COS 54-3 - Potential for forest loss following wildfire in far northeastern Siberia

Background/Question/Methods

Fire is a natural disturbance in high-latitude larch (Larix spp.) forests of Siberia, but increased fire activity associated with climate warming could trigger forest loss and transition to alternative vegetation functional types through changes in seed availability and environmental conditions. To explore factors affecting larch recruitment after fire, we used a combination of observational and experimental approaches at two locations in northeastern Siberia (Cherskiy and Yakutsk, Russia) that differed in regional climate, fire history, and prevalence of deciduous hardwoods. In summers 2018 and 2019, we quantified larch recruit density within early-successional (< 17 year old) fire perimeters in relation to distance from unburned forest near Cherskiy, where wind-dispersed larch is the primary tree species. In addition, in a common garden experiment, we experimentally manipulated both residual soil organic layer (SOL) depth and seed predator access in 1-m² plots where we sowed seeds (400 m^-2) in 2019 and 2020 and quantified seedling establishment during the subsequent growing seasons. Near Yakutsk, where hardwoods co-occur with larch, we characterized tree regeneration composition and abundance in 25 plots distributed across a mixed-severity fire (323 km²) that burned in 2002.

Results/Conclusions

Across studies, we found seed availability was the primary filter impacting post-fire larch recruitment, with environmental conditions being secondary filters. In the early-successional forests, larch recruit density was often < 1 tree m^-2 in the floodplain, and < 2 trees m^-2 in the uplands, and density usually decreased with increased distance from unburned forest edge. Of the 70 plots sampled, 50% had lower post-fire larch recruit density than pre-fire adult larch tree density, indicating a strong potential for forest loss. Experimental plots had 2-20 times more larch recruits when seed predators were absent, with increasing recruitment with increased soil burn severity. Near Yakutsk, recruit density increased with linearly with crown burn severity and presence of a few residual seed-bearing trees. Our findings indicate that larch recruitment likely benefits from heterogeneous, moderate severity fires that maintain some live trees as seed sources while reducing competition and maintaining safe sites for seeds. However, large, high-severity fires that kill seed trees and/or create groundlayer conditions that increase seed visibility while providing places for predators to hide could cause forests to transition to grasslands, shrublands, or deciduous hardwood forests, with implications for carbon cycling, permafrost stability, and energy/water fluxes, and thus, fire-vegetation-climate feedbacks.