OOS 39-6 - Increasing fire severity: Impacts on forest succession and permafrost soils in Siberian larch forests

Friday, August 12, 2016: 9:50 AM
Grand Floridian Blrm F, Ft Lauderdale Convention Center
Heather Alexander, Mississippi State University, Michael M. Loranty, Colgate University, Susan M. Natali, Woods Hole Research Center, MA, Michelle C. Mack, Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ, Sergey Davydov, Northeast Science Station, Cherskii, Russia and Nikita Zimov, Northeast Science Station, Russia

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.