Effects of thinning and coppice control on stand productivity and structure in a silvertop ash (Eucalyptus sieberiL. Johnson) forest

Summary The effects of commercial thinning on stand structure and productivity were investigated in a 28-y-old stand of silvertop ash (Eucalyptus sieberi) near Orbost, Victoria. Thinning reduced basal area by 40–50% in treated areas and stocking from about 1400 to 250 merchantable stems ha-1. Replicated experimental plots were established in unthinned, thinned-only, and thinned and coppice-treated forest to measure the effects of thinning and coppice competition on the growth of retained trees. Stand basal area increments averaged 1.76 m2 ha-1 y-1 and thinning did not significantly reduce stand basal area increment even in the first year after treatment. Over the 6 y of measurement, the relative basal area growth in thinned-only forest was 47–67% higher than in unthinned forest, and in thinned and coppice-treated forest basal area increases were 60–89% greater than in unthinned (a further 15–23% over thinned-only). Basal area increment for the potential sawlog trees (the largest 150 stems ha-1) in thinned-only forest exceeded basal area increment of potential sawlogs in unthinned forest by 40–60%. In forest where coppice competition was removed, growth rates were increased by a further 15–20% in all years. Smaller trees retained in thinned forest (potential pulpwood for a second thinning) grew 69% faster than similarly-sized trees in unthinned forest, and removal of coppice competition increased this response by a further 38%. Competition from coppice regeneration developing on cut stumps reduced potential basal area growth of retained trees by about 20% in all years following thinning. Coppice contributed extra basal area to the stand (from 0.7 m2 ha-1 y-1 in the first year after thinning to 1.3 m1 ha-1 y-1 in the sixth year). After 6 y, coppice regeneration from cut stumps persisted in thinned-only forest (but declined from 1100 to 500 coppice stems ha-1) and contributed about 16% of total basal area, but less than 1% in volume.