Effect of thinning on growth and allometry of Eucalyptus diversicolor

Karri (Eucalyptus diversicolor F. Muell.) forest is an endemic forest type of southwest Western Australia (SWWA), noted for having the tallest trees in the region, and providing commercial, ecological and conservation value. To inform management aimed at optimising these values, a thinning trial was commenced in 1992. The aim of this study was to evaluate the effects of thinning on individual tree growth, stand level growth and allometry of karri trees in even-aged stands. Mixed effect models were used to compare the effect of different levels of thinning on individual tree growth and stand level growth of karri. We developed allometric equations modelling the relationship between diameter at breast height (DBH) and height, and tested whether thinning affected this allometric relationship. Thinning enhanced DBH, height and crown width (CW) growth of individual trees by up to 149%, 56% and 108% respectively in heavily thinned stands, with stand basal area growth optimised at retained basal areas of 13 m2 ha−1. Thinning had a significant effect on allometry of DBH with height, height-DBH ratio (HDR) and CW. Reduction in stand density through thinning from below increased diameter growth of individual trees. Stand level basal area growth is optimised at an intermediate level of thinning. Thinning can be a good management tool for even-aged karri forest because it may promote the diameter and thereby basal area growth, good health of individual trees, and potential ecological benefits including accelerated development of larger tree crowns typical of mature forest.