Estimating the impacts of browsers on forest understories using a modified index of community composition

• Managers need better approaches for estimating impacts of increasing deer densities.
• We develop an indicator of plant composition to predict local-scale deer impacts.
• We find that our indicator FQAI positively correlates with deer densities.
• This is because deer remove fast-growing species with low conservation value.
• FQAI improves upon other methods for monitoring deer impacts.

Increases in deer densities throughout temperate regions are negatively affecting forest conservation, but approaches for estimating these impacts are labour intensive and thus prohibitive for many managers. Plant indicators provide a rapid and cost-effective approach for measuring the impacts of disturbances on forest communities, and may inform managers both about the need for conservation actions to mitigate deer disturbance and their outcomes. The Floristic Quality Index (FQI) is one metric that assesses plant community composition by assigning coefficients of conservatism (C) to individual species based on their life history traits, including responses to disturbances. FQI should therefore reflect the community-level impacts of browsing as a disturbance, but little information exists on its use within forests or along browsing gradients. Here, we improved the classical measure of FQI by generating a new Floristic Quality Abundance Index (FQAI) that better captures plant community responses to disturbance. We then tested whether it reflects local densities of white-tailed deer (Odocoileus virginianus) and their impacts on forest understories using ground cover data from twelve sites across south-eastern Ontario, Canada. By excluding deer at five sites, we found that they preferentially removed plant species with lower C values, thereby explaining positive relationships between deer densities and FQAI across our sites. We propose that FQAI is an assessment tool that may improve the effectiveness of limited conservation resources for monitoring deer impacts.