Fire-fuel-climate linkages in the northwestern USA during the Holocene

Variations in fire regimes can be inferred from changes in the abundance of sedimentary charcoal found in lake and bog sediments. When analysed with pollen data, inferences can be made about past vegetation dynamics and climate as well. The analysis of high-resolution charcoal records generally involves the decomposition of charcoal influx into (a) a slowly varying ‘background’ component that provides information about long-term changes in regional fire activity, biomass and/or depositional processes, and (b) a ‘peaks’ component that represents local fire events. In this study, 15 high-resolution charcoal records from the northwestern USA and associated pollen data were examined to describe the variations and controls of charcoal influx and background trends. Late-Holocene charcoal influx levels at each site were compared with late-Holocene sedimentation rates, vegetation and fire frequency, and with modern climate and physical site characteristics to better understand the spatial variability in charcoal abundance. Charcoal abundance was largely determined by physical site characteristics (eg, lake and watershed size) and the proportion of woody taxa. Background trends displayed regional similarities, and the subcontinental scale trend based on all records correlated closely with woody taxa proportions in the pollen spectra. Background charcoal and woody taxa proportions increased together from minima in the Late Glacial to maxima in the late Holocene. The strong similarity in these trends suggests that background charcoal influx is a function of fuel characteristics, which in turn are governed by climate and vegetation. Variations in sedimentation rate and fire frequency had little influence on background charcoal trends.