Effect of climate change on fire regimes in northwestern Minnesota

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Of all the impacts of projected climate change on forest ecosystems, perhaps the most difficult to forecast is the potential for altered fire frequency and intensity. Fire regimes in forests are poorly understood for lack of long-term evidence. Here I used petrographic thin sections to determine the annual production of charcoal within a lake catchment in northwestern Minnesota over the past 750 years providing the long and high-resolution record required to elucidate fire regimes. Maximum abundance and frequency occurred in the warm, dry fifteenth and sixteenth centuries. Fire importance decreased dramatically with the onset or intensification of the 'little ice age' about AD 1600. Fire cycles with harmonics corresponding to multiples of the 22-year drought cycles of the region and increased fire frequency at times when early successional stands were breaking up, suggest a synergistic influence of climate and fuel accumulation. The anomalously warm, dry twentieth-century climate would have produced substantially different fire regimes from the previous century in the absence of fire suppression.