Integrating fire effects on vegetation carbon cycling within an ecohydrologic model

Wildfire affects landscape ecohydrologic processes through feedbacks between fire effects, vegetation growth and water availability. Despite the links between these processes, fire is rarely incorporated dynamically into ecohydrologic models, which couple vegetation growth with water and nutrient fluxes. This omission has the potential to produce inaccurate estimates of long-term changes to carbon and water cycling in response to climate change and management. In this study, we describe a fire-effects model that is coupled to a distributed ecohydrologic model, RHESSys, and a fire-spread model, WMFire. The fire-effects model has intermediate structural complexity so as to be commensurate with the ecohydrologic model. The fire-effects model includes processes for litter and coarse woody debris consumption, processes for fire-associated vegetation mortality and consumption, and takes into account canopy structure (i.e., ladder fuels) for propagation of fire effects into a forest canopy. We evaluated the model in four Western U.S. sites representing different vegetation, climate, and fire regimes. The fire-effects model was able to replicate patterns of expected fire effects across different ecosystems and stand ages without being tuned to produce them; an emergent property of the model. Fire effects of shrubland and understory vegetation varied with surface fire intensity, by design, and fire effects in forest canopies were sensitive to parameters associated with the buildup of litter and understory ladder fuels. These findings demonstrate that the fire-effects model provides an effective tool for evaluating the post-fire changes to physical and ecological processes. Future work will project future fire regimes and improve understanding of watershed dynamics under climate change and land management via the simulation of the fire-effects model with fire spread and ecohydrology.