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

F S Chapin,M Wilmking,C C Fuller,A D Mcguire,I H Myers-Smith,J W Harden

doi:10.5194/bg-5-1273-2008

Abstract

Abstract. To determine the influence of fire and thermokarst in a boreal landscape, we investigated 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 ~600 years of vegetation succession with a transition from a terrestrial forest to a sedge-dominated wetland over 100 years ago, and to a Sphagnum-dominated peatland in approximately 1970. The shift from sedge to Sphagnum, 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. This concurrent wetland succession and reduced growth of black spruce trees indicates a step-wise 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 peatland. These observations and the peat profile suggest that future warming and/or increased fire disturbance could promote permafrost degradation, peatland expansion, and increase carbon storage across this landscape; however, the development of drought conditions could reduce the success of both black spruce and Sphagnum, and potentially decrease the long-term ecosystem carbon storage.

Highlights

The spatial patterns of vegetation in the low lying flood plains of Interior Alaska are controlled by disturbances such as fire, permafrost degradation, flooding, and drainage
Plant macrofossils and soil chemistry in the sylvic soil layers from both peatland and moat cores indicate that a terrestrial forest pre-dated the initiation of the collapse
The %C, %N, C:N, and density in the organic matter above the mineral horizon was similar in all of the cores from the transect, and woody peat and charcoal were found at the base of both the peatland and moat cores

Summary

Introduction

The spatial patterns of vegetation in the low lying flood plains of Interior Alaska are controlled by disturbances such as fire, permafrost degradation, flooding, and drainage. Because approximately one-quarter of the world’s soil carbon is currently sequestered in boreal peatlands (Gorham, 1991; Apps et al, 1993; Turunen et al, 2002), degradation of permafrost could result in significant carbon emissions Both carbon accumulation and methane emissions have been shown to increase in peatlands underlain by degrading permafrost as water tables increase and nutrients are released by the thawing soils (Robinson and Moore, 2000; Turetsky et al, 2000; Vitt et al, 2000; Camill et al, 2001; Turetsky et al, 2007). Future carbon accumulation in permafrost peatlands will depend on plant and microbial responses to a warming climate, but on vegetation succession driven by fire and permafrost thaw

Methods

Results

Conclusion