Basal area increment is unaffected by thinning intensity in young 'Eucalyptus dunnii' and 'Corymbia variegata' plantations across different quality sites

The subtropical eucalypt plantation estate in Australia now comprises more than 100,000 hectares in north-east New South Wales (NSW) and south-east Queensland (QLD). If a significant proportion of this resource is to be used for sawlog production, it is necessary to design effective silvicultural systems, particularly thinning procedures. A thinning trial was carried out at two sites in QLD and two in NSW. Each was planted with two species, 'Eucalyptus dunnii' and 'Corymbia citriodora' ssp. 'variegata'. The original stocking density of the plantations at planting was 1000-1300 trees per hectare. When the trees were 6-8 years of age, three treatments were applied in randomised complete block designs: an unthinned control, 550 and 300 stems per hectare residual stocking. Basal area increment after thinning was affected by site quality but was not affected by thinning treatments for either species. Mean diameter increment was significantly greater in the thinned stands of both species for all trees in the stand and for the largest 250 trees per hectare. The similarity of basal area growth and difference in diameter growth gives good management flexibility as all treatments at all sites were fully utilising site resources. Analysis of financial factors is needed to determine optimum management under different market scenarios.