Interaction of fire, climate and vegetation change at a large landscape scale in the Big Woods of Minnesota, USA

The Big Woods region of Minnesota is on the prairie-forest border and is a much studied model for the interaction of climate, fire and vegetation. The purpose of this study was (a) to document the extent and timing of changes in vegetation and fire over the past 2000 years and (b) to examine the link between charcoal influx and vegetation during the eighteenth and early nineteenth centuries. To reconstruct changes in vegetation, fire and climate, sieve charcoal (120 gm), pollen, environmental magnetism and loss-on-ignition were used. Charcoal deposition at prairie lakes was 3-4 times greater than for woodland or forest lakes, consistent with current understanding of the fire dependence of prairies, but charcoal influxes to forests were considerably higher than reported elsewhere, suggesting that ground fires were more common than assumed. Regionally, charcoal deposition at a subset of 17 lakes began to change at cal. AD 1100, dating to the period of the ‘Mediaeval Climate Anomaly’ (MCA; cal. AD 900-1250) but preceding the ‘Little Ice Age’ (LIA; cal. AD 1250). For seven of 14 lakes the MCA was characterized by a significant increase in charcoal deposition. This was followed by a decrease with the onset of the LIA, which for most sites continued into the nineteenth century. Relative timing of changes in charcoal and pollen and the other proxies differed from site to site, suggesting no one single response to climatic change. This variation most likely derives from local differences in fuels and topography, as compounded by strong positive feedbacks between fuels and fire.