Rapid Screening for Solid Wood Quality Traits in Clones of Loblolly Pine (Pinus Taeda L.) by Indirect Measurements

Abstract

Clonal forest has the potential to greatly improve the quality and uniformity of wood derived from pine plantations. However, conventional field sampling and laboratory analysis of wood samples are prohibitively time consuming and expensive to facilitate the necessary mass screening of wood quality in clones. New methods need to be assessed for providing indirect non-destructive measurements of wood quality that can be rapidly and reliably utilized in young clonal tests. This study assessed a drill resistance tool called the Resistograph and a time-of-flight acoustic tool called the TreeSonic for their efficiency at screening young clones of loblolly pine for three economically important solid wood properties: wood density, modulus of elasticity (MOE), and modulus of rupture (MOR).

A single clonal trail containing clones from three elite full-sib families of loblolly pine was used to assess the efficiency of indirect selection. Increment cores and static bending samples were collected along with growth measurements from the clones at age 8. Basic wood density was measured at breast height and for the whole-tree using x-ray densitometry on the increment cores, as well as volume and weight measurements on clear wood bending samples. MOE and MOR were determined from static bending tests on clear wood samples. Clone means for wood properties were moderately to highly repeatable and intense clonal selection resulted in genetic gains over the site mean ranging from 11.1 % for wood density to19.9 % for MOE.

After this adjustment for effects of friction, clone means for Resistograph amplitude values had moderately strong correlations with wood density at both breast-height (0.75) and for the entire stem (0.72). Genetic correlations between amplitude and density were quite strong, with estimates ranging between 0.92 and 1.00. Amplitude was weakly correlated with MOE and MOR at the phenotypic level but were moderately related at the genetic level. Clonal variation explained only 20 % of the phenotypic variance for amplitude, such that disparities between the genetic and phenotypic correlations were due to the low repeatability of clone means for amplitude. The efficiency of the Resistograph was high for screening clones for wood density, moderate for MOR, and rather poor for MOE.

TreeSonic stress wave speed measurements (SWS) were highly repeatable and had moderate and highly significant clone mean correlations with mechanical wood properties. SWS was largely uncorrelated with wood density. Thus, SWS was highly efficient at selecting clones for MOE, moderate for MOR, and poor for density; reverse of the results for amplitude. SWS and amplitude were unrelated and provided independent information regarding the variation in mechanical wood properties, such that combining them into a single index for selection increased selection efficiencies.

Simultaneous gain for growth and solid wood quality could be achieved using clonal selection indices based on indirect wood quality measurements. Substantial gain for both growth and wood quality required that moderate proportional economic weights be used. Simultaneous selection was complicated by negative genetic correlations between MOE and growth traits, as well as by relatively low variation among clone means for MOR and density compared to volume. Obtaining considerable gains for wood traits resulted in large reductions from optimal volume gains, and justifying such losses for volume gain in deployment populations may be difficult.

It appears that drill resistance and acoustic methodologies are currently developed to the extent that they can be reliably applied for screening and selecting clones for wood quality. These methods were shown to be rapid, inexpensive, and effective at providing desirable genetic gains. Reliability and selection efficiencies are expected to improve, especially for the Resistograph, as the sources contributing to extraneous environmental error in the measurements are identified and corrected.