Reconstructing the last 60,000 years climate-driven interactions of fire, vegetation, and megaherbivores at Fish Lake, Utah, USA

Fish Lake is located at 2700 m (a.s.l.) on the boundary of the Colorado Plateau and Great Basin geologic provinces in western North America. Climate forecast models suggest that this region will become warmer and drier during the 21st Century, which will likely intensify fire regimes and threaten biodiversity in this region, including the ancient Pando aspen clone located next to Fish Lake. Here we present a paleoenvironmental reconstruction from an 80-meter lake sediment core spanning the last 60,000 years. At the Last Glacial Maximum (LGM), the upland areas near Fish Lake (3200-3500 m asl) were heavily glaciated and plant communities were open and dominated mainly by herbs and conifers, such as grasses (Poaceae) and spruce (Picea spp.). During the LGM fire activity was low due to cold temperatures, low woody fuel abundance and connectivity, and the presence of megaherbivores (e.g., Mammuthus) as reconstructed from nearby fossil sites and the presence of coprophilous fungal spores (Sporormiella) in the Fish Lake sediments. In the Late Glacial Period, the demise of upland glaciers and megaherbivores was accompanied by a ‘release’ in woody vegetation, especially spruce and pine (Pinus spp.) and a rise in charcoal accumulation. During the Early Holocene, this rise in burning sustained and was likely enhanced by warming temperatures and the establishment of closed-canopy forests similar to modern composed of Engelmann spruce (Picea engelmannii), aspen (Populus tremuloides), and subalpine fir (Abies lasiocarpa). Fire activity in the Middle Holocene remained high, with a stepwise increase observed during the Late Holocene that occurred with increasing evidence of human activities and amplification of El Nino-Southern Oscillation (ENSO). Throughout the 60,000 record, aspen pollen is consistently present. While pollen alone does not provide direct evidence of the long-lived Pando aspen clone, this record does confer the presence of aspen growing near Fish Lake through contrasting climate periods and fire regimes. This long-term reconstruction offers new insights into the interactions of climate, vegetation, and herbivory in shaping wildfire in western North America to help support land management policies.