Pairing Test and Longitudinal Growth Strain: Establishing the Association
Where a log is sawn along the pith to produce two half-round posts the outward warp or bending has been attributed to the presence of growth stress in the standing tree. However this association has not been studied thoroughly. The paper compares the outward bending on sawing along the grain and corresponding growth strains in small diameter logs from 10-year-old Eucalyptus nitens. Sixty-three trees were felled and from each tree two logs, each 1.3 m long, were extracted. Longitudinal growth strain was measured at approximately mid-length on opposite sides of each log using the strain gauge method. The acoustic velocity along the logs was measured using a resonance-based tool to estimate modulus of elasticity. Subsequently logs were sawn into two half rounds along the length and the bending of the two halves was quantified by measuring the opening at both ends of the log. A strong positive relationship was observed between average growth strain in the logs and the opening on sawing within each group of logs i.e. butt logs and upper logs. The magnitude of opening in the upper log was about 2.5 times the opening in the butt log. This difference was found to be associated with the log dimensions and taper. A mathematical model was developed that calculated the force (growth stress) necessary to induce this warp. Its validity was tested against the experimental results. The model equation can effectively be used to predict the magnitude of distortion in logs on sawing knowing the growth strain and vice versa. This could be helpful in adopting the appropriate sawing/processing of each log based on its expected distortion. Alternatively, the equation can be used to predict the growth strain level in trees. Although totally destructive in nature, this approach could formulate an alternative method for the rapid screening or selecting young eucalyptus trees having low growth strain during the early stages of selective propagation in tree-breeding programs.