THE EFFECTS OF HOLOCENE CLIMATE CHANGE ON FIRE REGIMES IN IDAHO PONDEROSA PINE FORESTS

Ponderosa pine forests in the western U.S.A. have recently suffered extensive stand-replacing fires followed by major erosion and sedimentation. Tree-ring records in central Idaho indicate frequent, low-severity burns before fire suppression, but temperatures over this period [Little Ice Age (LIA) ~650-50 cal yr BP] were generally cooler than in recent decades. Increasing temperatures and severe droughts in the last several decades indicate that climate, along with fire suppression and other land-use change, may play a major role in recent severe fires. Historic burn area in the Boise National Forest shows a strong correspondence with instrumental records of summer drought. In order to investigate changes in fire regimes on Holocene timescales, we use post-fire sedimentation events preserved in alluvial fan deposits from small steep tributary basins in the Payette-Boise River area. We compare this record with similar records from the high-elevation lodgepole pine forests of Yellowstone National Park. Over millennial timescales, cooler periods promoted understory grass growth that fueled frequent low-severity fires in Idaho ponderosa pine forests, while in Yellowstone, high canopy moisture inhibited fires. Times of frequent surface fires in Idaho ponderosa forests during inferred cool intervals correspond with low fire occurrence in Yellowstone at ~350-500, 1200-1300, and 2800-3000 cal yr BP. These cool intervals correspond with cool episodes in the North Atlantic hypothesized to result from solar forcing (Bond et al., Science, 1997, 2001). In Idaho, large fire-related debris flows indicative of severe fires occurred during the ‘Medieval Climatic Anomaly’ (~1050-650 cal yr BP), showing that warmer drought episodes were also a major influence on past fire regimes. Although large fire-induced debris-flows are much less frequent in the overall record, over 20% of the total dated fan thickness was emplaced by only 8 large fire-induced debris flows between ~ 1000 and 800 cal yr BP. Sediment yield estimates from recent debris flow events (44,000 Mg/km²/yr) are orders of magnitude greater than 10,000-year average sediment yields (112 Mg/km²/yr; Kirchner et al., Geology, 2001) demonstrating the importance of single large events to Holocene average sediment yields.