The influence of site quality on timing of pruning in Eucalyptus pilularis and Eucalyptus cloeziana plantations

Maximising the production of knot-free (clear) wood can be achieved by early removal of branches from the lower crown. Ideally, this is carried out without reducing stem growth or the competitive status of the pruned tree. The decision surrounding the time to prune in Eucalyptus species is influenced by stem- and branch-diameter development, the rate of branch senescence and ejection on the lower stem, the timing of canopy closure and the proportion of leaf area that can be removed before growth is reduced. In this study, the effect of site quality on stem, branch and crown development was examined in 1.5- to 6.5-y-old Eucalyptus pilularis Sm. and E. cloeziana F.Muell. trees from plantations across north-eastern New South Wales, Australia. Results from 27 plantations established on former pastures showed that site quality influenced height, diameter growth, and live-crown rise in both species, but it influenced maximum branch diameter in E. cloeziana only. Using regression relationships and threshold values for stem, branch and crown variables considered critical to determine the timing of pruning, decision matrices for low (0–2.9 m), high (0–5.5 m), and multiple (0–2.9 m then 2.9–5.5 m) pruning regimes were developed to provide a practical guide to identify the age at which stands could be considered for pruning. For E. pilularis the rate of branch senescence will strongly influence the timing of pruning, while in E. cloeziana the timing of pruning will largely be driven by branch diameter growth on high-quality sites and branch senescence on medium- and low-quality sites.