Fire severity is increasing in boreal forests as climate warms. This could alter global carbon (C) cycling and create a positive feedback to warming depending on patterns of forest regrowth and permafrost degradation during the post-fire successional interval. Our overarching hypothesis was that post-fire soil organic layer (SOL) depth regulates C balance through impacts on forest regrowth and permafrost stability because of its role as a seedbed and thermal regulator. To assess fire severity effects on tree recruitment and underlying soils, we conducted plot-level experimental burns in July 2012 in a Siberian larch forest near Cherskii, Russia and tracked larch seed germination and seedling establishment and survival along with soil conditions over a 4-yr period. To understand how fire-driven changes in stand density influence C accumulation, we evaluated C pools within trees, shrubs, woody debris, soils, and roots across 20 stands of varying tree density (0.04-3.7 trees m-2) within a 75-year old fire scar.
High severity experimental fires led to a rapid, pronounced, and long-lasting (3+ yr) increase in permafrost thaw depth and higher larch tree recruitment. By 2015, thaw depth was ~ 40 cm deeper in high severity plots compared to controls. Larch recruitment was largely absent in controls and low severity plots but was ~3 seedlings m-2 in moderate and high severity plots; interannual survival was ~30-60%, suggesting that increased fire severity could create stands of similar density observed in the fire scar. Increased stand density increased C pools, with high density stands storing ~30% more C (~8,000 g m-2) than low density stands (~ 6,000 g m-2), mainly because high density stands stored ~7 times more C within live larch trees (1,880 g m-2) compared to low density stands. High density stands stored less C in large shrubs and coarse roots than low density stands, but these pools were relatively small (< 10% of total C pools). Soil C pools were similar across the density gradient (~ 2,500 g C m-2 for both SOL and mineral pools). Our findings suggest that increased fire severity is likely to alter forest successional dynamics, permafrost thaw, and C accumulation by decreasing SOL depth, thereby removing a barrier to larch recruitment and changing soil thermal properties.