Detecting local-scale fire episodes on pollen slides

Assessment of the impacts of future climate change on the boreal forest and forest-tundra biomes relies on a clear understanding of their past dynamics. Fire history information recorded in lake and peat sediments can be retrieved by counting charcoal particles on pollen slides, although it is still debated whether this is reliable at the scale of the watershed (i.e. local scale). Knowing that proportionately larger charcoal particles tend to settle closer to the fire compared to smaller particles, a new method of quantifying microscopic charcoal on pollen slides is presented, taking into account the size distribution of the charcoal fragments in each sample of a sediment core (the CSD method). The late-Holocene fire history of the surroundings of a small subarctic lake was reconstructed using this method and compared with results from four other methods (macroscopic charcoal counts, charcoal concentration, charcoal : pollen ratio, charcoal : Picea ratio). All methods were tested for accuracy by comparison with the fire history of the watershed reconstructed by dendrochronology and radiocarbon-dated soil charcoal. Only the CSD method allowed separation of local-scale fire episodes from background noise attributable to long-distance dispersal.