LANDSCAPE CHANGE ON THE SOUTHERN FLANK OF THE UINTA MOUNTAINS: THREE SEDIMENT-CORE RECORDS OF CLIMATE, VEGETATION, AND FIRE DURING THE LAST TWO MILLENNIA

Late Holocene climate variability has resulted in a complex history of landscape evolution and disturbance across the Rocky Mountains and Intermountain West. Global and regional reconstructions of paleoclimate during the last two millennia suggest long-term cooling, with shorter intervals of regional warming during the Medieval Climate Anomaly (MCA) and regional cooling during the Little Ice Age (LIA) embedded within the long-term trend. In this study we explore the linkages among climate, vegetation, and fire in the Uinta Mountains (northeastern Utah) during the MCA and LIA. We extracted two shallow cores from montane wetlands (Reader Lakes and Julius Basin sites) and one from a moraine-damned lake, recently drained (Heller Lake site), on the south flank of the range. In addition to establishing a radiocarbon-age model for each core, sediment samples were analyzed for fossil pollen and charcoal to characterize past vegetation and fire regimes. A comparison of the charcoal record from high- and mid-elevation sites suggest a lagged response in fire activity at higher elevations, in response to both the MCA and LIA climate forcing. Fire frequency generally increased during the MCA and decreased during the LIA. These results agree with regional fire reconstructions that suggest strong linkages between wildfire in the western US and paleoclimate, especially temperature and drought. The pollen-based vegetation reconstructions for our three sites suggest a more pronounced vegetation response to LIA cooling than to MCA warming, including an increase in grass species during the LIA. The climate, vegetation and fire histories preserved in the late Holocene stratigraphic record from the Uinta Mountains suggest that warmer climates result in greater frequency and magnitude of disturbance, therefore we should expect disturbances such as fire to increase in the Uinta Mountains in the future.