Rocky Mountain subalpine forests now burning more than any time in recent millennia

Philip Higuera, Ecosystem and Conservation Sciences, University of Montana, Missoula, MT, Bryan N. Shuman, Department of Geology and Geophysics, University of Wyoming, Laramie, WY and Kyra Wolf, Systems Ecology Graduate Program, University of Montana, Missoula, MT

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Background/Question/Methods
The 2020 fire season punctuated a decades-long trend of increased fire activity across the western United States, nearly doubling the total area burned in the central Rocky Mountains since 1984. Understanding the causes and implications of such extreme fire seasons, particularly in subalpine forests that have historically burned infrequently, requires a long-term perspective not afforded by observational records. We place 21st-century fire activity in subalpine forests in the context of climate and fire history spanning the past 2000 years using a unique network of 20 paleo-fire records.
Results/Conclusions
Due largely to extensive burning in 2020, the 21st century fire rotation period is now 117 yr, reflecting nearly double the average rate of burning over the past 2000 yr. More strikingly, contemporary rates of burning are now 22% higher than the maximum rate reconstructed over the past two millennia, during the early Medieval Climate Anomaly (770-870 CE), when Northern Hemisphere temperatures were c. 0.3°C above the 20th-century average. With 21st-century temperatures now exceeding those during the MCA, the 2020 fire season illustrates how extreme events are shaping subalpine forest response to a warming climate, with fire activity now exceeding the range of variability that shaped these ecosystems for millennia.