Growth responses of Eucalyptus globulus Labill. to nitrogen application and severity, pattern and frequency of artificial defoliation

Abstract

Defoliation from herbivores can reduce growth of affected plants, and this has been demonstrated for plantation-grown Eucalyptus globulus. We undertook a field study to examine: (1) the influence of severity, pattern and frequency of defoliation on E. globulus stem growth, (2) whether fertiliser application influences growth responses to defoliation and (3) whether rate or timing of fertiliser application is important in determining growth responses. Two rates of nitrogen (N) (100 or 300 kg ha −1) were applied either pre- or post-defoliation or both. Defoliation treatments examined the effects of pattern, frequency and severity of defoliation on growth. In the absence of fertiliser, removal of 25% of leaf area from throughout the crown reduced diameter and height increment over the following 20 months. Removal of all leaf area from the upper 50% of crown had a much more dramatic effect on stem growth than did removal of all leaf area from the lower 50% of crown. More frequent defoliation increased the effect of defoliation on stem growth, and there was more of an effect following defoliation of the upper rather than lower crown. While there was no fertiliser response in undefoliated trees at the site, pre-defoliation fertiliser application maintained diameter and height increment of defoliated trees at a level similar to that of undefoliated trees. The growth response to fertiliser following defoliation was greater with pre- than post-defoliation applications of N, although there was still a significant response to post-defoliation applications. While foliar N content increased with rate of N application in undefoliated trees, defoliation generally reduced foliar N content and total phenolics. It was concluded that fertilising with N, even on sites with no discernible N deficiency, is a potential management tool for dealing with defoliation events. Management implications of the results are discussed.