Effects of partial defoliation on closed canopy Eucalyptus globulus Labilladiere: Growth, biomass allocation and carbohydrates

Herbivory caused by leaf-eating insects continues to be a severe risk to forest trees and forest stands. Besides quantifying the extent of defoliation, the quantification of the trees’ response to the loss of biomass is a challenge to plant ecologists and foresters alike, and an important precondition for the application of appropriate silvicultural measures. While many defoliation studies target small trees as model systems, little is known about the effect of defoliation on larger trees. In the present study, we investigated the effects of 45% removal of leaf area on growth, biomass allocation and carbohydrates of 13m tall, four-year-old, plantation Eucalyptus globulus Labill. in southern Tasmania. Responses were measured in three crown zones (lower, middle, upper) over a period of 11 months. Height increment was unaffected by defoliation, but diameter increment was significantly reduced 155 days after treatment. Defoliation treatment had no effect on stem volume and biomass partitioning compared with the control treatment. Trees responded to defoliation by decreased branch senescence in the lower crown, greater leaf area development in the mid crown and increased specific leaf area. Defoliation reduced concentration of soluble sugars (SS) in foliage by 22% and the pools of SS in the coarse roots by 34%. Decrease in root SS was only observed in 10–15mm diameter class and the rootball. We concluded that this four-year-old E. globulus stands with a closed canopy was able to tolerate a single, partial artificial defoliation event, which is similarly observed with younger trees.