Abstract

Sample collection and preparation

Sample preparation was guided by ASTM D143‐09 (ASTM International 2009), a widely used standard for testing mechanical properties of small clear timber specimens. An alternative would have been ASTM D198‐15 (ASTM International 2015), which describes similar tests for structural lumber. However, the former was chosen for two reasons: (1) Kretschmann (2010) cites ASTM D143 as being the primary testing standard used to determine static bending properties, and (2) the most likely end use of J. scopulorum is not in structural lumber. ASTM D5536‐94 (ASTM International 2010) describes methods of tree selection and processing to prepare samples used in ASTM D143‐09 (2009). The random sampling method was chosen because it is appropriate for tests meant to establish general allowable lumber stresses for a particular species. The nature of juniper trees presented several challenges in obtaining knot-free specimens of sufficient size. For this reason, the secondary test method in ASTM D5536‐94 (2010) was followed. This method calls for specimens with a cross section of 2.54 by 2.54 cm (1 by 1 in.), and a length of 40.64 cm (16 in.).

Trees were harvested from Ben Delatour Scout Ranch, located in the Red Feather Lakes area of northern Colorado. The site sits at 2,286 m (7,500 ft) in elevation and consists of rolling hills stocked with ponderosa pine (Pinus ponderosa) and other tree species. The area receives an average of 46.7 cm (18.4 in.) of precipitation annually and year-round average temperatures range from −1.6°C to 10.9°C (29.2°F to 51.7°F; Red Feather Lakes Colorado 2014). Ten J. scopulorum trees with diameter at breast height greater than 20.3 cm (8 in.) were harvested. Logs were transported back to the Colorado State Forest Service headquarters and soaked with water to keep them green until milling. A small bandmill was used to mill the logs into flitches of 7.62 cm (3 in.) or smaller width, so they could be further processed with a table saw and jointer. Half of the flitches were kept in water tanks to ensure they stayed green, and half were stacked to air-dry.

Flitches were processed into 2.54 by 2.54 by 40.64-cm (1 by 1 by 16-in.) specimens (sticks) at the Colorado State University Industrial Sciences Laboratory. Flitches were edged on a jointer and cut to within 0.159 cm (1/16 in.) of target size on a table saw. Care was taken to prepare sticks that were free of knots and other defects, but that was not always possible when working with this material. If specimens failed at knots during testing, the test was dropped and excluded from calculations. Green sticks were kept submerged in water to prevent loss of moisture. Air-dried flitches were left stacked for 3 months to dry to an approximate moisture content of 12 to 16 percent and then stored outside after processing.

Testing and data collection

Specific gravity and shrinkage tests

Guidance for specific gravity testing came from ASTM D2395‐14 (ASTM International 2014). After being used in static bending tests, green specimens were cut into 2.54 by 2.54 by 7.62-cm (1 by 1 by 3-in.) blocks. Both volume and weight of specimens were recorded before and after oven-drying at 103°C. Volume was measured using the water immersion method (ASTM International 2014); ovendried samples were first placed in a paraffin bath to seal them. The density of water (1 g/cm³) was used to convert wood density to specific gravity. The following equations were used:

$${\text{SG}}_{\text{G}}={\left({\text{WT}}_{\text{OD}}/{\text{Vol}}_{\text{G}}\right)/1\text{\hspace{0.17em}g}/\text{cm}}^3$$ $${\text{SG}}_{\text{OD}}=\left({\text{WT}}_{\text{OD}}/{\text{Vol}}_{\text{OD}}\right)/1\text{\hspace{0.17em}g}/{\text{cm}}^3$$ $$\text{Shrinkage\hspace{0.17em}}(\%)\text{\hspace{0.17em}=\hspace{0.17em}}\left({\text{Vol}}_{\text{G}}-{\text{Vol}}_{\text{OD}}\right)/{\text{Vol}}_{\text{G}}\times 100$$

where

 SG_G = green specific gravity,

 SG_OD = ovendry specific gravity,

 WT_OD = ovendry weight,

 VOL_G = green volume, and

 VOL_OD = ovendry volume.

Air-dry strength and stiffness

The mean MOE value for air-dry specimens was 4,611 MPa (668,800 psi) and the mean MOR was 68,460 kPa (9,929 psi; Table 1). The test specimens displayed a range in moisture content of 11.4 to 14.9 percent (mean, 13.0%). However, despite the range in moisture content, the overall coefficient of variation (CV) for both tests was lower than the 50 species average CVs of MOE (22%) and MOR (16%) reported in the Wood Handbook (Kretschmann 2010).

Table 1 Air-dry strength and stiffness summary results for 87 samples.^a

^a Range in moisture content for specimens was 11.4 to 14.9 percent and the mean was 13.0 percent. CV = coefficient of variation; MOE = modulus of elasticity; MOR = modulus of rupture.

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The air-dry MOE values for J. scopulorum are low compared with other commercial softwoods in the Rocky Mountains, which may preclude the species from various structural applications. However, J. scopulorum is unlikely to be used in structural applications because of its small diameter and prevalence of knots. The low stiffness value does reduce the wood's chances for use in products such as tool handles, an application for which it would otherwise be highly suitable. On the other hand, J. scopulorum has a high air-dry MOR relative to other commercial softwoods in Colorado. For comparison, the following air-dry values are reported in Kretschmann (2010): ponderosa pine MOE of 8,900 MPa (1.29 × 10⁶ psi) and MOR of 65,000 kPa (9,400 psi) and Douglas-fir MOE of 12,600 MPa (1.83 × 10⁶ psi) and MOR of 87,000 kPa (12,600 psi).

The ideal use for the wood would be one that takes advantage of its flexibility, strength, and natural decay resistance. One such use would be in highway materials such as guardrails and signposts. In these, a low MOE may be desirable so that it may reduce the impact borne by vehicles in collisions (Burke 2001). Faller et al. (2009) tested the suitability of ponderosa pine and Douglas-fir as posts supporting steel W-beam guardrails. They found that the material performed well and recommended that other species be tested to provide more outlets for wood from small-diameter thinning projects. There is certainly a market for this in Colorado; Lynch and Mackes (2001) reported that Colorado highways used over 3.5 million new board feet of combined highway signage and guardrail posts in the year 1997.

Green strength and stiffness

The mean MOE value for green specimens was 3,846 MPa (557,900 psi) and the mean MOR was 46,760 kPa (6,782 psi; Table 2). These results have implications on trees in urban settings. High green MOR values suggest that in general J. scopulorum trees are better suited to handle high levels of wind and snow loading than species with lower values. A more accurate, site-specific evaluation can be made using knowledge of a tree's elastic limit. Coder (2005) describes a nondestructive process where a winch is used to apply force to a tree and evaluate the wind load that the tree could withstand before breakage. The elastic limit of a species can be calculated from its MOE and maximum crushing strength.

Table 2 Green strength and stiffness summary results for 75 samples.^a

^a CV = coefficient of variation; MOE = modulus of elasticity; MOR = modulus of rupture.

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Specific gravity and shrinkage

The mean specific gravity for the test specimens was 0.4545 (ovendry basis) and 0.4184 (green basis), and the mean volumetric shrinkage value was 7.956 percent (Table 3). The shrinkage tests had very high variance relative to the other tests in this study. According to Glass and Zelinka (2010), the average coefficient of variability for volumetric shrinkage in wood is approximately 15 percent, but in these tests it was 26 percent. One possible explanation for this is the irregular growth rings exhibited by the specimens. Although the specimens were all approximately the same size, their distance in the tangential direction varied. Because of the anisotropic nature of wood, one would expect a specimen with shorter tangential distances (end grain with parallel growth rings) to shrink less than one with longer tangential distances (end grain with curved growth rings).

Table 3 Specific gravity (SG) and shrinkage results for 82 samples.^a

^a CV = coefficient of variation.

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Usage of juniper biomass (bioenergy feedstock, animal bedding, mulch, etc.) would be consistent with restoration projects that involve mastication. Specific gravity is positively correlated with biomass yield (Chave et al. 2005), so results of this study suggest that the wood could be suitable for these uses. Additionally, the higher heating value of J. scopulorum (19.59 MJ/kg, 8,430 BTU/lb) is within the normal ranges used in bioenergy processes reported by Wright et al. (2009). To estimate the total biomass yield that a specific project or region could produce, total wood volume and specific gravity are necessary. Much work has gone into equations to predict the aboveground volume in juniper trees of all species (Chojnacky 1985). Chojnacky and Moisen (1993) extended this work to provide formulas for converting total juniper volume to biomass using specific gravity. However, the authors did not report the specific gravity of J. scopulorum. With values provided by the current study, there is now enough information to estimate large-scale juniper biomass yield.

The density of wood not only influences its external strength, but also the strength of the xylem conduit in living trees. Wood density is positively correlated with resistance to xylem implosion from negative pressure (Hacke et al. 2001), which helps a tree tolerate drought conditions. Hacke et al. (2001) specifically mention that this phenomenon may explain juniper's rapid expansion throughout the west. With density being such an important predictor of drought tolerance, knowledge of density may be useful in developing models to predict future distribution of species taking climate change into account.