Legacy C

Increasing fire severity and the loss of legacy carbon from forest and tundra ecosystems of northwestern North America

Climate warming in northern latitudes has led to an intensification of wildfire disturbance. Increased fire frequency, extent, and severity is expected to strongly impact the structure and function of northern ecosystems. As a large proportion of organic carbon (C) in Arctic tundra and boreal coniferous forests resides in the soil organic layer (SOL), combustion of this layer can lead to large C emissions. In fact, increased depth of SOL burning associated with an intensifying fire regime, might shift northern ecosystems across a C cycle threshold: from net accumulation of C from the atmosphere over multiple fire cycles, to a net loss. For this shift to occur, burning must release old C that escaped combustion in one or more previous fires. We term these old SOL C pools ‘legacy C’ because they are carryover of C pools sequestered prior to past disturbance events or under past climate. Fires that burn legacy C are expected to push ecosystems across thresholds in C cycling, permafrost stability, and plant composition.

What We Do
We are studying recent extreme fire activity in tundra ecosystems of Alaska, USA and in the boreal conifer forest of Northwest Territories, Canada. Our research combines scalable SOL consumption metrics with radiocarbon techniques for aging soil C to determine the ecosystem, landscape, and regional controls on the combustion of legacy C in forest and tundra regions. We have examined multiple fire scars in different regions, which provides a natural gradient in depth of burning. This gradient will allow us to address the vulnerability of legacy C to combustion along with the consequences of its loss. We plan to combine our understanding of legacy C loss with post-fire observations to assess the consequences of legacy C loss on permafrost dynamics, vegetation regeneration, and the initiation of successional trajectories.

Photo Credit: Xanthe Walker

Photo Credit: Xanthe Walker

Related Publications

Walker, X. J., J. L. Baltzer, S. G. Cumming, N. J. Day, C. Ebert, S. J. Goetz, J. F. Johnstone, S. Potter, B. M. Rogers, E. A. G. Schuur, M. R. Turetsky, M. C. Mack. 2018. Increasing wildfires threaten historic carbon sink of boreal forest soils. Nature. In press.

Walker, X. J., B. M. Rogers, J. L. Baltzer, S.G. Cumming, N. J. Day, S. J. Goetz, J. F. Johnstone, E. A. G. Schuur, M. R. Turetsky, M. C. Mack. 2018. Cross-scale controls on carbon emissions from boreal forest megafires. Global Change Biology, 24(9): 4251-4265. doi:0.1111/gcb.12893

Walker, X. J., J. L. Baltzer, S. G. Cumming, N. J. Day, J. F. Johnstone, B. M. Rogers, K. Solvik, M. R. Turetsky, M. C. Mack. 2018. Soil organic layer combustion in boreal black spruce and jack pine stands of the Northwest Territories, Canada. Int J Wildland Fire 27:125–34.

Grants supporting this work:
NASA’s Arctic Boreal Vulnerability Experiment (ABoVE)