Late Holocene fire-climate relationships of the western San Juan Mountains, Colorado

In recent decades, warming temperatures and severe drought have contributed to large and severe wildfires in the south-western United States. To put current wildfires in a long-term context, we reconstructed fire events with alluvial stratigraphy methods in south-western Colorado, and compared with paleoclimate records over the late Holocene. The chronology of 32 fire-related sedimentation events from six tributary basins was established using 48 radiocarbon dates. Based on deposit characteristics, we found episodes of increased high-severity fire for 2750-2350 cal yr BP (800-400 BCE); 1400-1175 cal yr BP (550-775 CE); 1050-700 cal yr BP (900-1250 CE); and 525-250 cal yr BP (1425-1700 CE). There were peaks in low-severity fires in 2350-2000 cal yr BP (400-50 BCE) and 300-70 cal yr BP (1650-1880 CE). Two of the four episodes of high-severity fire corresponded with multi-decadal droughts, including clusters of extreme drought years. The most recent fire episode was preceded by wetter conditions and decreased frequency of extreme drought years, which facilitated the build-up of fuel loads. However, previous high-severity episodes were not consistently preceded by wetter conditions. Steep slopes and overall rugged terrain were also likely contributing factors to fuel accumulation and high-severity fires. A recent wildfire followed a fire-free interval of greater than 120 years and likely contained more extensive areas of high-severity burned area than fire events over the late Holocene.