Examination of the direct and indirect effects of white-tailed deer (Odocoileus virginianus) in a deciduous forest community
White-tailed deer (Odocoileus virginianus) have increased in range and population density since European settlement and are altering forest communities. Deer impacts include direct effects, such as overbrowsing, and indirect effects, such as increased soil compaction or altered soil nutrient regimes. These effects may differ between native species that are adapted to specific conditions in the forest and invasive species that may be more tolerant of a greater range of conditions, decreasing native species abundance or richness in the community. The goal of this study was to determine how deer affect nutrient and physical characteristics of the soil and the composition of the herbaceous plant community in a deciduous forest in the Shenandoah Valley of Virginia. We measured the effects of deer on the soil characteristics by comparing the level of soil compaction and soil nutrient pools and fluxes between seven pairs of fenced and open plots after 2-3 years of deer exclusion. To document the change in plant composition between fenced and open plots, we used the initial composition of each plot as a baseline and calculated a quantitative Sorensen similarity index over successive years.
We observed significant effects of deer exclusion on two important abiotic variables. Exclusion significantly reduced soil compaction at the surface and to a depth of 15.24 cm. Deer exclusion also resulted in a greater nitrate flux, the dominant nitrogen ion in the system, indicating greater nitrogen availability for plant growth. Other soil characteristics (acidity/alkalinity, soil inorganic nutrients, and soil organic matter) measured at the onset of the growing season after 2-3 years of deer exclusion did not show differences between deer exclosures and open plots.
We did not find any significant effects of deer exclusion on the plant community composition, total abundance, or overall plant richness. Although there were no significant effects of white-tailed deer on the plant community composition, these results do indicate that overabundant deer populations are negatively impacting some components of this system. By increasing soil compaction and altering nutrient cycling, the native forest plants may be subjected to conditions they are not adapted to, decreasing their ability to compete with exotic species introduced to the system, resulting in a long-term change in the plant composition.