Climate and human influences on historical fire regimes (AD 1400–1900) in the eastern Great Basin (USA)

Abstract

High fire activity in western North America is associated with drought. Drought and fire prevail under negative El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) phases in the Southwest and with positive phases in the Northwest. Here, I infer climate effects on historic fire patterns in the geographically intermediate, eastern Great Basin and seek out evidence of human influence on reconstructed fire regimes. Surface fire chronologies were constructed for 10 sites using tree-ring-based fire scars. Regional (67) and local (247) fire years and no-fire (187) years were identified from 1400 to 1900 CE. I compared fire chronologies with indices of the Palmer Drought Severity Index (PDSI), ENSO, and PDO. Regionally, fires were significantly more common during drought and were associated with negative ENSO and positive-to-negative PDO transitions while no-fire years were associated with positive ENSO and negative-to-positive PDO transitions. Conditions were significantly wetter 2 years prior to regional fire years and drier 4 years prior to no-fire years, providing evidence that fires were historically fuel limited. Local fire years occurred under a broad range of climate conditions. Most sites showed either persistent late or bimodal (early and late) fire seasonality patterns. These patterns are distinct from the mid-season peak observed for modern lightning-caused fires, suggesting a human influence on historical ignition patterns. Results demonstrate that climate was an important synchronizer of fire at the regional scale and that locally fire regimes were the product of climate-regulated fuels and some combination of human and lightning ignition patterns.