Shedding Light on the Problem of Deer Overgrazing in Carolinian Forests

In nearly a decade of research we have carried out a multi-faceted study of the impact of deer grazing in three major Carolinian parks: Point Pelee National Park, and Rondeau and Pinery Provincial Parks. This research has had a direct impact on management policy. We will review key findings of the research program and highlight what we consider to be our most general research finding, namely that the state of the overhead canopy in Carolinian forests appears to have a major impact on the composition of understorey plant communities. We suggest that deer overgrazing has initiated a process that has signiticantly altered understorey light conditions. Our hypothesis is that increased canopy gaps, initially caused by deer preventing forest regeneration, have led to trees being more susceptible to wind throw. This further opens the canopy, leading to increased light levels in the understorey, which in turn drive changes in the vegetation. Non-native, invasive species can take advantage of the increased light conditions and appear to replace and suppress native woodland species, which are adapted to shade. The forest may then switch to some alternative stable state. Currently, analysis of long-term data sets is aimed at evaluating this hypothesis. Our current research aims to quantify the relationship between understorey light levels and the plant community, and it will establish whether there is some threshold light level beyond which many vulnerable native understorey species cannot survive, and are suppressed by exotics. In this respect it is of general interest to anyone working in a degraded, highly disturbed forest, with an interest in habitat restoration.