Does hot and dry equal more wildfire? Contrasting short- and long-term climate effects on fire in the Sierra Nevada, CA

This paper uses a new model, RHESSys-WMFire, that integrates ecohydrology with fire spread and effects to simulate a 60-yr time series of vegetation, fuel development, and wildfire in a 6572-ha watershed in the Southern Sierra Nevada, USA, with a factorial design of increased temperature and severe drought. Climate and wildfire are closely linked. Climate regulates wildfire directly over short timescales through its effect on fuel aridity and indirectly over long timescales through vegetation productivity and the structure and abundance of fuels. Prediction of future wildfire regimes in a changing climate often uses empirical studies that presume current relationships between short-term climate variables and wildfire activity will be stationary in the future. This is problematic because landscape-scale wildfire dynamics exhibit non-stationarity, with both positive and negative feedback loops that operate at different temporal and spatial scales. This requires that such feedbacks are accommodated in a model framework from which wildfire dynamics are emergent rather than pre-specified.

All climate scenarios had an initial pulse of elevated area burned associated with high temperature, low precipitation, and high fine fuel loading. There were positive correlations between annual area burned and mean annual maximum temperature and negative correlations with annual precipitation, consistent with understood direct effects of climate on wildfire in this system. Decreased vegetation productivity and increased fine fuel decomposition were predicted with increased temperature, resulting in long-term reduced fine fuels and area burned relative to baseline. Repeated extreme drought increased area burned relative to baseline and over the long-term had substantially reduced overstory biomass. Overstory biomass was resilient to repeat wildfire under baseline climate. The model system predicts that the short-term direct effects of climate on wildfire can differ from long-term indirect effects such that the simple maxim hotter/drier equals more wildfire can be both true and false, depending on scale.