White-tailed deer induced changes in the germinable seedbanks of Ontario's Carolinian (deciduous) forest communities

Background/Question/Methods A long-term (18-year: 1991-2009) study of Carolinian forest dynamics in southwestern Ontario has documented the impact of white-tailed deer in several provincial and national parks, and private woodlots. This region of Canada has the highest human population density, the lowest extant natural habitat cover and some of the highest number of listed rare and endangered species. We examined the effects of deer density and forest size on plant community composition and on germinable seedbank composition in sites throughout the region, both inside and outside of deer exclosures and in deer browsed and grazed control plots. We asked how long plant communities in heavily grazed and browsed sites take to shift towards the species composition found in long-term deer exclosures (built in 1978) and in forests with low deer densities (<8 deer km-2). We predicted that in parks with clear browse lines, from which many native spring ephemerals were absent, that the absence of a germinable seedbank would create a long-term lag in community response and recovery, following deer control. Results/Conclusions High deer densities and herbivory levels significantly altered the structure and species composition of forest plant communities, in favour of graminoids and ruderal species. Plant community composition did not shift significantly after 9 years, in areas where deer densities were reduced via exclosures or herd reduction, towards those in sites with long-term reduced deer densities, in which native spring ephemerals were present. This could be attributed to impacts of deer herbivory on forest structure and light levels (increased), and on seedbank composition. Germinable seedbanks were dominated by ruderal species in sites with high deer densities, and native spring ephemerals were absent. However, after 18 years of deer density reduction, species richness, including the presence of native species has significantly increased in many sites. Surprisingly, the germinable seedbank in small-sized woodlots with an understorey plant community comprised of native plant species, such as the spring ephemeral functional group, were dominated by introduced and ruderal species. This may present challenges for habitat managers in the future.