The physiological and pathological implications of pruning eucalypts

A major focus in Tasmania since 1988 has been research for the management of temperate eucalypt plantations for solid wood. Unpruned plantation eucalypts do not deliver high-quality appearance-grade sawlogs. The extent to which trees can be pruned before their growth is significantly reduced is a function of species and site. An understanding of the processes that optimise clearwood production after pruning is required to make informed decisions about the type and timing of silvicultural inputs. The effect of fertiliser addition on pruning-associated decay was investigated at two nitrogen (N)- and phosphorus (P)-deficient sites. At one site, pruning of Eucalyptus nitens increased the level of decay and degrade in clearwood formed after pruning. Improved nutrition increased the longevity and size of branches in trees, and led to a higher incidence of decay infections compared to trees at a lower level of nutrition. At the second site, the responses of Eucalyptus globulus and E. nitens to pruning in two lifts and different rates of N-fertiliser application were compared. Pruning reduced growth, but the final measured volume (at ~ age 6 years) of pruned trees in fertiliser treatments was greater than in the unpruned trees with no fertiliser. E. nitens exhibited superior growth over the course of the experiment with a larger volume response to applied N than E. globulus. E. nitens had a higher incidence of decay infections in pruned stubs because of its tendency to have larger branches than E. globulus, though the overall incidence of decay in this experiment was very low. A rapid rate of occlusion of the wound created by pruning is required to maximise the production of clearwood. Five years after pruning, trees over a range of early and late fertiliser treatments were harvested and dissected to assess branch occlusion and clearwood production. The occlusion rate of pruned stubs was low, being delayed by the exudation of kino from branch stubs and by thick bark. Branches growing higher in the tree were more likely to have occluded than lower branches. The amount of clearwood produced depended on branch height, status and diameter, stub length, growth before and after pruning, and the distance required for a stub to occlude. Fertiliser treatments did not significantly affect these relationships, which suggested that applications at age four years of age or later can be made as required for increasing growth rates and clearwood production. Physiological responses to second-lift pruning have been investigated less intensively than those of first-lift resulting in less certainty in linking pruning strategies to outcomes. In an experiment to address this gap in knowledge, trees of E. nitens were pruned in two lifts. The responses to fertiliser nitrogen application of pruned and unpruned trees were also investigated. The physiological response to pruning was dependent on resource availability. Pruning did not initiate a physiological response in trees which did not receive fertiliser N. Where N fertiliser was applied, pruning increased levels of photosynthetic activity in comparison to unpruned trees. Fertiliser N also stimulated the production of leaf area. Due to low water availability post second lift pruning, it is hypothesised that the trees response to pruning involved an interaction between the effects of N on leaf area, increases in stomatal conductance due to pruning and the effect of N on the trees ability to increase photosynthetic activity. Thesis results are discussed in relation to adoption of management strategies to maximise growth and solid wood products such as the application of supplementary fertiliser and the best timing and severity for pruning operations.