VARIABILITY OF FIRE REGIMES AND GEOMORPHIC RESPONSE IN PONDEROSA PINE FORESTS OF CENTRAL IDAHO

As of July, 2002, the number of acres burned in the western U.S. was already nearly triple the 10-yr average, much of it in ponderosa pine forests. Tree-ring studies show that before European settlement, frequent low-severity fires (RI ~several yr to a few decades) characterized fire regimes in ponderosa forests, suggesting that recent catastrophic fires and resulting debris flows are unprecedented and primarily a result of fire suppression. Tree-ring records extend only through the past 500 yr, however, and the presettlement record is influenced by generally cooler climates of the Little Ice Age (LIA, ~1200-1900 AD). Because climate is a primary control, the full range of natural fire regimes and geomorphic response in ponderosa pine forests is not fully understood. We interpret and radiocarbon-date Holocene fire-related sedimentation preserved in tributary alluvial fans in the Payette-Boise River region of the western Idaho batholith. Debris-flow and sheetflood sediments with abundant coarse, angular charcoal indicate sedimentation in response to fire. Burned soil surfaces directly mark fire events, and overlying sediments are likely fire-related. Preliminary results show that much fire-related sedimentation occurs as frequent small events with recurrence intervals of several decades, consistent with a regime of light surface fires where many burns produce little or no geomorphic response. Such regimes appear prevalent during cooler climates, including the LIA. Between 950 and 730 cal yr BP, however, at least 4 fans contain fire-related debris flows or burned soil surfaces underlying major debris flow deposits. These dates correspond to the Medieval Warm Period of ~1050-750 cal yr BP, a time of widespread and severe droughts in the western U.S., and also of major fire-related debris-flow activity in the lodgepole pine-dominated forests of Yellowstone. Vegetation in these fan basins varies from rangeland with few trees to open ponderosa pine and denser mixed ponderosa pine and Douglas-fir. The concurrence of events in different vegetation communities suggests that this period is characterized by widespread high-severity fires and major geomorphic response. Studies of fire regimes during warm dry periods of the past can inform future fire management in ponderosa pine forests in the western U.S.