Sorting Eucalyptus nitens plantation logs using acoustic wave velocity

Acoustic wave velocity (AWV) was evaluated as a predictor of wood stiffness in plantation-grown Eucalyptus nitens. To represent the resource currently being directed to the structural timber market, five harvest sites were selected in NE and NW Tasmania spanning two age classes (8 y and 13-15 y) and three productivity classes. A total of 155 sawlogs, 5.5 m long, were cut from 137 harvested trees. AWV was measured in both standing trees and in sawlogs in the mill yard. Disk samples were collected from the butt end of each log to determine green and basic density. Logs were then sawn, and structural boards dried and finished according to commercial processing practice. One sample board per log was then tested for stiffness, bending and shear strength and hardness.

Sites differed significantly (P < 0.001) in standing tree AWV, log AWV and all wood properties of the butt logs, with the 13-15-y age class displaying higher AWV, wood basic density, stiffness and hardness than the 8-y age class. Log AWV² explained 54% of the variance in board static modulus of elasticity (MoE_stat) for the pooled data. At the age class level, 47% of the variance was explained for the 13-15-y logs, but the correlation was much poorer-explaining only 6%> of variance in MoE_stat-for the subset of 56 logs from the two 8-y-old sites. Dynamic MOE (MoE_dyn, the product of green density and log AWV²), gave useful predictions of MoE_stat for the younger age class. MoE_dyn calculated using log AWV 2 explained 56% of the variation in MoE_stat, facilitating the segregation of logs into three stiffness classes. Tree AWV² explained 40% of the variation in MoE_stat for the pooled data, similarly enabling segregation of boards into three stiffness classes. Relative to segregation at the log level, a similar percentage of low-stiffness material was identified; but ability to identify higher-stiffness material was reduced. A significant (P < 0.01) positive correlation of 0.30 was found between board stiffness and hardness, indicating that segregation based on increasing stiffness would also improve hardness.