The limits of statistical climate-fire modeling: what goes up must come down

Climate and fire are strongly linked, although the relationship between them is contingent on fuels and thus fire responses to climate variability and change vary considerably across ecosystems, fuels management, and land use. By comparing relationships between climate and wildfire in the western U.S., we evaluated the standard conceptual model of fire response to climate change, which is essentially that increasing temperature will cause longer fire seasons and increased area burned. However, the observational data indicate that this hypothetical response is too simple. When considered as a whole across the western U.S., fire climate responses across a gradient of water balance deficit (potential minus actual evapotranspiration) area burned varies non linearly as a function of drought, first increasing with increasing deficit but then declining with further increasing deficit. This result implies that future fire projections based on climate change scenarios must be approached with caution, as fire cannot be expected to increase indefinitely with deficit. Instead, the relationships between fuel availability and continuity likely mediate the fire-climate relationship at both local and regional spatial scales. We explore the consequences of approaching future fire projections as linear responses to climate and present a qualitative classification of ecosystem proximity to threshold changes in fire-climate response.