Landscape–lake interactions in the Beartooth Mountains, Wyoming: a 350-year fire history reconstruction

Wildfire is the dominant natural disturbance agent in forests of the Greater Yellowstone Ecosystem, and can have long-lasting impacts on local landscapes and limnologic processes. We applied a multi-proxy paleolimnological approach to infer the impact of forest fires on the catchment of Island Lake, Wyoming (USA), over the last ~ 350 years. Specifically, we examined whether fossil diatom assemblages recorded indirect, limnological impacts from fire, including changes in water clarity, nutrient input, and lake mixing. The objective of this study was to determine if post-fire lake changes could be reconstructed effectively using fossil diatom assemblages from Island Lake, Wyoming, by comparing the diatom record to dendrochronological and macroscopic charcoal records. Local and regional fire years AD 1747 and 1900 were determined from tree-ring records, which were corroborated with sediment accumulation rates and magnetic susceptibility in Island Lake sediments. Inferred lacustrine conditions that were associated with fire events, particularly regional, mixed-severity fire year 1900, included increased turbidity, greater water column mixing, and possible augmented influx of total phosphorus to the system. Thus, the fossil diatom assemblage in Island Lake appears to have recorded forest fire activity indirectly, particularly when fire activity was severe or widespread within the catchment. An increase in small colonial Fragilaria (sensu lato) species since 1975, in association with the rise in abundance of Asterionella formosa and Tetracyclus glans, suggests recent changes in thermal stratification patterns, epilimnion temperatures, and available nutrients.