Effects of white-tailed deer on populations of an understory forb in fragmented deciduous forests

The effects of grazing by white-tailed deer (Odocoileus virginianus) on populations of Trillium spp. were examined in remnant, old-growth patches of the highly fragmented Big Woods forest ecosystem in southeastern Minnesota. We conducted three separate studies involving an exclosure experiment, transplant experiments, and comparisons of Trillium populations among study sites. The highest grazing intensity was observed where deer occurred at high overwinter concentrations (similar to 25-35/km(2)); significantly lower grazing intensities occurred at low overwinter density (similar to 5-10/km(2)). Deer focused their grazing on large, reproductive plants; at sites with high deer density, Trillium population structure was skewed toward small plants, and deer consistently caused over 50% reduction in reproduction during the growing season. Protection of individual plants from deer for two growing seasons resulted in dramatically increased flowering rates and significantly greater leaf area compared to control plants. No significant impact of current-year herbivory on reproduction in the following year was detected. Nevertheless, flowering rates at one site with high overwinter deer densities for at least the past 5 years suggest that the cumulative effects of grazing over several years can reduce reproduction in subsequent years. Transplant experiments with Trillium grandiflorum also showed that deer had significant effects on growth and reproduction where deer occur at high density. Our results suggest that changes in landscape structure and local deer abundance have altered plant-deer relationships such that grazing can lead to the local extirpation of sensitive forbs such as Trillium spp. As a result, active, long-term management of deer at low densities appears necessary for the conservation and restoration of fragmented forest communities in eastern North America.