Browsing of Eucalyptus nitens seedlings by marsupial herbivores

Browsing by marsupials on Eucalyptus nitens (Shining gum) was studied in two newly-planted forestry plantations in north west Tasmania, Australia. Sodium monofluoracetate (1080) poison was used at one site (Pearsefield), but not the other (Clipper West). Five seedlots of E. nitens sourced from Southern New South Wales (NSW), Toorongo, Northern NSW, Macalister and Glen Tunnel were planted at each coupe to identify any seedlot preferences by herbivores and to assess differences in levels of damage. Initial heights of the seedlots differed. Three components of seedling damage were analysed: impact of browsing on seedling height, severity of damage and type of damage. Herbivores in the area were recorded in spotlighting surveys. Five months after planting, 55% of seedlings at Clipper West sustained over 50% damage to their foliage and stem. Glen Tunnel and Macalister seedlings were significantly more damaged than the Southern NSW seedlings. At Pearsefield, only 28% of the seedlings sustained over 50% damage. Glen Tunnel and Toorongo seedlings were significantly more damaged than the other three seedlots at this plantation. At Clipper West, 78% of the damaged seedlings had their apical buds removed, compared with only 50% at Pearsefield. Five months after planting, mean height of all seedlings was less than at planting due to browsing. Reductions in height were greater at Clipper West than at Pearsefield. In terms of the amount of stem eaten, herbivores preferred the Toorongo and Glen Tunnel seedlots at both plantations. The Southern NSW seedlings were the least preferred. The Tasmanian pademelon (Thylogale billardierii) was the most frequenfly observed herbivore at Clipper West (7.0/hectare (ha)), compared to 1.2/ha for the Bennett's wallaby (Macropus rufogriseus) and 0.7/ha for the common brushtail possum (Trichosurus vulpecula). Herbivore densities were lower at Pearsefield, and the brushtail possum (0.4/ha) was the most frequently observed. 1080 poison was presumably effective in reducing herbivore numbers, but its effect appeared to be temporary. Feeding trials, using a captive colony of Tasmanian pademelons, also examined preference and damage to seedlots of the five seedlots E. nitens. Tasmanian pademelons preferred Glen Tunnel seedlings on the basis of the amount of stem eaten, and damaged them more than any other seedlot. This study suggests that smaller sized seedlings may be more prone to damage, because removal of even a small amount of plant represents a large reduction in a seedling's total biomass. Planting larger seedlings may be a simple means of reducing the impact of herbivores. Management implications of all results are discussed with respect to forestry in Australia.