Publication Date

5-2010

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Supervisory Committee Chair

Jennifer Pierce, Ph.D.

Abstract

Fire history of the high-elevation forest of the Sawtooth Valley in central Idaho was reconstructed using tree rings (providing annual to decadal resolution over the past ~400 years) and radiocarbon dating of charcoal (providing centennial to millennial resolution over the past ~8000 years). Fourteen annually resolved fires were reconstructed between 1632 and 1933 AD from fire-scarred Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta var latifolia). Stand ages of lodgepole pine indicate that at least five annually resolved fires in the ~28,000 ha study area were stand-replacing(SR). Multi-watershed SR fires burned in 1632 and 1842, and single-watershed SR fires burned in 1739, 1783, and 1933. Comparison of fire dates with independent reconstructions of drought (PDSI) and July temperature indicates that most fires burned during summers that were significantly warm and dry. Drought-induced fires (PDSI < -1, n = 11) burned at the tail end of a positive surge of spring Pacific Decadal Oscillation (PDO), which included a significantly positive (> 95% CL) anomaly the year before the fire. Eight of 11 drought-induced fires burned during combined positive phases of spring PDO and the Atlantic Multidecadal Oscillation (AMO) Index, a centered 10-year moving average of AMO anomalies. This subset of fires coincided with significantly positive spring PDO the year of the fire and the year before (> 95% and 99% CL, respectively). The multi-watershed SR fires of 1632, the most extensive (at least 4 watersheds) reconstructed in this study, were preceded by the longest protracted La Niña event (11 years) reconstructed for the past 484 years. Multidecadal periods of low frequency for all fires (n = 20) coincided with combined cool phases of the AMO and July temperature in central Idaho. Charcoal samples from nine soil sites and two incised alluvial sites provide millennial-scale records of fire. Summed probabilities from 16 calibrated radiocarbon dates show relatively high (compared with the early and middle Holocene) fire probabilities during the past ~1500 years, with peaks in ~720 AD and ~1630 AD. The ~1630 peak in the charcoal record likely corresponds with multi-watershed fires in 1632 from the fire-scar record, indicating good correspondence between these two proxy records of fire. A cool period between ~1280 and 1350 AD coincided with a deep trough in radiocarbon probabilities of fire in the Sawtooth Valley and the nearby montane forests of the South Fork Payette drainage.

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