Landscape-Scale Impacts of Deer on Tree Ferns in South-Eastern Australia

Tree ferns (order Cyatheales) are a key component of wet forests globally, providing critical forest understorey structure and ecosystem functions. Tree ferns may be impacted by ungulates in novel habitats, but the extent and severity of these impacts are often uncertain. We aimed to determine the impact of introduced deer on tree ferns in wet forests of south-eastern Australia. Using both broadscale deer abundance and impact surveys and targeted tree fern assessments, we surveyed browsing impacts on tree ferns at over 200 sites across a range of wet forest types in south-eastern Australia where deer are present. Tree fern species, plant height and estimates of foliage biomass removed by browsing were recorded for over 4500 individual tree ferns including 1871 Cyathea australis, 2622 Dicksonia antarctica and 41 Todea barbara. Browsing impacts on tree ferns were recorded at 96% of surveyed sites, with a third to a half of tree ferns typically impacted by browsing at each site. There were no differences in recorded impact between tree fern species. Browsing of tree ferns was strongly height dependent, regardless of species, and associated with indices of deer density. Tree ferns < 100 cm were often highly impacted (mean > 20% foliage browsed), with impact declining approximately linearly with height to 200 cm, typically low 200 to 300 cm, and absent thereafter. The widespread and in many cases severe browsing on tree ferns recorded can be largely attributable to introduced deer. These impacts potentially threaten tree fern populations and diminish the vegetation structure and ecosystem function of these wet forests. Management interventions to reduce deer populations in the wet forests of south-eastern Australia are critical to protect forest integrity and function.