Projecting Future Fire Regimes in a Semiarid Watershed of the Inland Northwestern United States: Interactions Among Climate Change, Vegetation Productivity, and Fuel Dynamics

Fire regimes are influenced by both exogenous drivers (e.g., increases in atmospheric CO2 and climate change) and endogenous drivers (e.g., vegetation and soil/litter moisture), which constrain fuel loads and fuel aridity. Herein, we identified how exogenous and endogenous drivers can interact to affect fuels and fire regimes in a semiarid watershed in the inland northwestern United States throughout the 21st century. We used a coupled ecohydrologic and fire regime model to examine how climate change and CO2 scenarios influence fire regimes. In this semiarid watershed, we found an increase in burned area and burn probability in the mid-21st century (2040s) as the CO2 fertilization effect on vegetation productivity outstripped the effects of climate change-induced fuel decreases, resulting in greater fuel loading. However, by the late-21st century (2070s), climatic warming dominated over CO2 fertilization, thus reducing fuel loading and burned area. Fire regimes were shown to shift from flammability- to fuel-limited or become increasingly fuel-limited in response to climate change. We identified a metric to identify when fire regimes shift from flammability- to fuel-limited: the ratio of the change in fuel loading to the change in its aridity. The threshold value for which this metric indicates a flammability versus fuel-limited regime differed between grasses and woody species but remained stationary over time. Our results suggest that identifying these thresholds in other systems requires narrowing uncertainty in exogenous drivers, such as future precipitation patterns and CO2 effects on vegetation.