Permafrost supports post-fire recovery of black spruce dominance in subarctic boreal forests

Le résumé fourni par la source

Abstract Black spruce (Picea mariana) dominates the boreal forest of North America, provides important wildlife habitat, and contributes substantially to long-term carbon storage. Wildfire shifts forest composition immediately following fire, but under climate change an intensifying wildfire regime may hinder black spruce regeneration and drive persistent compositional change away from black spruce. Further, wildfire can lead to permafrost thaw with unknown implications for forest regeneration. We monitored post-fire regeneration in black spruce stands in the Northwest Territories, Canada in permafrost and permafrost-free areas and explored how permafrost thaw rate (increased active layer thickness) impacted tree composition and density. We measured tree composition during three time periods: pre-fire, 2-4 years post-fire, and 9 years post-fire. In permafrost areas, pre- and post-fire tree composition demonstrated greater similarity due to smaller shifts away from black spruce compared to permafrost-free areas. When evaluating post-fire tree densities with increasing time after fire, we found that regeneration outcomes became apparent more slowly in permafrost areas compared to permafrost-free areas. For example, in permafrost areas black spruce relative densities and dominance (>50% relative density) increased with time after fire, driven by mortality of trembling aspen and jack pine seedlings rather than ongoing black spruce recruitment. In permafrost-free plots, post-fire black spruce relative densities were more stable over the same period. In permafrost areas permafrost thaw rate also influenced post-fire black spruce densities; areas with rapid and moderate thaw had higher relative densities than areas with stable permafrost depths, though this effect diminished over time. Our study shows regeneration outcomes in forests underlain by permafrost are determined over a longer regeneration period than in permafrost-free areas. While permafrost persists in this landscape, permafrost presence and thaw determines post-fire regeneration outcomes, but climate-driven permafrost loss may exacerbate forest compositional shifts away from black spruce towards trembling aspen and jack pine.

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