Extreme fires under warmer and drier conditions inferred from sedimentary charcoal morphotypes from Opatcho Lake, central British Columbia, Canada

Predictions of the extent of forest fires under warmer and drier conditions in boreal regions require knowledge of long-term relationships between fire and climate. However, many long-term studies that utilize the remains of total charcoal in lacustrine sediments fail to demonstrate a relationship between climate change and fire activity. A new approach to reconstruct the relative-area burned based on specific types of charcoal particles (the charcoal-morphotype (CM) fire index) has shown significant correlations to recorded forest fires. Here we assess the utility of the CM derived from an analysis of charcoal morphotypes in sediment cores from Opatcho Lake (British Columbia, Canada) using two independent paleoclimate proxies over the last 400 years, and since the mid Holocene. Over the past 400 years, significant correlations between the CM fire index and independent climate reconstructions (diatom-inferred salinity, dendroclimatic reconstructions of temperature and precipitation, and reconstructions of the Pacific Decadal Oscillation) range from 0.35 to 0.42. Similarly, since the mid Holocene the correlation between the CM fire index and independent proxies of past climate (diatom-inferred salinity and temperature inferences from chironomids from the southern interior of British Columbia) range from 0.70 to 0.76. These significant correlations strongly contrast with the very low and insignificant correlations between the CM fire index and total charcoal, suggesting that this approach provides paleofire information not available from traditional techniques. The CM fire index suggests that fires were at least two-and five-fold larger than those observed during the instrumental period, over the last 400 and 6000 years, respectively.