Linking tree-ring and sediment-charcoal records to reconstruct fire occurrence and area burned in subalpine forests of yellowstone National Park, USA

Reconstructing specific fire-history metrics with charcoal records has been difficult, in part because calibration data sets are rare. We calibrated charcoal accumulation in sediments from three medium (14-19 ha) and one large (4250 ha) lake with a 300 yr tree-ring-based fire-history reconstruction from central Yellowstone National Park (YNP) to reconstruct local fire occurrence and area burned within a 128 840 ha study area. Charcoal peaks most accurately reflected fires within 1.2-3.0 km of coring sites, whereas total charcoal accumulation correlated best with area burned within 6.0-51 km (r2=0.22-0.62, p<0.05). To reconstruct area burned for the entire study area, we developed a statistical model based on a composite charcoal record. The model explained 64-79% of the variability in area burned from AD 1675 to 1960 and was robust to cross-validation. Reconstructed area burned from AD 1240-1975 was significantly higher during periods including extreme annual drought (p=0.05), and area burned varied significantly at ~ 60 yr timescales (p<0.05), similar to the variability in an independent precipitation reconstruction covering the same period. Widespread burning (>10 000 ha) occurred at 150-300 yr intervals, and at the site level, fire probability increased with stand age (composite Weibull c parameter = 1.61 [95% CI 1.36-2.54]), both suggesting that post-fire stand development played an important intermediary role between climate and fire by increasing fuel abundance and probability of fire spread. Our study illustrates the possibility of reconstructing area burned with multiple charcoal records, and results imply that future fire regimes in YNP will be governed by direct impacts of altered moisture regimes and by vegetation dynamics affecting the abundance and continuity of fuels.