White-Tailed Deer (Odocoileus virginianus) Exclusion Shifts Soil Carbon Dynamics in Mature Oak-Dominated and Hemlock-Dominated Forest Stands

Abstract While the direct effects of white-tailed deer ( Odocoileus virginianus ) on vegetation have been intensively studied, less is known about the indirect and interactive effects of herbivory on lower trophic levels, such as soil microbes and their processing of carbon pools. We explored how carbon dynamics shift with release from over-browsing by white-tailed deer in two mature stands of oak and hemlock trees. We measured soil carbon pools (for example, soil organic matter, carbon stocks, litter biomass, and litter stabilization) and fluxes (for example, soil respiration, methane uptake, microbial substrate use, and litter decomposition) using a spatially balanced survey design inside and outside two 24-year-old deer exclosures, one in each forest stand. Soil carbon pools were higher inside the exclosures than in deer-browsed plots in both forest stands, but the effect of deer herbivory on fine-scale spatial patterning of soil carbon pools and mean carbon fluxes varied by forest type. Release from deer herbivory in the oak stand increased the patchiness of soil pools and led to higher litter decomposition, soil respiration, and methane uptake rates. Release from deer herbivory in the hemlock stand did not affect the spatial structure of soil pools, had little effect on methane uptake, and had negative effects on litter decomposition and soil respiration. These differences may be due in part to the interactive effects of two herbivores, deer and the hemlock woolly adelgid ( Adelges tsugae ), that appear to be limiting regeneration and promoting the proliferation of monodominant hay-scented fern ( Dennstaedtia punctilobula ) in the hemlock-dominated stand. Our work suggests that future efforts consider multiple zoogeochemical stressors simultaneously, in addition to variation in environmental templates, to explain uncertainties in carbon pools and fluxes in temperate forested ecosystems.