Millennial-Scale Fire and Vegetation Change from a Rare Mid-Latitude Permafrost Fen (Beartooth Plateau, WY, USA)

Long-term fire histories are well documented across most North American temperate forest systems, yet the fire regimes of high-alpine treeline environments remain poorly understood. Here, we present a millennial-scale fire history from the Sawtooth Fen Palsa (SFP), a rare permafrost fen palsa located in the high-alpine treeline ecotone of the Beartooth Plateau, Wyoming, a permafrost system now unraveling due to recent decades of rapid warming. Analysis of paleoenvironmental proxies from peat sediments overlying permafrost reveals that a multi-century peak in fire activity occurred around 10,000 cal yr BP, coinciding with the afforestation of newly deglaciated, ice-free sites. This initial surge in fire activity was followed by a decline when orbitally driven increased growing-season temperatures likely promoted forest expansion at high elevations where moisture was not limiting. High severity fire activity increased again during the mid- Holocene (approximately 6,700–5,000 cal yr BP), when effective moisture increased, favoring subalpine forest expansion and increased connectivity of woody biomass (sagebrush and forest), enhancing the potential for canopy fire spread. Generally cooler and wetter conditions and possible year-round coverage of the fen palsa with snow and ice drove a near-absence of woody biomass burning at 5,000 cal yr BP. Rapid warming in recent decades has triggered the formation of dozens of thermal collapse ponds across the fen palsa. The frequency of these features has more than doubled since 2000 AD, underscoring the degradation of underlying permafrost in response to changing climatic conditions. Continued warming is expected to cause the complete loss of the permafrost lens, with far-reaching implications for ecosystem dynamics, disturbance regimes, and carbon and nutrient cycling.