Large herbivores are well known to have profound effects on grassland processes. Yet the mechanisms that underlie their important impacts on grasslands are not well understood. In this study, we investigated how herds of migratory ungulates influenced several important biogeochemical pathways that control plant growth in grasslands of Yellowstone National Park (YNP). For three years (2012-2014), we measured plant aboveground biomass, leaf nitrogen (N) concentration, and soil moisture and N availability each month during the growing season (April-October) inside and outside replicated exclosures in dry and mesic grasslands. The influence of herbivores on each plant and soil variable was examined using a mixed linear effects model with the variable as the fixed effect and year and month as random effects. Results were used to develop a Bayesian model to examine the effects of YNP ungulates on the different biogeochemical pathways that control plant production. We were particularly interested in how grazer effects on soil moisture, soil N availability, leaf N content, and the size of the plant canopy regulated monthly time steps of plant growth.
Results/Conclusions
Results revealed markedly different mechanisms for grazer control of shoot production at the dry vs mesic grasslands. At the mesic site, herbivores fed back positively on aboveground production by stimulating soil N availability and leaf N concentration, which both facilitated shoot growth. At the dry site, where low soil moisture severely limited plant and soil processes, herbivores inhibited productivity by reducing the amount of photosynthetic tissue, the primary control on the monthly rate of shoot production. These findings indicate the important role of soil moisture in governing consumer effects on topographically variable YNP grassland production, allowing for a positive feedback on plant-soil N dynamics and plant production when soil water is plentiful, and a negative feedback when low soil water inhibits that feedback.