Correlated response of pulpwood profit traits following differential fertilisation of a Eucalyptus nitens clonal trial

Silvicultural treatments that are aimed at increasing plantation growth rate may also impact directly or indirectly on wood properties. We examined this impact in a fertiliser ‚àöv= clone trial in northwestern Tasmania, Australia. Nitrogen (N) and phosphorus (P) fertilisers were applied at planting to three Eucalyptus nitens (Deane et Maiden) Maiden clones and one F1 hybrid clone of E. nitens and E. globulus Labill. in a factorial design with each clone exposed to two levels of nitrogen (0 and 23 kg N/ha) and two levels of phosphorus (0 and 21 kg P/ha) spot-applied close to each seedling. The trial comprised four replicates per treatment with 5 ‚àöv= 5 tree clonal plots. Height was measured at ages one and two years, and diameter at breast height over bark at age 11 years. Increment cores at breast height were obtained from one fastand one slow-growing ramet within each clonal plot to determine corewood basic density, near infrared-predicted kraft pulp yield, cellulose content, and extractives content. No significant interactions among main treatments were detected for any of the growth- or wood-property traits. Nitrogen application increased cellulose content (p < 0.05). Phosphorus application significantly increased diameter (p < 0.01), but resulted in lower wood density (p < 0.001). Within clonal plots, large trees had lower wood density (p < 0.001) and a higher extractives content (p = 0.004) than the corresponding small trees. Pulpwood production per hectare (calculated from plot volume, mean whole-tree adjusted density and mean plot kraft pulp yield) indicated that: (i) the choice of germplasm had a much larger effect on plantation profitability than did the starter fertiliser application; and (ii) that failure to account for adverse changes in wood properties in calculating pulp fibre production would result in over-estimation of the gain in pulp production due to starter phosphorus application by up to 0.6 t/ha or 20% per 12 year rotation.