Using a Thermokarst Bog Chronosequence to Examine Post-thaw Changes in Net Carbon Balance and the Interactions Between Permafrost, Vegetation, and Carbon

Abstract:

Northern forest soils and wetlands have served as carbon (C) sinks for thousands of years. The boreal region contains 50% of the world’s soil organic C, with northern peatlands accounting for 30% of that pool. However, climate change in this region, in the form of warming air temperatures, has the potential to release a significant portion of this C due to changes in ecosystem structure and function. In particular, permafrost thaw in low-lying, moderately ice-rich areas results in the formation of collapse-scar bogs, dramatically altering the C cycle. Recent studies have shown that the transition from permafrost plateau to thermokarst bog results in the rapid loss of silvic (forest) peat, followed by a slow accumulation of C in post-thaw bog peat. Results from these studies suggest that this transition may turn these areas from net C sinks to C sources in the decades to centuries following thaw. Here we examine a bog chronosequence located within the Tanana River floodplain of Interior Alaska to determine if this pattern of C loss and gain holds true. Peat cores were taken to mineral soil from a permafrost plateau and three bogs with different ages of thaw (within the last several decades, within the last century, and within the past several centuries). All sites were located within the Bonanza Creek Long-term Ecological Research (LTER) site near Fairbanks, AK. We examined how the complex history of these thermokarst features can affect the C cycle. Macrofossil analysis reveals that most cores contained multiple cycles of permafrost aggradation and degradation, with the permafrost aggradation occurring epigenetically after peat initiated from a floodplain fen. Differences in vegetation communities that form peat, and the respective bulk densities associated with fens, permafrost plateaus, and collapse-scar bogs, resulted in different C accumulation rates. These data will provide insight into the fate of C within thermokarst bogs with complex permafrost histories in Interior AK, ultimately providing a better understanding of past and present controls on the regional C balance.