Effects of pattern and severity of pruning on growth and branch development of pre-canopy closure Eucalyptus nitens

Abstract

In order to produce appearance grade timber from plantation-grown Eucalyptus nitens, green pruning is necessary. However, studies of decay associated with green pruning E. nitens determined that there is a high incidence and severity of decay resulting from pruning large diameter (≥30 mm) branches. Observations in pre-canopy closure plantations suggest that branches of this size can develop before the time that first lift pruning would normally occur (canopy closure), meaning that there is a high probability of decay if trees with large branches are pruned. This paper describes an experiment that investigated pre-canopy closure pruning treatments that may reduce the incidence of unacceptably large branches at the time of first lift pruning. The effects of both pattern (removing all branches to a predetermined stem diameter versus removing a selection of larger branches from throughout the crown) and severity (0–80% leaf area removal) of pruning on stem growth and branch development were investigated at three sites over a 15-month period. While even 20% leaf area removal reduced stem volume, only 80% leaf area removal changed the growth trajectory such that long-term growth was likely to be reduced. An amount of 20% pruning increased branch diameter increment. Pattern of pruning had only a small effect on stem growth or branch diameter distribution, but removing a selection of branches from throughout the crown resulted in more dead branches than did removing all branches to a predetermined stem diameter which is important from a wood quality perspective. Branch studies indicated that pre-canopy closure pruning of E. nitens is unlikely to reduce the number of trees with unacceptably large branches except at very fast growing sites. The results of the experiment suggest that such sites may still have high numbers of trees with large branches at the time of first lift pruning even if pre-canopy closure pruning is performed. It was concluded that a more effective management strategy would be to schedule first lift pruning based on an understanding of patterns of branch development. Hence, at the high productivity site first lift pruning 3 months after pre-canopy closure pruning was scheduled would have resulted in the rejection of virtually no trees based on branch size.