Genetic variation in Eucalyptus nitens pulpwood and wood shrinkage traits

Eucalyptus nitens plantations are generally established for pulpwood production but an increasing area is being managed for solid wood. Genetic variation in, and correlations among, three Kraft pulpwood traits (diameter at breast height, basic density and near-infrared-predicted cellulose content) and three 12-mm wood-core shrinkage traits (recoverable collapse, net shrinkage and gross shrinkage) were examined, utilising data from two 9-year-old first-generation progeny trials in Tasmania. These trials contained approximately 400 open-pollinated families (over 100 of which were sampled for wood properties) representing three central-Victorian E. nitens races. Significant genetic variation at the race and/or within-race level was identified in all traits. Within races, relative levels of additive genetic variation were higher for shrinkage traits, although narrow-sense heritabilities were lower and the expression of genetic variation less stable across sites than for other wood property traits. Heterogeneous intertrait genetic correlations were identified across sites between growth and some wood property traits. However, where significant, genetic correlations indicated that within-race selection for growth would adversely affect core basic density and all core shrinkage traits. Furthermore, results based on cores suggested that within-race selection for higher basic density would favourably impact on cellulose content and collapse but selection for either higher basic density or cellulose content would adversely affect net shrinkage. Most within-race genetic variation in gross shrinkage appeared to be due to genetic variation in collapse. The implications of these results for sawn timber breeding will depend on the strength of genetic correlations between core traits and rotation-age objective traits and objective trait economic weights.