Variations in fire frequency and climate over the past 17 000 yr in central Yellowstone National Park

A 17000 yr fire history from Yellowstone National Park demonstrates a stron g link between changes in climate and variations in fire frequency on mille nnial time scales. The the history reconstruction is based on a detailed ch arcoal stratigraphy from Cygnet Lake in the rhyolite plateau region. Macros copic charcoal particles were tallied from contiguous 1 cm samples of a 6.6 9-m-long core, and the data were converted to charcoal-accumulation rates a t evenly spaced time intervals. Intervals of high charcoal-accumulation rat es were interpreted as local fire events on the basis of information obtain ed from modern charcoal-calibration studies in the Yellowstone region. The record indicates that fire frequency was moderate (4 fires/1000 yr) during the late glacial period, reached highest values in the early Holocene (>10 tires/1000 yr), and decreased after 7000 calendar Sr B.P. The present fire regime (2-3 fires/1000 yr) was established in the past 2000 Sr The charcoal stratigraphy correlates well with variations in July insolation through ti me, which suggests that regional climate changes are responsible for the lo ng-term variations in fire frequency. In the early Holocene, summer insolat ion was near its maximum, which resulted in warmer, effectively drier condi tions throughout the northwestern United States. At this time, the fire fre quency near Cygnet Lake was at its highest. After 7000 calendar ST B.P., su mmer insolation decreased to present values, the regional climate became co oler and wetter, and fires were less frequent. The Cygnet Lake record sugge sts that long-term fire frequencies have varied continuously with climate c hange, even when the vegetation has remained constant.