COS 4-3
Fire severity effects on larch forest regrowth and permafrost thaw in northeastern Siberia
Global change models predict increased fire activity in boreal forests as climate warms and dries, which could alter global carbon (C) cycling and create a positive feedback to warming. The magnitude of this feedback, however, will ultimately depend on fire effects on forest regrowth and permafrost degradation during the post-fire successional interval. The primary objective of this research was to increase our understanding of post-fire C dynamics in larch forests of the Siberian arctic. 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 test this hypothesis, a fire severity gradient was experimentally created in a mature larch stand, and Circumpolar Active Layer Monitoring (CALM) grids were established in two early-successional fire scars in Cherskii, Russia. Larch seeds were sowed in experimental plots in fall 2012/2013 and subsequent larch seedling establishment, survival, and growth tracked over two summers. Thaw depth, soil moisture, and soil temperature were measured to determine severity effects on permafrost thaw. In CALM grids, permafrost characteristics and microtopography were coupled with assessments of seedling establishment in relation to surrounding vegetation, microhabitat, and soil parameters.
Results/Conclusions
One year post-burn, larch recruitment was 10 times greater on severely burned plots than those unburned. By only one week post-fire, thaw depth was 15-25 cm deeper in plots burned at high severity (55 cm) compared to other treatments (30-40 cm). These differences in thaw depth with burn severity were maintained during two subsequent growing seasons and were associated with increased soil temperature and moisture. In the CALM grids, larch seedlings were most often found in areas with a shallow SOL, little grass competition, and near a mother tree. They were also found in ‘safe sites’ associated with tussock clumps, downed logs, and base of tree boles, suggesting that larch recruitment post-fire depends on microsite conditions that favor both germination and establishment. Our findings suggest that increased fire severity has the potential to alter forest successional dynamics, permafrost thaw, and patterns of C accumulation by decreasing soil organic layer depth, thereby removing a barrier to larch recruitment and changing soil thermal properties.