Fire and soil erosion history in East Canadian boreal and temperate forests

Fire-made soil erosion should trigger (i) an increase of inorganic sedimentation within lake-basins and (ii) a change of magnetic susceptibility if the burn depth is strong enough to reach the mineral soil and to modify the magnetism of mineral particles. Magnetic susceptibility will also change with the flux of mineral sediments even without a change of their magnetism. Here, we test the role of fire on soil erosion by measuring the mineral accumulation and the magnetic susceptibility in sediments from seven small lakes' and two dunes' profiles from East Canada over the Postglacial. Four sites are located in the boreal forest south of James Bay, two in the eastern maritime Quebec and one in the cold temperate south-eastern Ontario. Charcoal accumulation rate is used as a proxy of biomass burning based on the assumption that higher the biomass burning, higher is the charcoal accumulation. The mineral accumulation, deduced from loss-on-ignition residues, is a proxy of erosion process in the lake catchment areas. No relationship is observed between sediment types, sedimentation, magnetic susceptibility and charcoal concentrations in lakes. The patterns of erosion proxies do not match with those of fire, except in dunes. The results suggest that fires have no significant impact on soil erosion in East Canadian forest ecosystems, except in dry-sandy areas. This fact can result from fire severity that is not strong enough to completely burn the humus layer, especially in northern boreal forest characterized by thick soil organic layers. Fire is thus not a significant process affecting the lake sedimentation by soil material input, nor a factor of soil dynamics by rejuvenation of top most soil centimeters over the Postglacial, except in dry sandy areas where dune activity is obviously controlled by burning. (c) 2006 Elsevier Ltd. All rights reserved.