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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. [ABSTRACT FROM AUTHOR]

Abstract: Quantitative analyses of variations in morphological features of charcoal were undertaken in a 210Pb-dated sediment core from Prosser Lake (British Columbia, Canada). Seven morphological types of charcoal were defined by particle shape, major–minor axis ratio, apparent porosity and progradation to unburned material. The distribution of morphotypes and total charcoal abundances were assessed as a proxy for fire events recorded between 1919 and 2000 and to subsequent mechanisms of transportation–sedimentation to lake sediments. Charcoal morphotypes showed distinct relationships to recorded area burned by fires. Fragile charcoal fragments with highly irregular porosity (termed Type M) displayed the strongest correlation to burned area (r 2 = 0.51; P = 0.0001) and did not produce any false-positive signal for fires recorded within a radius of 20 km around the lake. We infer that high porosity and low density Type M fragments are aerially transported and directly deposited on the lake, and that the fragility of Type M charcoal prevents significant quantities from being secondarily transported and incorporated into the sedimentary record. We propose that charcoal morphology is an important but underutilized technique that can yield important insights into fire type, proximity and transportation–sedimentation processes. [Copyright &y& Elsevier]