Impacts of Wildfire on Active Layer Depth and Carbon Stocks of Permafrost Plateaus in the Sahtu Region of the Northwest Territories

Abstract:

Northern permafrost ecosystems store approximately 60% of global terrestrial carbon; however, recent changes in wildfire activity may increase the availability of these carbon stocks to decomposition and release as greenhouse gases by triggering local permafrost decay. Consequently, understanding the interactive effects of wildfire and permafrost thaw on belowground carbon stocks will be critical for anticipating the stability of northern carbon stores. Here, we quantify the response of belowground carbon stocks to wildfire in five intact permafrost plateaus in the Sahtu region of the Northwest Territories, Canada, ranging from 19 –75 years post fire. Permafrost and active layer soil cores were collected and subdivided in the lab at 5 cm intervals to quantify bulk density, carbon fraction, and carbon stocks, while active layer thickness and organic layer thickness were determined in the field with a frost probe and mineral soil depth measurement respectively. Preliminary results indicate a trending decline in both active layer thickness and total carbon stocks in the decades following a wildfire event, with no associated change in peat bulk density or organic layer thickness. These results may reflect fire-initiated active layer thickening and suggest that carbon stocks are declining because of long-term impacts of wildfire on permafrost thaw. Our preliminary work indicates permafrost system C losses of 603 +/- 336 gC/m² per year following fire for 75 years following a fire event. By evaluating the responses of C stocks and active layer thickness to wildfire in the Sahtu region of NWT, we can better predict how the large C stocks of northern permafrost ecosystems in the discontinuous permafrost zone will respond to a changing fire regime.