Isolating the effects of white-tailed deer on the vegetation dynamics of a rare maritime American holly forest

Effects of multiple herbivores were isolated in a rare, old-growth, maritime forest that has been affected by high white-tailed deer (Odocoileus virginianus) populations for several decades. Three decades of change in the plant community was described using permanent plots and nearly two decades of recovery using deer exclosures. Additional short-term exclosure experiments were used to separate the effect of rabbit and vole herbivory and the influence of canopy type on understory vegetation. We assessed whether natural enclosures formed by high densities of greenbrier (Smilax rotundifolia and S. glauca) provide spatial refugia for plant species from deer. Significant differences in fenced plots indicate that deer, rabbits and voles individually affect the understory vegetation, though rabbits and voles are secondary to deer that are the major herbivore in the American holly (Ileex opaca) forest. Plant cover decreased within permanent plots from 1967 to 1986; during the same time period deer densities increased drastically island-wide. Since plots were fenced in 1986 no additional significant changes were detected, but this is likely due to an inadequate sample size. The cover of all species depended on the interaction of fencing and canopy type with significantly higher plant cover in fenced plots beneath mixed or exclusively deciduous canopy. The proportion of woody stems surpassing 0.5 m tall was significantly higher when stems were fenced. Browsing impacts were apparent on approximately a quarter of the vegetation in June 2003. Plant cover, richness and diversity were higher within natural greenbrier enclosures. Overall the exclosure studies indicate the potential natural recovery that could occur within the forest if deer herbivory were limited. Few species have been extirpated, although several are confined to the greenbrier enclosures. Under the current level of herbivory, the maritime holly forest composition will be altered, changing the characteristic canopy of a critically imperiled plant community.