Historical Climate and Suppression Effects on Simulated Fire and Carbon Dynamics in the Conterminous United States

Abstract

Active management for ecosystem restoration should accommodate future changes in climate. Simulation modeling is one of the few tools available to explore the effects of changing climate on fire and ecosystem dynamics. MC1, a dynamic global vegetation model (DGVM) for simulating the broadscale response of ecosystems to climate, includes a fire disturbance module that generates process-based simulations of fire occurrence, behavior, and effects. MC1 simulations for the historical period in the conterminous United States demonstrated the skill of the fire module. Annual trends in simulated and observed area burned were significantly correlated, and observed seasonal and interannual shifts in the distribution of fire occurrence were also successfully simulated. The model also successfully reproduced the observed relationship between large fire years and drought, as well as the observed pattern in fire occurrence associated with longer-term climatic cycles in the western United States. The results indicate that the annual area burned in the conterminous United States over the last 50 years would have been 8 times greater on average without fire suppression. The simulation without fire suppression indicated that biomass accumulation during relatively wet years is nearly balanced by biomass consumption during relatively dry years, demonstrating the regulatory function of unconstrained fire in the net biological production of ecosystems. With fire suppression, the model simulated a carbon sink of 180 Tg/yr over the last half-century consistent with empirical estimates of the current sink due to fire exclusion. The simulated carbon sink accumulated primarily as aboveground woody biomass in semiarid regions of the western United States where increased woody vegetation density was widely observed over the recent historical period. The greater intensity and severity of modern wildfires due to higher fuel loadings and more multilayered stand structures was also reproduced by the model simulations.