HOLOCENE FIRE REGIMES AND GEOMORPHIC RESPONSE IN CONIFER FORESTS OF THE NORTHWESTERN UNITED STATES: EVIDENCE OF MILLENNIAL-SCALE CLIMATE CHANGE

Fires result in increased runoff, erosion, and slope instability and are often recorded in the stratigraphy of small alluvial fans as charcoal-rich debris-flow and flood deposits and charred litter layers. We use this record to compare the timing and severity of Holocene fires in xeric ponderosa pine (Pinus ponderosa) forests of central Idaho with cool, moist mixed-conifer forests Yellowstone National Park (YNP). The high-elevation Pinus contorta-dominated forests of YNP burn infrequently in large, severe fires with recurrence interval (RI) of ~150-300+ yr, and fire extent is strongly dependent on drought and wind conditions. Stand-destroying fires consume most litter and leave ash-laden soil surfaces that promote runoff and debris flows. In Idaho ponderosa forests, tree-ring fire scars show frequent low-severity surface fires (RI ~10-22 yr) before Euro-American settlement, in contrast with recent catastrophic crown fires and resulting debris flows that are attributed to forest density increases due to fire suppression and other land use. Tree-ring records extend back only 500 yr, however, and the presettlement record is influenced by cooler climates of the Little Ice Age ~1200-1900 AD. Thin charcoal-rich sheetflood deposits are common in fan stratigraphy and likely represent low-severity surface fires. Since many such fires produce no deposition, fan stratigraphy yields a minimum recurrence interval for fire events. The maximum at-a-site frequency of fire-related events in Idaho (RI of several decades) occurs around 300, 1400, 2900, and 7000 cal yr BP. At these same times, fire-related sedimentation was at a minimum in YNP, and local paleoclimate records and the Bond et al. (1997, 2001) North Atlantic ice-rafted debris record indicate cooler climates. In contrast, from ~1050 to 750 cal yr BP (Medieval Warm Period) and 2300 to 1600 cal yr BP, fire-related sedimentation was common in YNP, and fire-induced debris flows occurred at widespread sites in central Idaho. On a millennial timescale, warmer climates with intervals of severe drought appear to promote high-severity fires in both areas. Similar conditions are likely in part responsible for the large, severe fires that occurred in a variety of western U.S. ecosystems in 2002 and other recent years, despite regional climatic and land management differences.