Growth and physiological responses of Eucalyptus globulus Labilladière following defoliation

Many defoliating herbivores cause damages to Eucalyptus globulus Labill. plantations, reducing the quality and value of the wood products they source. This damage ranges from mild to severe removal of leaf surface, and can result in significant reductions in yield. Our knowledge of physiological responses to defoliation in this species is incomplete, with previous studies focusing on very young trees and ignoring the potential interaction of abiotic stress with defoliation. Projections of future climates in many of the eucalypt-growing parts of the world suggest that abiotic stress, particularly degree of water stress will increase. In addition, there is strong evidence that changing climate, and particularly increasing temperature, is likely to favour many of the defoliating pests commonly associated with E. globulus. The objectives of work described in this thesis were to investigate the physiological strategies adopted by E. globulus in response to a defoliation event, determine their effects on growth and water relations, and examine the interaction with limited water supply, with the aim of improving our capacity to predict the impact of defoliation on tree productivity and water use. The main studies investigating tree responses to defoliation have utilised artificial defoliation treatments rather than natural defoliation because of logistical constraints. Although artificial manipulations are assumed to have significant advantages, the adequacy of the artificial method has been questioned in term of accuracy and differences in the plant responses. I tested that E. globulus respond differently to both artificial and natural defoliations. The results showed that the directions of response to artificial and insect defoliation were very similar. However, the influence of differential magnitude of the responses was more difficult to ascertain. I conclude that artificial defoliation may not accurately reflect the full strength of effects from insect defoliation, and caution must be exercised in extrapolating results of simulated herbivory experiments. Most previous studies of E. globulus responses to defoliation have focused on young, pre-canopy closure trees, and none have examined physiological responses of older trees to defoliation. Substantial defoliation can occur post-canopy closure. The effects of a single defoliation event on 4-year-old E. globulus were investigated on growth, photosynthetic and water relation responses in non-limited water supply conditions. The trees responded to removal of 45% of leaf area by a transient change in stem growth, change in crown architecture, the up-regulation of photosynthesis likely via the improvement of tree water status. It was concluded that 4-year-old E. globulus were able to compensate for the loss of foliage. Plantations of E. globulus are being established increasingly on lower rainfall sites, and in addition drought conditions are projected to increase in many areas of Australia over the next century. There is little understanding of the interactions of water stress and defoliation, although some results suggest that defoliation may be beneficial to trees growing under water limitation. I tested the hypothesis that partial defoliation would alleviate the effects of water stress. The effect leaf removal on 75% of crown length of 1-year-old E. globulus on growth, gas exchange and water use was examined in irrigated and rain-fed plots. Over a short-term period, trees responded to the interaction of limited water supply and defoliation by maintaining tree growth, increasing tree transpiration rate per unit leaf area, canopy conductance and hydraulic conductance, while maintaining the gradient of leaf water potential constant. It was concluded that defoliated trees were able to ease the effect of water stress by ii improving plant water status. Also, the findings were meeting the requirements of the theoretical hydraulic model.