Abstract: Holocene fire history, inferred from sedimentary charcoal analysis, is often explained in terms of the history of climate and changes in vegetation flammability. Therefore, Holocene fire histories may differ substantially at opposing ends of elevational gradients where fuel types differ substantially. We used charcoal analysis to reconstruct the fire histories of a open pine forest at 1585 masl and at a dense mixed conifer dense at 2050 masl site, both located within 40 km of each other in the northwest periphery of the Northern Great Basin in Oregon. Charcoal recovered from sediment cores was analyzed in continuous 1-cm intervals. We found a reverse pattern of fire activity between the sites, with the period between 10,000 to 8000 cal. yr BP experiencing increased biomass burn frequency at the high-elevation site and very low fire frequency at the lower site. The period following the deposition of Mount Mazama at 7630 cal. yr BP had greatly reduced biomass burning reduced at the high-elevation site compared to the lower site, which saw a significant increase in fire frequency. These contrasting patterns may be partly explained by moisture-controlled changes that simultaneously affect fuel availability at low elevations and fire season length at high elevations. However, the 25 cm Mazama tephra deposition had an abrupt effect on reducing fire history at the dense forest site, suggesting tephra may have directly affected fuel availability.
