Tree growth-competition relationships in thinned Eucalyptus plantations vary with stand structure and site quality

Growth responses to thinning can vary with site quality and age, however, the direction of the response varies. An understanding of the mechanisms behind thinning responses could help forest managers optimise production as well as inform modellers and ecologists about the functioning of tree stands. Thinning was used to create a range of stand densities in eleven Eucalyptus plantation stands on seven sites in south-eastern Australia. Basal area periodic annual increment (PAI; cm(2) year(-1)) of individual dominant and codominant trees was then related to competition, such that PAI = a + b ln(Competition). Competition was defined as the sum of the basal area of neighbouring trees within a radius of 6.5 m. The relative (%) and absolute (cm(2) year(-1)) responses to competition were quantified using b, which was correlated with site quality and stand structure of unthinned stands. Stand structure was quantified using statistics or parameters that describe the diameter distribution for the given age, species and site, including skewness, the coefficient of variation and parameters of the Weibull probability distribution. Relative and absolute responses both increased with increasing site quality and in stands with more negatively skewed diameter distributions (higher proportion of larger trees) or lower coefficients of variation. Absolute thinning responses often increased with increasing size class, while relative thinning responses often decreased. Variables describing diameter distributions (size-class structure) were able to describe some of the variation in competition responses that site quality could not. This indicates why stands on similar site qualities but with different stand structures can have correspondingly different thinning responses. Stand structural variables may be more useful predictors of thinning responses in stands that experience large temporal changes in diameter distributions compared with site quality, which is a more static variable. Thus, knowledge about the development of diameter distributions might help to refine thinning regimes.