(1) Dept. of Geography, University of Oregon, Eugene, OR 97403, USA, (2) Montana State University, Bozeman, MT 59717, USA.

Holocene studies of charcoal and pollen records from lake sediments have shown that climate and vegetation have shaped northwestern U.S. fire regimes on multiple time scales. We examined 15 high-resolution lake-sediment charcoal records along with pollen data from this region to identify broad scale fire-fuel-climate linkages. Modern climate and site characteristics were analyzed to determine their impact on recent levels of charcoal influx into the lakes. Widespread trends in Holocene fires and fuels were investigated by analyzing charcoal accumulation rate (CHAR) and pollen data. CHAR data was decomposed into “peaks,” which represent discrete local fire events, and slowly varying “background” trends, which represent changes in regional fire activity, biomass, and depositional processes. Differences in modern climate and vegetation (expressed as growing degree-days) and lake/watershed properties largely account for recent variations in charcoal abundances among lakes. Variations in local fire-episode frequency (reconstructed from “peaks” in the CHAR) have been spatially heterogeneous, but have responded to millennial- and centennial-scale climate changes. For example, higher-than-present fire-episode frequency in the early Holocene is registered in several sites in the summer-dry PNW and occurred at a time when summers in that area were warmer and drier than at present. Concurrently, lower-than-present fire-episode frequency in the early Holocene characterizes sites in summer-wet regions of the northern Rockies and Yellowstone region. These areas experienced greater summer moisture in the early Holocene. Slowly varying “background” charcoal trends were more regionally coherent than the fire-episode frequency data, and showed trends similar to those of arboreal pollen percentages (used as a proxy for regional changes in woody fuels). Background charcoal trends probably reflect changes in fire size and severity at broad spatial scales, and intervals with high charcoal accumulation (e.g., during the MCA, ca. 1000-700 cal yr BP), may indicate a period of large, severe fires.