Pruned Plantation-Grown Eucalyptus nitens: Effect of Thinning and Conventional Processing Practices on Sawn Board Quality and Recovery

Thinned and pruned plantation-grown Eucalyptus nitens (H.Deane & Maiden) Maiden has potential to supplement native forest 'ash' eucalypt logs for the Tasmanian sawmilling industry. Processing methods developed for native forest eucalypts were applied to logs from 22-year-old pruned stands thinned at age six years to 100, 200, 300, 400 stems/ha and unthinned control stands with a stocking of 700 stems/ha at harvest. Consistent with existing industry requirements for the two sawing methods, 42 trees with diameter at breast height over bark (DBHOB) < 43 cm were selected for back-sawing (flat-sawing), and 39 trees with DBHOB > 43 cm for quarter-sawing. For each sawing method, sets of trees were selected to provide trees evenly distributed across the target diameter range and, as near as possible, with sets matched across thinning treatments for size. From each tree two sawlogs, nominally 2.7 m in length, were cut from the pruned part of the stem. Both logs from individual trees were either back-sawn or quarter-sawn with single-saw log breakdown and re-saw systems. The boards were dried, dressed and graded to meet the requirements of Australian Standard AS 2796. Potential value-limiting defects were recorded, recovery calculated and measurements made of docked volume due to board end splits, flitch deflection, shrinkage and other processing related characteristics. Thinning treatment affected some shrinkage traits but for all other measures of processing performance was not significant, for logs of matched size. Likewise, a basal area measure of localised competition experienced by individual trees had no significant effect on processing performance. This indicates that the thinning intensity was not critical for processing with either a back-sawing or quarter-sawing strategy in conventional sawmills, as trees of equivalent size grown under different competitive regimes did not differ substantially in their processing performance. Quarter-sawn logs had lower total recovery, but higher recoveries of select and standard grades, than back-sawn logs. Upper logs produced higher recoveries of select and standard grades than butt logs for both sawing methods. These differences were primarily due to the severity of drying-related defects and the volume of wood docked to eliminate board endsplits. For both sawing methods the total recoveries from all logs, and the recoveries of select and standard grades from the upper logs, were similar to recent comparable studies in native forest sawlogs. However, the recoveries of select and standard grades from the butt logs were lower. Poor sawing accuracy contributed to the high occurrence of surface checking and product under-sizing for both sawing methods. Together with improved material handling during drying, greater sawing accuracy has the potential to improve not only recoveries but also product quality from both the butt and upper logs. © 2009 New Zealand Forest Research Institute Limited, trading as Scion.