Eucalypts : picking the winners : a breeding objective for plantation eucalypts and early-age selection towards this objective

The optimum age for early selection of eucalypts in breeding to minimise the total cost of producing kraft pulp was investigated. The effect of future changes in density and pulp yield on the cost of converting roundwood to unbleached kraft pulp was modelled by defining cost functions for each stage of the kraft pulping process. The pulping cost function was combined with cost functions for growing, harvesting, and transporting plantation-grown eucalypt wood to derive a production function for the total cost of kraft pulp production in relation to the biological traits of growth rate, stem form, basic density and pulp yield. The economic value of trait improvement was combined with assumed genetic parameters to determine expected gain towards the objective for a given breeding effort in each trait. Density and growth were shown to be the most important traits for breeding, followed by pulp yield and stem form, with relative values of 3.6, 3.2, 1.9 and 1 respectively. Breeding to improve only density and growth was demonstrated to achieve 95% of the gain possible from breeding for all four traits. Pith-to-bark density profiles were determined for 588, 7-year-old plantation grown Eucalyptus nitens trees. Results were aggregated to derive age-age correlations for density which were shown to be high. Age-age correlations for growth were determined across a greater sample using height measured at 20 months and diameter at 1.3 m measured at four years and seven years. Based upon dual trait selection for density and growth, the optimum economic age for early selection, the selection age where the present value of all future gains is maximised, was determined to be three years. The optimum biological selection age, the selection age where average annual gain is maximised, was determined to be four years. Assessing wood properties is expensive and time consuming, reducing considerably the use of these traits in selection programs. The thesis demonstrated the application of the Pilodyn in the indirect selection of density in E. nitens, with very high genetic correlation (rg=-0.92) between Pilodyn penetration and density. The application Near Infrared Reflectance Analysis (NIRA) in the indirect selection of pulp yield was explored, however problems with calibration left results inconclusive. This thesis clearly points to an increasing emphasis on wood properties, in particular wood density, in selection decisions and to a reduction in selection age to as early as three years of age. Such practices are expected to impact significantly on the overall costs of producing eucalypt kraft pulp in Australia.