Long-term response ofEucalyptus regnansF.Muell. to commercial thinning in a regrowth eucalypt forest in Tasmania

Summary A 24-y-old stand of Eucalyptus regnans (F.Muell.) in the Plenty Valley, Tasmania was thinned to residual basal areas of about 14, 21 and 28 m2 ha−1, representing basal area reductions of 65%, 50% and 33% (heavy, moderate, and light thinnings). Unthinned control plots were also established. Mean stand basal area in the light thinning plots was equivalent to that of the unthinned plots within about 20 y of thinning, and in the moderate thinning plots within about 30 y. At age 55 y, 31 y after thinning, stand basal area in the heavy thinning plots was about 8 m2 ha−1' (about 17%) less than in the unthinned plots. Increment advantages over the controls of 0.59, 0.47 and 0.44 m2 ha−1 y−1 were related to moderate, heavy, and light thinning respectively. Although there were no significant differences (P≤0.05) among treatments, mean diameter for the largest 150 stems ha−1 increased with heavier thinning. Mean net merchantable volume increments and sawlog volumes in all three thinned treatments exceeded those in the unthinned plots, and sawlog volume relative to total volume increased consistently with increased thinning intensity. The moderate thinning plots experienced negligible mortality, produced comparable basal area and more merchantable volume than the unthinned plots, and resulted in increased individual tree sizes and total sawlog volume. This was the only treatment which provided improvement for all these parameters over the ‘do nothing’ (unthinned) option.