Deer Browsing Effects on Temperate Forest Biogeochemistry, Plant Community Composition, and Plant Chemistry

Abstract

Herbivores influence ecosystem nutrient dynamics in many ways. Impacts on soil biogeochemical processes often include increasing nutrient cycling rates under high nutrient availability and decreasing nutrient cycling rates under low nutrient availability. Herbivores also alter plant communities as well as concentrations of nutrients and secondary defense chemicals in plant tissues, which further impact decomposition and nutrient cycling. These patterns are far from universal, and interactions between soil fertility and herbivory are under continuing investigation. This dissertation focusses on the interactions between herbivores and primary producers, more specifically the white-tailed deer (Odocoileus virginianus) and temperate forest nutrient cycles. Increasing deer populations have been posing threats to their habitats for decades. Deer overabundance affects forest regeneration and understory vegetation cover and composition, with consequences across trophic levels such as insects and birds. Research thus far, however, has found inconsistent effects of deer overabundance on soil biogeochemical processes as well as on plant community structure. The net effects of deer browsing on soils remains difficult to predict, which is reflected in inconsistencies within and across studies. By sampling inside and outside deer exclosures across a network of forest sites in southeast Michigan, I investigated deer browsing effects on temperate forest soil nitrogen (N) and carbon (C) cycling along gradients of soil and litter C:N and canopy litterfall. Deer browsing increased net N mineralization rates in high nutrient environments and decreased N mineralization rates in low nutrient environments, whereas browsing decreased soil CO2 respiration in high nutrient environments and increased soil CO2 respiration in low nutrient environments. My work shows that deer browsing can have significant effects on net N mineralization and C respiration in temperate forest soils and that the direction and magnitude of deer browsing effects on soil N and C cycling vary across fertility gradients. Differences in deer browsing effects on soil processes could be mediated by plant responses to herbivory across gradients of resource availability. Deer effects on plant communities were significant but did not vary with environmental factors. Browsing significantly decreased several plant structural measures, including plant percent cover, tree seedling cover, and sapling density, but had no detectable effects on plant community composition overall. Other important browsing effects on plant community composition, however, are reflected in significant changes in percent cover of nearly 25% of plant species and 3 plant functional types with deer browsing. Furthermore, my detailed analyses of plant chemical composition in temperate forest understory plant communities showed that deer browsing alters plant chemistry and that plant species vary in their response to browsing. Together with inherent differences among plant species in chemical and nutrient concentrations, this work highlights the importance of both selective browsing and browsing-induced defenses for ecosystem nutrient dynamics. Foliar chemistry did not vary with environmental variables, indicating that among-species differences are more important than within-species responses to browsing in driving plant community chemical responses to browsing. Understanding the factors that contribute to changes in forest C and N cycling continues to be critically important, especially considering predicted scenarios of climate change. Overall, my dissertation provides support and insights into how interactions between above- and belowground processes are important drivers of ecosystem functioning.