Wetland succession in a permafrost collapse: interactions between fire and thermokarst

To determine the influence of fire and thermokarst in a boreal landscape, we investigated ~600 years of vegetation succession from peat cores within and adjacent to a permafrost collapse feature on the Tanana River Floodplain of Interior Alaska. Radioisotope dating, diatom assemblages, plant macrofossils, charcoal fragments, and carbon and nitrogen content of the peat profile indicate that succession proceeded from a terrestrial forest to a sedge-dominated wetland over 100 years ago and to a <i>Sphagnum</i>-dominated bog in approximately 1970. The shift from sedge to <i>Sphagnum</i>, and a decrease in the detrended tree-ring width index of black spruce trees adjacent to the collapse coincided with an increase in the growing season temperature record from Fairbanks. The concurrent wetland succession and reduced growth of black spruce trees indicates a non-linear ecosystem-level response to a change in regional climate. In 2001, fire was observed coincident with permafrost collapse and resulted in lateral expansion of the bog. These observations and the peat profile suggest that future warming and/or increased fire disturbance could promote permafrost degradation and bog expansion, and increase carbon storage in the collapse; however, the development of drought conditions could reduce the success of black spruce and <i>Sphagnum</i>, decreasing long-term ecosystem carbon storage in the adjacent landscape.