Effects of thinning on the longitudinal and radial variation in wood properties of Eucalyptus nitens

Abstract

Eucalypt plantations in Tasmania have been managed predominantly for fibre production, but there is also growing interest in the production of solid wood products. For solid wood production, stiffness and basic density are key wood properties as they define the suitability of the timber for particular products and ultimately value. To inform processing options available for targeting high value wood products there is a need to understand how wood properties vary within a tree and how thinning impacts wood quality to foster efficient processing. Three thinning trials of 20-22-year-old plantation grown Eucalyptus nitens were used to assess stiffness and basic density longitudinally from the base to 20 m height in the tree and radially at a fixed height of 2.5 m. Longitudinally and radially, wood properties varied more within the tree than the variation which arose as a result of thinning. Stiffness was lowest at the bottom of the tree irrespective of thinning treatment and the highest stiffness was located from 7.5 to 15 m height depending on thinning and site. Commercial thinning to 300 trees ha-1 had no effect on stiffness in the bottom of the tree but resulted in lower stiffness in the upper logs. Trees in thinned stands had slightly lower basic density and that reduction was consistent within the tree and across sites. Thinning resulted in significant radial change in wood properties and the thinning effect was apparent soon after the thinning treatment. The results demonstrate that thinning has an adverse impact on wood properties, but not to a degree that hinders the benefits thinning brings to maximizing wood growth. However, the high variation in wood quality within the tree suggests that it would be valuable segregating logs within a tree to maximize solid wood product value.