Biomass and fire dynamics in a temperate forest-grassland mosaic: Integrating multi-species herbivory, climate, and fire with the FireBGCv2/GrazeBGC system

Landscape fire succession models (LFSMs) predict spatially–explicit interactions between vegetation succession and disturbance, but these models have yet to fully integrate ungulate herbivory as a driver of their processes. We modified a complex LFSM, FireBGCv2, to include a multi-species herbivory module, GrazeBGC. The system is novel in that it explicitly accommodates multiple herbivore populations, inter- and intra-specific spatial forcing of their forage demands, and site-specific dietary selectivity to interactively modify biomass, fuels and fire behavior across a landscape and over time. A factorial experiment with five grazing regimes, three climates and two fire-management scenarios generated interactive influences on undergrowth biomass (shrub, herb, total), surface-fire (fire-line intensity; flame length; scorch height; soil heat; CO, CO2, CH4, and PM2.5 emissions), and the landscape’s fire-return interval. Herbivory’s effects increased with biophysical site potential and herbivore forage demand, but its effects were also contingent on climate and fire-suppression. Multi-species grazing modified biomass and fire within stands and biophysical sites, but regimes involving only wildlife or livestock were less effectual. Multi-species herbivory affected the landscape’s fire-return interval, but otherwise it did not “scale up” to significantly modify total landscape respiration, primary production, carbon, or the total area burned by individual fires. As modeled here, climate change and the effectiveness of future fire suppression exerted stronger effects on landscape metabolism and carbon than did herbivory.