CLIMATE DRIVERS OF HISTORICAL SURFACE FIRES IN THE NORTHERN ROCKY MOUNTAINS, USA: PRELIMINARY RESULTS

HEYERDAHL, E. AND RISERT II, J. P.

USDA Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT 59808

We will present preliminary results of a study of the climate drivers of historical surface fires (1650-1900) at 14 sites in mixed-conifer forests in the Northern Rocky Mountains (Idaho and western Montana), USA. Over the period of record, we dendrochronologically dated 5920 fire scars (plus 444 injuries and abrupt changes in ring width that were synchronous with fire scars) from 438 trees (97% ponderosa pine, 3% Douglas-fir and western larch). On average, summers were significantly drier than average during years of synchronous fires (PDSI »1 standard deviation below 0 for years with >20% of recording trees scarred at ³25% of recording sites) and significantly wetter than average during years with no fire (PDSI below 0 during years when no sites with >20% of recording trees scarred). On average, NINO3 was positive during years of synchronous fires, but not significantly different than other years. However, NINO3 was significant high (El Niño years) during synchronous fire years that occurred during the positive phase of the Pacific Decadal Oscillation (NINO3 »one standard deviation above 0). The Atlantic Multidecadal Oscillation, as well as antecedent conditions for any climate parameter, did not differ significantly from 0 during synchronous fire years, or non-fire years, at our sites. We will assess the strength of these relationships again after we add fire histories from another 12-14 sites (6 from two published studies in eastern Oregon and Washington, and 6-8 sites new sites). We will also assess the strength of climate drivers in sub-regions of the Northern Rocky Mountains.