The reconstruction of boreal forest fire history from lake sediments: A comparison of charcoal, pollen, sedimentological, and geochemical indices

Abstract

Fossil charcoal, fossil pollen, sedimentological and geochemical analyses of lake sediments have been used previously to reconstruct a history of local fires and resulting vegetation change. The rationale behind these approaches is described and the usefulness of each technique for reconstructing fire history in the boreal forest is assessed empirically. Historical and dendrochronological records provide regional and local fire histories for a site in Wood Buffalo National Park, Alberta, Canada. The local and regional history of fires is compared with the microscopic charcoal content, macroscopic charcoal content, elemental carbon content, fossil pollen content, sedimentology, and geochemistry of annually laminated sediments from a small lake. There is no significant correlation between the abundance of microscopic charcoal, macroscopic charcoal and total elemental carbon content of the sediments. Automated measures of microscopic charcoal abundance made with an image analysis system are correlated significantly with optical counts of microscopic charcoal. None of the charcoal measures provide unequivocal records of local fire activity and the abundance of microscopic charcoal appears to be influenced by variations in regional fire activity. However, the highest depositional rate of macrofossil charcoal occurred during the time of a fire that burned within the drainage basin. Variations in sedimentological measures and geochemistry do not correlate with local fire activity. Fossil pollen percentages and accumulation rates display a pattern of variation that is consistent with observed vegetation responses to fire in the boreal forest. What is likely apparent in the pollen record are the results of a series of burns of sufficient extent and intensity to kill most of the above-ground biomass of the vegetation in an area at least as great as the drainage basin. The inability of evidence from lake sediments to provide detailed histories of past fire activity is not surprising given the wide range of variation in the spatial extent, proximity, intensity and impact of individual fires.