado State Forest Serv (CFS). 2001. Red zone study. Colorado State Univ., Fort Collins, CO.

Conklin, D.A. 2000. Dwarf mistletoe management and forest health in the Southwest. USDA Forest Serv., Southwestern Region, Albuquerque, NM. 30 pp.

Dhubhain, A.N., J.A. Evertsen, and J.J. Gardiner. 1988. The influence of compression wood on the strength properties of Sitka spruce. Forest Prod. J. 38(9):67-69.

Dunnett, C.W. 1980. Pair-wise multiple comparisons in the unequal variance case. J. Am. Statistical Assoc. 75(372):796-800.

Gerhards, C.C. 1979. Time related effects of loading on wood strength: A linear cumulative damage theory. Wood Sci. 11(3):139-144.

Gorman, T.M., D.L. Lynch, and K.H. Mackes. 1999. Report on test results for Colorado white fir. Dept. of Forest Sciences, Colorado State Univ., Fort Collins, CO.

Kretschmann, D.E. and B.A. Bendtsen. 1992. Ultimate tensile stress and modulus of elasticity of fast-grown plantation loblolly pine lumber. Wood and Fiber Sci. 24(2):189-203.

Lynch, D.L. and K.H. Mackes. 2003. Evaluating costs associated with forest restoration fuel hazard reduction projects in Colorado. In: Proc. from Conf. on Fire, Fuel Treatments, and Ecological Restoration. Proc. RMRS-P-29, Rocky Mountain Res. Sta., Fort Collins, CO.

MacPeak, M.D. 1990. Comparison of grade, yield, and mechanical properties of lumber produced from young fast-grown and older slow-grown planted slash pine. Forest Prod. J. 40(1):11-14.

Maffei, H.M. 1989. Southwestern dwarf mistletoe damage to multi-aged ponderosa pine stands of the Colorado front range. Dept. of Plant Pathology and Weed Sciences, Colorado State Univ., Fort Collins, CO. 112 pp.

Pearson, R.G. and R.C. Gilmore. 1971. Characterization of the strength of juvenile wood of loblolly pine. Forest Prod. J. 21(1);23-31.

_______ and _______. 1980. Effect of fast growth rate on the mechanical properties of loblolly pine. Forest Prod. J. 30(5):47-54.

Pellerin, R.F., P. Koch, and J.J. Vogt. 1989. Mechanical properties of lodgepole pine: 6- and 9-inch diameter stems. Forest Prod. J. 39(11/12):13-20.

Piirto, D.D., D.L. Crews, and H.E. Troxell. 1974. The effects of dwarf mistletoe on the wood properties of lodgepole pine. Wood and Fiber 6(1):26-35.

SAS Institute Inc. 1999. SAS/STAT User's Guide, Version 8. SAS Inst. Inc., Cary, NC. 3884 pp.

USDA Forest Service, Forest Products Laboratory (USDA). 1999. Wood Handbook: Wood as an Engineering Material. Forest Prod. Soc., Madison, WI. 463 pp.

Zobel, B.J. and J.R. Sprague. 1998. Juvenile Wood in Forest Trees. Springer-Verlag, Berlin, Germany.

Kurt Mackes*

Wayne Shepperd

Christopher Jennings*

The authors are, respectively, Assistant Professor, Dept. of Forest, Rangeland and Watershed Stewardship, Colorado State Univ., Fort Collins, CO (mackes@cnr.colostate.edu); Research Forester, USDA Forest Serv., Rocky Mountain Res. Sta., (wshepperd@fs.fed.us); and Research Associate, Dept. of Forest, Rangeland and Watershed Stewardship, Colorado State Univ. (jennings@lamar.colostate.edu). This paper was received for publication in September 2003. Article No. 9761.

*Forest Products Society Member.
Table 1. -- Mean variation in SG, MOE, and MOR for sections along the
length of trees.

Tree Sample SG MOE
section size Mean SD Sig. (a) Mean SD Sig. (a)
 (E + 6 psi)

Normal
 1 67 0.407 0.038 0.772 0.217
 2 76 0.394 0.035 0.848 0.175
 3 66 0.391 0.043 0.816 0.169
 4 25 0.390 0.038 0.752 0.162
Open grown
 1 68 0.402 0.035 0.416 0.145 2,3
 2 40 0.407 0.042 0.526 0.149 1
 3 11 0.382 0.031 0.633 0.124 1
 4 2 0.405 0.035 0.652 0.103
Mistletoe infested
 1 81 0.414 0.046 3 0.675 0.210 2,3
 2 58 0.408 0.037 0.779 0.171 1
 3 38 0.393 0.028 1 0.794 0.108 1
 4 10 0.406 0.040 0.719 0.187

Tree MOR
section Mean SD Sig. (a)
 (psi)

Normal
 1 4,782.4 1,098.2
 2 4,939.8 875.0
 3 4,918.5 951.3
 4 4,616.0 1,008.3
Open grown
 1 3,171.6 748.9 2,3,4
 2 3,820.8 736.0 1,4
 3 4,073.2 536.5 1
 4 4,432.3 808.1 1,2
Mistletoe infested
 1 4,338.5 1,102.1
 2 4,703.0 962.5
 3 4,781.6 712.0
 4 4,777.4 982.2

(a) Multiple comparisons using Dunnett T3 Test where the mean difference
is significant at the 0.05 level. Numbers in this column represent tree
sections that have mean values that are significantly different from the
section in column 1.

Table 2. -- Mean variation in SG, MOE, and MOR through the cross
sections of butt logs (first log section) from trees harvested for
study.

Distance Sample SG MOE
from pith size Mean SD Sig. (a) Mean SD Sig. (a)
 (in) (E + 6 psi)

Normal
 Pith 14 0.482 0.069 1,2 0.376 0.093 1,2
 1 35 0.405 0.039 Pith 0.851 0.180 Pith,2
 2 28 0.406 0.036 Pith 0.678 0.224 Pith,1
 3 4 0.433 0.049 0.746 0.242
Open grown
 Pith 13 0.447 0.070 0.296 0.091 1,2
 1 30 0.399 0.035 0.459 0.160 Pith,3
 2 26 0.402 0.036 0.407 0.141 Pith
 3 11 0.413 0.033 0.328 0.052 1
Mistletoe infested
 Pith 17 0.482 0.072 1,2 0.404 0.101 1,2,3
 1 37 0.413 0.045 Pith 0.741 0.222 Pith,3
 2 30 0.408 0.036 Pith 0.643 0.199 Pith
 3 11 0.441 0.073 0.542 0.096 Pith,1

Distance MOR
from pith Mean SD Sig. (a)
 (in) (psi)

Normal
 Pith 3,536.9 693.5 1
 1 5,036.5 849.7 Pith
 2 4,347.6 1,231.1
 3 5,602.5 1,137.6
Open grown
 Pith 2,685.7 603.3
 1 3,281.9 764.8
 2 3,174.4 806.5
 3 2,904.6 548.7
Mistletoe infested
 Pith 3,454.1 762.0 1
 1 4,514.5 1,121.4 Pith
 2 4,179.1 1,136.5
 3 4,126.8 892.4

(a) Multiple comparisons using Dunnett T3 Test where the mean difference
is significant at the 0.05 level. The numbers and the word pith in this
column represent tree sections that have mean values that are
significantly different from the section in column 1.

Table 3. -- Mean data for SG, MOE, and MOR.

 SG MOE MOR
Tree no. Sample size Mean SD Mean SD Mean SD
 (10E + 6 psi) (psi)

Normal
 1 13 0.403 0.032 1.005 0.807 5,543.5 586.0
 7 15 0.434 0.029 0.863 0.166 5,392.4 412.6
 12 13 0.392 0.035 0.830 0.179 5,462.0 634.7
 14 15 0.379 0.020 0.696 0.144 4,261.9 729.7
 22 11 0.399 0.028 0.682 0.176 4,508.2 727.0
 23 12 0.399 0.013 0.905 0.097 5,225.9 460.5
 27 15 0.384 0.041 0.637 0.164 4,307.9 778.5
 28 11 0.356 0.026 0.768 0.107 4,013.5 637.0
 29 9 0.381 0.029 0.764 0.190 4,534.9 792.3
 32 11 0.359 0.011 0.725 0.084 4,494.9 570.2
 35 12 0.433 0.038 0.898 0.114 5,512.5 641.1
 38 8 0.451 0.062 0.954 0.175 5,847.2 847.9
 39 10 0.376 0.013 0.804 0.045 4,762.1 352.2
 55 12 0.394 0.022 0.779 0.171 4,428.6 1,078.3
 57 5 0.420 0.017 0.952 0.097 5,445.9 635.5
 59 10 0.430 0.025 0.928 0.176 5,380.1 1,309.2
 60 14 0.393 0.041 0.830 0.157 4,994.1 556.8
 61 12 0.370 0.024 0.619 0.134 3,969.9 367.6
 65 11 0.445 0.018 1.015 0.255 5,866.0 1,616.8
 67 15 0.365 0.023 0.689 0.153 3,988.1 974.2
Subtotal 20 trees 0.398 0.029 0.817 0.121 4,897.0 638.8

Open grown
 3 4 0.435 0.006 0.301 0.003 2,874.4 123.5
 15 9 0.436 0.040 0.338 0.058 2,497.9 844.7
 16 2 0.395 0.021 0.290 0.011 2,678.7 173.0
 21 7 0.421 0.029 0.516 0.090 3,425.9 833.2
 31 11 0.429 0.023 0.538 0.141 4,023.2 535.0
 41 5 0.380 0.007 0.687 0.050 4,239.8 350.4
 44 7 0.366 0.025 0.581 0.113 3,864.7 324.0
 45 5 0.386 0.024 0.227 0.027 2,060.2 138.7
 49 7 0.390 0.045 0.507 0.151 3,538.0 525.4
 51 6 0.380 0.023 0.356 0.126 3,092.8 604.9
 64 14 0.389 0.015 0.657 0.108 4,389.7 300.8
 80 11 0.396 0.036 0.382 0.077 3,061.2 338.2
 90 9 0.400 0.073 0.546 0.091 3,825.3 391.2
 91 4 0.440 0.020 0.544 0.050 4,138.0 484.4
 92 9 0.410 0.020 0.463 0.134 3,304.7 752.6
 93 11 0.388 0.018 0.438 0.118 3,351.9 755.7
Subtotal 16 trees 0.403 0.023 0.458 0.133 3,397.9 666.3

Mistletoe
 infested
 33 11 0.384 0.015 0.754 0.171 4,460.4 638.5
 63 13 0.398 0.018 0.668 0.114 4,140.5 475.5
 70 6 0.435 0.022 0.426 0.152 2,993.8 651.9
 72 8 0.454 0.024 0.578 0.165 4,256.4 769.9
 73 20 0.430 0.039 0.750 0.148 4,550.9 659.4
 75 9 0.369 0.019 0.560 0.133 3,209.6 1,091.0
 76 16 0.389 0.015 0.808 0.132 4,734.3 714.4
 77 4 0.418 0.013 0.772 0.209 4,766.1 747.2
 78 2 0.380 0.000 0.628 0.146 3,644.1 1,162.6
 81 14 0.377 0.023 0.835 0.099 4,915.4 317.7
 82 6 0.475 0.069 0.672 0.066 5,372.0 842.7
 94 6 0.425 0.048 0.916 0.073 6,047.1 725.4
 95 5 0.374 0.021 0.694 0.089 4,343.8 329.7
 96 8 0.400 0.023 0.515 0.112 3,750.6 624.0
 100 5 0.380 0.021 0.806 0.147 5,205.8 608.4
 101 8 0.383 0.030 0.767 0.137 4,630.4 1,031.7
 102 7 0.411 0.013 0.625 0.156 4,145.2 1,296.3
 103 13 0.389 0.018 0.741 0.139 4,727.6 342.6
 104 12 0.471 0.032 1.014 0.197 6,003.1 830.1
 105 9 0.404 0.021 0.745 0.188 4,408.9 805.4
 106 5 0.414 0.021 0.769 0.195 4,486.7 1,009.0
Subtotal 21 trees 0.408 0.031 0.716 0.134 4,513.5 770.0

All groups 57 trees 0.403 0.028 0.679 0.194 4,334.9 919.1
 combined
 total

Table 4. -- Statistical significance for property differences between
normal (N), open-grown (O), and mistletoe-infested (M) trees.

Tree groups SG MOE
being compared t-value Adj. prob > t Sig. (a) t-value Adj. prob > t

O to M 0.84 0.6817 5.79 <0.0001
O to N 0.07 0.9977 8.02 <0.0001
M to N 0.84 0.6828 -2.44 0.0469

Tree groups MOE MOR
being compared Sig. (a) t-value Adj. prob > 1 Sig. (a)

O to M * 4.70 <0.0001 *
O to N * 6.21 <0.0001 *
M to N * -1.66 0.2293

*Asterisk indicates mean difference is significant at the 0.05 level.
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Author:Mackes, Kurt; Shepperd, Wayne; Jennings, Christopher
Publication:Forest Products Journal
Geographic Code:4EUUK
Date:Oct 1, 2005
Words:7466
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