We inferred climate drivers of regionally synchronous surface fires from 1651 to 1900 at 15 sites with existing annually accurate fire-scar chronologies from forests dominated by ponderosa pine or Douglas-fir in the inland Northwest (interior Oregon,Washington and southern British Columbia).Years with widespread fires (35 years with fire at 7 to 11 sites) had warm spring - summers and warm-dry summers, whereas years with no fires at any site (18 years) had the opposite conditions. Spring climate likely affected the length of the fire season via the effects of snowmelt on soil and fuel moisture, whereas summer climate influenced fuel moisture during the fire season. Climate in prior years was not a significant driver of regionally synchronous surface fires, likely because fuels were generally sufficient for the ignition and spread of such fires in these forests. Fires occurred significantly more often than expected by chance when the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) were both warm phase and less often when they were both cool phase. Interactions between large-scale climate patterns influenced fire synchrony in the inland Northwest because phases of ENSO and PDO were associated with changes in the frequency of warm-dry v. cool-wet spring-summer climate.
Heyerdahl, Emily K.; McKenzie, Donald; Daniels, Lori D.; Hessl, Amy E.; Littell, Jeremy S.; Mantua, Nathan J. 2008. Climate drivers of regionally synchronous fires in the inland northwest (1651-1900). International Journal of Wildland Fire. 17: 40-49.