Sensitivity and resilience of high-severity fire regimes to climatic variability from centuries to millennia
Robust links between climate and wildfire activity at annual timescales suggest that climatic warming will lead to increases in fire frequency and severity. However, feedbacks and interactions with vegetation, in response to climate itself and altered fire regimes, will mediate the direct impact of climatic change on wildfire regimes. Understanding these mechanisms is challenging, particularly in high-severity fire regimes, because their dynamics evolve over multiple decades to centuries. Retrospective analyses utilizing fire history records offer one of the best ways to assess fire-regime sensitivity to climatic variability across multiple time scales. We use historical and paleo records of fire, climate, and vegetation to highlight themes from high-severity fire regimes from western North America relevant for anticipating fire-regime response to future climate change.
At millennial time scales, the paleo record indicates that fire regimes can be particularly sensitive to climate-induced changes in vegetation. In the absence of large-scale vegetation change, the millennial-scale average rate of burning in many paleo records is surprisingly complacent despite evidence of notable climatic change. This long-term complacency is observed in Holocene records from the southern Rocky Mountains north to the Alaskan arctic. At small spatial and temporal scales, high variability exists and can often be attributed to the direct impacts of climatic variability on summer moisture deficits, consistent with fire-climate relationships in stand-replacing fire regimes at annual timescales. Feedbacks among climate, vegetation, and fire are also apparent at these shorter temporal scales, with vegetation changes limiting or promoting flammable fuels at landscape scales, and subsequently mediating the links between climate and fire activity. The paleo record supports predictions that 21st-century warming will likely lead to increased burning, but it further suggests that fire-regime response will be more complicated than expected based on direct, annual-scale fire-climate relationships.