Recent increases in fire activity highlight major uncertainties about how disturbances will interact with ongoing climate change. In the western U.S., shifting disturbance regimes are predicted to lead to long-lasting directional changes or shifts in biogeochemical states, influencing carbon and nitrogen balance over large spatial and temporal scales. While these ideas have a strong conceptual basis and empirical support on decadal timescales, data have been lacking to test them over longer timescales - and to consider their implications for future projections - until only recently. Here we present preliminary finding from a new project investigating coupled climate-fire-ecosystem dynamics over the past several millennia across some of the most fire-prone regions in the Rocky Mountains. The project utilizes lake-sediment records in the Northern and Southern Rockies to reconstruct the frequency, severity, and spatiotemporal synchrony of past fire activity and the biogeochemical impacts, from landscape to regional scales. Coupled climate-fire-ecosystem dynamics will be examined through a suite of ecosystem simulations comparing paleo-informed to equilibrium scenarios. Here we present a 4500-year paleoecological record of fire activity from a Colorado subalpine forest with ecosystem modeling to investigate how fire-regime variability impacts soil carbon and net ecosystem carbon balance in a single subalpine watershed.
In a lodgepole-pine dominated subalpine forest in the Southern Rockies, we found that carbon trajectories in a paleo-informed scenario differed significantly from an equilibrium scenario (with a constant fire return interval), largely due to variability in the timing and severity of past fires. Paleo-informed scenarios contained multi-century periods of positive and negative net ecosystem carbon balance, with magnitudes significantly larger than observed under the equilibrium scenario. Further, the variability in past fire activity created legacies in soil carbon trajectories that lasted for millennia, and which were of greater magnitude than changes from simulations with a 2 °C climate warming (and constant fire return intervals). Our results imply that fire-regime variability is a major driver of carbon trajectories in stand-replacing fire regime, with compounding impacts that can last over multiple disturbance intervals. Predicting carbon balance in these systems, therefore, will depend strongly on the ability of ecosystem models to represent a realistic range of fire-regime variability over the past several centuries to millennia.