Holocene fire regimes, vegetation and biogeochemistry of an ecotone site in the Great Lakes Region of North America

Abstract

AbstractQuestionAs anthropogenic change pushes ecosystems towards historically novel states, understanding how landscape disturbances interact with climate variability becomes increasingly important. We reconstructed the Holocene vegetation and fire history of a small watershed to investigate the linkages among disturbance regimes, biogeochemical cycling and ecosystem structure. Specifically we asked: (i) how do compositional and structural changes in forest ecosystems modify fire regimes; and (ii) how do changes in fuel type and fire regimes impact biogeochemical cycling?LocationComstock Lake, Wisconsin, USA, a small kettle lake at the modern prairie–forest ecotone in the Great Lakes Region of North America.MethodsWe analysed a lacustrine sediment core from Comstock Lake, Wisconsin, for paleoecological proxies including charcoal quantity and morphotype, pollen composition, magnetic susceptibility, organic matter concentration and stable carbon and nitrogen isotopes. These paleoecological variables were used to reconstruct climate‐mediated shifts in vegetation, fire disturbance and biogeochemical cycling during the last 13 200 calendar years before present (cal yr BP).ResultsForest composition in the landscape surrounding Comstock Lake varied among coniferous forest, deciduous forest and savanna over the Holocene. Independent of the vegetation changes, five distinct fire regimes were identified, ranging from frequent, low‐intensity surface fires to infrequent, high‐intensity crown fires. The main feature of our vegetation reconstruction is a shift from Pinus to Quercus forest that occurred between 10 000 and 11 000 cal yr BP. However, changes in fuel source and forest structure were apparently more important than shifts of species composition in determining fire regimes. We observed an inverse relationship between non‐arboreal fuel sources and biomass burning throughout all five fire regimes. The biogeochemical consequences of fires were apparent during some, but not all regime transitions. Generally, the sediments were composed of relatively small clastic inputs from the catchment and mostly autochthonous organic sources.ConclusionThe location of Comstock Lake on the prairie–forest boundary meant that regional climate signals during the Holocene were filtered through vegetation. Both compositional changes in dominant tree taxa and structural changes in canopy openness influenced fire regimes.