Effects of white-tailed deer over-browse on soil microclimate and leaf litter decomposition

Background/Question/Methods

            Ungulate herbivores influence decomposition in forested ecosystems through several pathways. Most models rely on the ability of herbivores to drive changes in litter nutrient composition by modifying resource availability to decomposing organisms. However, herbivore-driven changes in soil environments may represent another mechanism through which herbivores affect carbon cycling. White-tailed deer (Odocoileus virginanus) are overabundant throughout the eastern United States, reaching densities as high as 75/km². At high densities deer drive changes in forest composition including loss of understory strata and suppression of seedling recruitment, halting gap succession. Our objective was to assess whether deer over-browse influences soil carbon balance by altering the soil physical environment.

            Four site pairs, each consisting of a deer exclosure and a site exposed to deer browse, were established in the Piedmont region of New Jersey. A litterbag experiment was undertaken at these site pairs to test the hypothesis that increased soil temperature, resulting from increased light due to reduced vegetation structure, would accelerate decomposition rates at the exposed sites. Litterbags containing Acer rubrum litter were deployed at 5 plots within each site, and  collected at 3 month intervals. Vegetation structure and soil temperature were measured at each plot. Litter quantity, litter C:N ratio, and soil C:N ratio were also measured to assess the influence of deer over-browse on microbial resource availability. Decomposition curves were fitted for all sites, and the relative contribution of soil temperature and the soil resource variables to differences in decomposition rate between treatments was assessed using Bayesian analysis of variance.

Results/Conclusions

            Our results show that overabundant deer are capable of influencing litter decomposition rate in temperate deciduous forests, but not by influencing soil temperature or microbial resource availability. The vegetation survey found differences in vertical profile structure, but these results did not correspond with significant differences in soil temperature, litter quantity, litter C:N, or soil C:N ratio between site types. Preliminary results from the litterbag experiment demonstrate a moderate decrease in decomposition rate (3-10 % mass loss) in exposed sites relative to deer exclosures after 6 months. This finding is contrary to our hypothesis that exposed sites would experience faster litter decomposition. Ongoing work includes monitoring of soil moisture status and measurement of related soil variables (soil bulk density, organic matter content, root density) to assess whether deer driven changes in the rhizosphere, through soil compaction and/or root loss, present a mechanism to explain the results of our decomposition experiment.