Understanding when hotter and drier means more or less wildfire

Understanding when hotter and drier means more or less wildfire. Modeling processes governing feedbacks among vegetation, fuels, climate, and wildfire.

It is obvious and true that, all else being equal, hotter and drier summers tend to be associated with increased wildfire burn area and more severe fire effects. But can this relationship be extrapolated to the future? The key is whether "all else" will actually be equal in novel future climates. The correlation between climate variables and annual area burned characterizes the direct effect of climate on fire season weather. Climate also affects wildfire indirectly through long-term changes to productivity, phenology, mortality, and decomposition, which modify fuel load and continuity. We coupled the Regional Ecohydrological Simulation System (RHESSys) with a fire spread and fire effects model to represent the full set of feedbacks among climate, vegetation, fuels, and wildfire. Through a series of simulations in the Big Creek watershed in the Sierra Nevada we found that increased temperature alone decreased projected annual area burned relative to baseline, likely through increased fuel constraints. Drought plus warming increased annual area burned. A key mechanism, and often overlooked uncertainty, identified in these simulations is the role of decomposition in fine fuel accumulation. Decomposition itself is related to climate, increasing with increased temperature and moisture, and fine fuel loading is sensitive to decomposition rate. One imperative to achieve reliable projections of future wildfire regimes is to understand and reduce uncertainty in fine fuel accumulation.