Holocene Forest Fire Records as Evidence of Millennial-Scale Climate Change in the Northwestern United States

Both modern observations and Holocene fire proxy records indicate that the magnitude and timing of wildfires is significantly affected by climatic change. Continental-scale drought episodes appear to be linked strongly to temperature, so that in 2002 and other recent warm years, major fires occurred in a variety of coastal and interior forest ecosystems despite regional precipitation differences and land management effects. We examine the timing of Holocene fires in central Idaho and Yellowstone National Park through interpretation and radiocarbon-dating of burned soil surfaces and fire-related sediments preserved in small alluvial fans. A large sample of alluvial fan stratigraphic sections within each region provides an essentially continuous and high-resolution record of fire-related sedimentation, including indications of fire severity. Major fire-related sedimentation occurred in cool, high-elevation mixed-conifer forests of Yellowstone between 1250 and 730 cal yr BP. In lower and warmer ponderosa forests in Idaho, at least 4 sites experienced fire-induced debris flows between ~950 and 730 cal yr BP. Dates from these fire-related sedimentation events correspond to the Medieval period, as does evidence of synchronous fires in diverse climatic zones and ecosystems from other studies in the western U.S. Throughout the Holocene, minima in fire-related sedimentation in Yellowstone correspond with maxima of ice-rafted debris and cool episodes on ~1500 year cycles in the North Atlantic (Bond et al., 1997). At these times, floodplain widening along streams in Yellowstone and low-severity fires in Idaho are consistent with higher effective moisture and cooler conditions. Even though these studies cannot reconstruct absolute variations in temperature and precipitation, they reflect millennial-scale temperature variations in the western U.S. that are clearly consistent with the North Atlantic cycles. Other paleoclimate proxies from the western U.S. including lake-level fluctuations, paleovegetation records, and glacial advances also provide evidence of notable warm and dry episodes in the Medieval period as well as Little Ice Age cooling. Nonetheless, late 20th-century warming and fires may exceed previous episodes in extent and severity.