Defining deer overabundance and threats to forest communities: From individual plants to landscape structure

Changes in habitat and reduction in predation and hunting pressure are two primary causes of high-density populations of white-tailed deer (Odocoileus virginianus) in many areas of eastern North America. Despite increasing recognition of the major effects deer exert on forest communities, deciding when deer are overabundant remains a major challenge to managers charged with conserving native plant communities. In this context, we define overabundance as a condition where deer are causing the local extinction of a native plant species. Because this definition is difficult to apply from a management perspective, we outline an approach using native understory forbs. Indicator species are selected based on combined criteria of palatability to deer, leaf and flower morphology, and demographic characteristics. Four indices related to plant population viability are derived from understory survey data: flowering rate, mean stem height, stage-class distribution, and deer browsing pressure. We apply this analysis to Trillium populations from forests in Minnesota (highly fragmented agricultural landscape with varying deer densities), Pennsylvania (forested and fragmented landscape with long-term high deer densities), and New York (forested and fragmented landscape with long-term low deer densities). We observed two distinct types of deer-affected plant populations. In sites with moderate or recent increases in deer density, Trillium populations were characterized by low mean plant size, high browsing rates, intermediate flowering rates, and a size class distribution lacking large, reproductive plants. Sites affected by long-term high deer densities exhibited low browsing rates on Trillium, low Trillium flowering rates, a population structure lacking both large and small plants, and high summer browsing pressure on woody saplings. We suggest these combined indices be used to assess deer browsing impact, and we discuss the role of landscape structure and deer density in defining deer overabundance.