Review of growth and wood quality of plantation-grown Eucalyptus dunnii Maiden

Abstract

Summary Forests NSW manages Eucalyptus plantations on the north coast of NSW, Australia, for high-value timber production. One species increasingly being planted both in Australia and overseas is Eucalyptus dunnii Maiden. For a species to be considered suitable for forestry, criteria to be met include successful establishment, growth and suitability for end use, be that pulp, solid wood or veneer production. Historic data from E. dunnii plantations aged from 3 to 34 y were reviewed. Growth and wood quality using a range of non-destructive and destructive measurements are reported. Eucalyptus dunnii typically grew equally well as some alternative species, although species ranking was affected by the growing site. Eucalyptus dunnii produced high-quality wood chips with average pulp yield from three NSW plantations aged 8–10 y of 53% and 265 kg m−3. This yield is comparable with that of 10-y-old E. globulus plantation material from Tasmania. Wood density increased with tree age from about 500 kg m−3 at age 10 y to 600 kg m−3 at age 25 y, and more slowly beyond that age. Many solid-wood quality traits such as hardness and strength could be positively correlated with both tree age and basic density. This has implications for the timber industry as it is intended that plantation trees will be harvested at younger ages than native forest trees, but wood quality in such younger material may not satisfy minimum product performance requirements. However, trees selected for higher density achieved strength group ratings at age 9 y that would normally be achieved at age 25–30 y. It is not known if similar improvements can be made in other wood quality traits. Quality traits requiring further examination are growth stresses and end splitting of logs, and shrinkage of sawn timber. Collapse (reversible shrinkage) and non-reversible shrinkage are positively related to wood density, but a greater concern is the high ratio of tangential shrinkage compared to radial shrinkage. This ratio, which can be 2.5 or greater in E. dunnii, leads to excessive distortion in sawn material. It may be possible to reduce overall wood shrinkage and the ratio of tangential to radial shrinkage, as well as other unfavourable wood quality traits, through genetic selection as these traits in related eucalypts (e.g. blackbutt, E. pilularis) are heritable.