Comparative methods for reconstructing fire histories at the stand scale using charcoal records in peat and mineral soils

Analysis and ¹⁴C dating of charcoal fragments ≥2 mm buried in mineral soils make it possible to obtain a stand-scale portrait of Holocene fires that occurred in well-drained, fire-prone environments, as well as changes in forest stand composition over time, based on botanical identification of charcoals. However, it is not always possible to reconstruct all fire events, due to disturbances that altered soil stratigraphy. In order to evaluate the efficacy of this approach, we conducted a comparative analysis with a proximal environment that presents an a priori continuous stratigraphy of charcoal fragments. For two sites in the coniferous boreal forest of eastern North America, the charcoal record of a forest soil was compared with that of an adjacent peatland margin situated at a distance of 20 m. For both types of sedimentary environments, a similar fire history was reconstructed for a large part of the Holocene. The greatest differences were for the early Holocene period, for which a smaller number of fires was detected in the forest soil compared to the peatland soil. Retracing the oldest fires using mineral soils in a fire prone environment is more difficult, due to charcoal decay that results from repeated fire events. However, forest soils reveal a relatively accurate fire history for subsequent millennia if the number of charcoals dated is sufficiently large. Any accurate reconstruction of the fire history of proximal peatland environments is strongly dependent on continuous stratigraphic units of peat and charcoal. Indeed, the age of charcoal fragments in peat may be different from that of the sedimentary layer in which they are buried due to allogenic disturbances like erosion events caused by deep burning of the organic horizon and other mass wasting events. Despite the large number of ¹⁴C dates it requires, analysis of soil macrocharcoal yields a realistic picture of the fire history at the stand scale. The concurrent analysis of macrocharcoal from adjacent peatland deposits may be used as a complement to more accurately record the oldest fire events.