FIRE, CLIMATE, AND VEGETATION HISTORY IN THE NORTHERN ROCKY MOUNTAINS DURING THE LAST 13,000 YEARS

MITCHELL J. POWER (1), CATHY WHITLOCK (2), AND PATRICK BARTLEIN (1)

(1) Department of Geography, University of Oregon, Eugene, OR 97403 (2) Department of Earth Sciences, Montana State University, Bozeman, MT 59717 (Whitlock@montana.edu)

Foy Lake, a low-elevation site located near the forest-steppe border in northwestern Montana, provides records of annual-to-decadal-scale landscape change for the last 13,000 cal yr BP. Sedimentary charcoal and pollen analyses were used to document fire, climate, and vegetation linkages in the Northern Rocky Mountains (NRM) at annual-to-centennial timescales. Annually-sampled charcoal in lake-sediment cores is compared with evidence of known fires identified from historic fire atlases and fire-scarred tree-ring studies during the last ~150 years. Historical records of fire and climate near Foy Lake, including seasonal climate conditions that led to the AD 1910 fires that burned over two million acres in Idaho and Montana, are used as a baseline for analysis of sedimentary charcoal influx during Holocene fire episodes. The long-term record of sedimentary charcoal and pollen at Foy Lake shows variability that likely reflects variations in vegetation and fire regime due to climate change. Fire activity (inferred from the frequency of charcoal peaks) was initially low from 13,000 and 11,500 cal yr BP, and increased from 10 episodes/1000 years to 20 episodes/1000 years after 11,500 cal yr BP. Between 6200 and 2200 cal yr BP fire activity was low relative to present, averaging 10 episodes/1000 years. Fire activity has increased during the last 2200 cal yr BP, with a Holocene maximum of 30 episodes/1000 years at 500 cal yr BP. The pollen record suggests Foy Lake has alternated between steppe and open forest throughout the Holocene. These changes in vegetation generally correspond with changes in fire activity.