Identifying factors related to the severity of mammalian browsing damage in eucalypt plantations

Abstract

Several mammalian herbivores damage young eucalypts growing in commercial Tasmanian plantations. Damage severity varies between plantations and damage control programs could be improved if the factors influencing this spatial variation were identified. In this study, measures of damage, herbivore abundance, plantation characteristics and the characteristics of forest adjacent to plantation areas were collected at 32 Eucalyptus nitens plantations during the first year of growth. Two exploratory analyses were conducted on two separate suites of site descriptors. The first, which modelled damage as a function of vegetation cover and herbivore scat counts, explained 71% of the between-site variation in damage severity. Damage was positively related to scat deposition rate for the brushtail possum and grass cover, and negatively related to fern (mainly bracken) cover. The second analysis modelled damage as a function of plantation characteristics that could be measured before planting and which may, therefore, be useful to forestry managers as damage predictors. This model explained 47% of between-site variation. Damage was negatively related to area to perimeter ratio and the degree of canopy closure in forest adjacent to plantations. Damage was positively related to the proportion of the perimeter that adjoined forest. Our results suggest that the importance of the brushtail possum, as a contributor to variation in damage between plantations, needs to be reviewed and that several measures of plantation characteristics have potential for use in future development of risk assessment systems.