An 840-year record of fire and vegetation in a boreal white spruce forest

Boreal forest fire and vegetation history was examined using an 840-yr fossil pollen and charcoal record from a site dominated by Picea glauca in Alberta, Canada. The record was created using contiguous 5-yr samples of annually laminated sediments from a 2.7-ha lake (Rainbow Lake A in Wood Buffalo National Park). Following a peak in the microscopic charcoal accumulation rate (CHAR), there is a sequence of peaks in sediment thickness and in 14 of the 21 common pollen and spore taxa. The general pollen sequence is an initial peak in herbs, then shrubs, then deciduous trees, and finally conifers and Sphagnum. The peak value for each taxon differs following individual fires, suggesting that a site does not undergo the same postfire vegetation sequence following all fires. Based on the CHAR, pollen, and sediment thickness records, at least 12 large, local fires are apparent. The average time interval between the 12 fires is 69 yr. Significant periodicities in CHAR and in 11 of the pollen taxa range between 95 and 185 yr. These estimates of the mean fire interval are broadly similar to the 71-142 yr fire cycle for Picea glauca forests, estimated by dendrochronological analysis of regional forests. The pollen taxa also exhibit long-term changes in relative abundance. An initial dominance by Picea cf. glauca is terminated by a fire around AD 1185 and is replaced by Populus dominance between AD 1250 and 1550; the fire is followed by an 800-yr period of increasing abundance of Picea cf. glauca. Herbs and shrubs generally exhibit a peak in abundance between AD 1350 and 1750. These long-term patterns may reflect climatic change, response to fires with different characteristics, or rearrangement of the forest patches around Rainbow Lake A.