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Pre-commercial thinning and fertiliser enhance growth in young native Eucalyptus obliqua (L'Hérit.) stands in Tasmania

Summary The objective of this study was to determine the effects of pre- commercial thinning and fertiliser application on the wood production rates of young regrowth Eucalyptus obliqua stands in southern Tasmania. Research plots in three stands aged 16–21 y were established and monitored for 4 y after treatment. About 50% and 75% of the original stand basal area (BA) was removed using stem injection of glyphosate herbicide and the remaining trees were fertilised with 300 kg N ha−1 plus 75 kg P ha−1 in a factorial experiment. Basal area increment rates for the 50% BA removal plots were not significantly different from those in their respective controls in two out of three stands (about 1.5 m2 ha−1 y−1 basal area), whereas for all 75% BA removal plots they were reduced significantly. Mean diameter increment improvements of final crop trees (about 250 stems ha−1) over the control ranged from about 0.24 to 0.34 cm y−1. However, there were no significant differences in diameter increment between the two thinned treatments for this stratum, implying that no significant growth advantage was associated with an early thinning to this density, as was done in the 75% BA removal treatment. Pre-commercial thinning alone, and fertiliser alone, increased stand basal area by 47–18% compared to control plots. The differences, though not always statistically significant, were consistent across sites despite different numbers of retained stems. Injecting eucalypts < 10 cm in diameter had no significant effect on basal area increment of the crop tree stratum. This study indicates that removal of 50% of the basal area is an effective pre-commercial thinning option for E. obliqua stands to enhance diameter growth of the potential final crop trees.