Interactive effects of white-tailed deer and invasive plants on temperate deciduous forest native plant communities

Background/Question/Methods
The relationship between white-tailed deer (Odocoileus virginianus) populations, herbivory and invasive species proliferation has received little study, despite its potential impacts on native biodiversity and natural areas management. To address this important ecological issue, we initiated a controlled field experiment in 2005 at three high deer density study sites in mid-Atlantic temperate upland deciduous forest (Conservation and Research Center, Front Royal, VA; Great Falls National Park, VA; and the Goldmine tract of the Chesapeake and Ohio Canal National Historical Park, MD). Following initial baseline vegetation surveys (all herbaceous plants and woody plants ≤ 30 cm in height) of 333 4x4-meter randomly located plots, control, deer exclusion (fenced) and invasive species removal (hand pulling) treatments were applied in a 2x2 factorial design at each of the study sites. Plots were resurveyed in 2007 to estimate the understory vegetation response in terms of native species richness, diversity and woody stem counts. Response variables were analyzed as mixed-model repeated measures analyses of covariance using SAS version 9.0 software.

Results/Conclusions

The forest understory plant community responded positively after 1.5 years of exposure to the experimental treatments, and the non-woody components were largely responsible for the significant changes thus far. Native herbaceous species richness increased significantly at two of the three study sites, with the invasives removal treatments having the greatest response. Significant increases in native forb species diversity occurred at the same sites, with the invasives removal treatments again showing the largest increases but mainly in plots that also had high initial invasive species cover. Woody species richness, diversity and stem numbers displayed significant relationships with the canopy species richness covariate and showed increasing trends at all sites over time, but significant treatment effects did not appear by 2007. The results for non-woody native plants, particularly in those plots that were treated with hand-pulling of invasives only, support the conclusions that invasive plant cover negatively impacts their survival and that collateral damage to native species did not occur. The similar response in the invasives removal treatments indicates the primary inhibitor for most non-woody natives is the presence of invasive plants and not deer herbivory. Detection of significant treatment effects on woody species may occur after a scheduled plot resurvey in 2009, although positive trends were documented here. Deer management, such as fenced exclusion or population reduction, in the absence of invasive plant removal, may therefore be insufficient to promote restoration of the native plant community.