Vegetation and climate interactions in controlling permafrost aggradation and degradation on decadal to centennial timescales

High latitude warming is increasing the vulnerability of permafrost to thaw, which can result in wholesale ecological and biogeochemical transformations. Particularly in zones of discontinuous permafrost, where mean annual temperatures are close to 0 degrees C and often regional climate is not cold enough to sustain permafrost everywhere, permafrost integrity is strongly influenced by surface vegetation, hydrology, winter snowfall, and disturbance such as fire. In permafrost peatlands within the discontinuous zone of permafrost, thermal properties of peat can sustain permafrost, even when mean annual air temperatures exceed 0 degrees C. However, interactions of vegetation and climate influence fire frequency and intensity, that can result in permafrost collapse due to increased albedo and heat penetration into mineral soils if the organic layer is consumed. The recovery of the permafrost depends on the proper hydrological conditions that allow for peat accumulation, as well as the rate of vegetation succession to shrubbier species that can help protect permafrost in the summer by increasing shading and by allowing heat convection from the ground surface through snow cavities to the atmosphere in winter. We studied a collapse scar bog in Interior Alaska and determined that fire facilitated two collapse previous permafrost collapse events during the last millennium. The first occurred 700 years ago, and the combination of peat accumulation and a cooler, drier climate that fostered rapid shrub invasion on the peatland, which resulted in permafrost aggradation on the order of decades. A second permafrost collapse occurred 525 years ago has remained permafrost-free to present day, as a result of wetter conditions that have sustained Sphagnum riparium and sedge communities under a wetter, warmer climate. However, margins of a nearby collapse scar show recruitment of shrubs and larches, and aggradation of new permafrost is evident, despite continued climate warming.