Browsing rates and ratios provide reliable indices of ungulate impacts on forest plant communities

Abstract

Ungulate browsing affects many plant species, shifting patterns of relative abundance, plant community diversity, and ecosystem processes. Despite the strength and diversity of ungulate impacts, we lack comprehensive methods and a unified network for tracking and comparing ungulate impacts across the Great Lakes region. The Great Lakes Network Office (GLN) of the National Park Service has identified white-tailed deer as a keystone herbivore that could threaten the health of Great Lakes forests. To supplement their permanent vegetation monitoring program, GLN monitors deer and moose browsing pressure on woody and herbaceous plants at nine national parks in the region. We used data from these surveys to investigate relationships among patterns of browsing across species and sites with the goal of inferring efficient and reliable indicators of ungulate impacts. We also investigated the relationships between patterns of browsing on woody vegetation and among-site variation in the presence and abundance of understory species. Our results showed that ungulates clearly prefer certain woody species, but these vary across the region in response to local community composition. Ungulate impacts on understory species are correlated with woody browsing impacts but also vary with local ecological conditions. To build a more efficient and accurate composite indicator of overall ungulate impacts, we combined data on woody plant browsing with data on the abundance of specific understory species to infer overall levels of threat from ungulate browsing. Such combined indicators improve our ability to monitor the impacts of ungulate browsing, particularly if such methods can be expanded and standardized across the region.