ALLUVIAL FAN RECORDS OF CLIMATE-DRIVEN VARIABILITY IN HOLOCENE FIRE REGIMES IN PONDEROSA PINE FORESTS OF CENTRAL IDAHO

PIERCE, J.L. (1) AND MEYER, G.A. (2)

(1) Department of Geosciences, Idaho State University Pocatello, Idaho 83209, (2) Department of Earth and Planetary Science, University of New Mexico, Albuquerque, New Mexico 87131

Interpretation and radiocarbon dating of fire-related alluvial-fan deposits in the South Fork Payette River (SFP) drainage, Idaho reveals a ~8000-year record of fire-related sedimentation events following fire in Pinus ponderosa forests. Analysis of deposit characteristics provides an indication of the magnitude of geomorphic response and inferred fire severity. Alluvial fan records show maxima in frequent, small fire-related sedimentation events in Idaho ~350-500, 1200-1400, and 2800-3000 cal yr BP. The ~350-500 cal yr BP peak corresponds with the LIA; prior episodes may correspond with paleoclimatic indicators of generally cooler climates in the western USA. We infer that during cooler intervals in the SFP, high canopy moisture content inhibited stand-replacing fires while increased understory grass growth fueled frequent, low- to moderate-severity burns during intervals of relative drought. In contrast, the Medieval Climatic Anomaly (MCA, ~1050-650 cal yr BP) is characterized by large fire-related events at a range of ecosystems in the SFP and a peak in fire-related sedimentation in lodgepole-pine dominated forests of Yellowstone National Park. We hypothesize that dry canopy conditions resulting from widespread and severe drought during the MCA led to infrequent but severe stand-replacing fires in Idaho ponderosa and Yellowstone lodgepole-dominated forests. Charcoal macrofossils indicate similar forest compositions existed over the last ~4000 years in SFP basins; between ~4000-7000 yr ago, however, limited data suggests Pinus ponderosa was either sparse or absent in two basins where currently present.