| White-tailed deer populations have recovered from low population numbers in eastern North America and Southern Canada during the early 20 th Century, that were caused by high hunting pressure. At high densities, white-tailed deer are considered keystone species that significantly alter the plant community composition and ecosystem dynamics of forests. Increasing white-tailed deer populations across southern Ontario, the most densely populated, industrialized, urbanized and intensively farmed part of Canada, have led to the development of the Provincial Strategy for Managing Human-Deer Conflicts. While deer populations have been extensively studied in rural natural habitats, there have been few studies of deer impacts in natural habitats located in urban and suburban areas.;In this study, a diverse set of metrics was used to determine the impacts of grazing and browsing by white-tailed deer on the vegetation of three Environmentally Significant Areas in London, Ontario, Canada. The three sites: Sifton Bog, Meadowlily Forest and Medway Forest, varied in deer population density, from high (Sifton Bog) to low (Meadowlily Forest). Deer exclosures were built in both upland forest (all sites) and bog sites (only at Sifton), and also in interior forest and edge locations at Meadowlily. Understory plant community composition was measured in spring and summer 2008 and 2009 in both grazed (control) sites and within exclosures. Trillium grandiflorum, an indicator of deer herbivory pressure, was measured in 2008. Woody stand structure and available winter deer browse were measured in 2007. In areas where deer densities were low, current annual growth and available winter deer browse were significantly higher than in areas with high deer densities. Forest plant communities in Sifton Bog and Medway Forest were characteristic of those in other sites that have experienced high levels of long-term deer herbivory: overall numbers of T. grandiflorum plants were extremely low (n ≤ 5), indicating high levels of physical disturbance and selective grazing. T. grandiflorum densities and heights were greatest in the interior forest at Meadowlily Woods, while deer herbivory also affected plant community composition at the forest edges of the Meadowlily site. Species diversity in the 0-2 m forest layer was low in these sites, and there was an absence of forest regeneration, leading to a missing shrub layer, as was the case at both Medway and Sifton forest areas. While all plant species recorded in the site with lowest deer densities (Meadowlily) were native, many non-native, plant species, some of which are known to be invasive, were found in sites with high deer densities. Buckthorn (Rhamnus) species were common in many plots. The absence of T. grandiflorum plants from areas with high deer densities was similar to the situation in Point Pelee National Park in the early 1990s, prior to deer herd reductions. Furthermore, the lack of any vegetation response in the deer exclosures over two growing seasons, indicated that there is a low to absent germinable forest seedbank. If deer population densities were to be reduced, any forest recovery, in terms of species diversity and woody stand regeneration is likely to take decades and to be influenced by the presence of non-native species in the regional and local seed rain. In the absence of deer population reductions, there will be a continued lack of forest of regeneration and decline in biodiversity. |
