Long-term Impacts of Fire on Permafrost Vulnerability and C loss in Siberian Larch Forests

Abstract:

In Boreal forests, which contain large stocks of terrestrial carbon (C), fires have been increasing and are expected to continue to do so as the climate becomes warmer and dryer. Here we studied the indirect, long-term effects of fire on ecosystem C cycling via changes in stand density, organic layer and thaw depth in, in Siberian larch (Larix cajanderi) forests, underlain by continuous permafrost, near Chersky, Russia. Understory net ecosystem exchange (NEE), ecosystem respiration (R_eco) and thaw depth were measured for 3 growing seasons from density plots (related to fire severity) found within a 75-yr burn scar and from experimental burn plots. In 2015, R_eco was partitioned, using a dual-isotope approach, to determine how fire severity alters the contribution of autotrophic and heterotrophic respiration. R_eco gas samples were collected from static chambers, and at the density gradient we also collected carbon dioxide (CO₂) from R_eco sources (organic and mineral layer soils, above and belowground vegetation). We expect that differences in thaw depth, vegetation and organic layer related to stand density, will impact the contribution of old C sources to R_eco. In the experimental burn plots, the severity of the burn and thaw depth were positively correlated, and promoted loss of old C. Following the fires in 2012, higher intensity burns decreased R_eco, but 3 years later, R_eco was similar across burn treatments. In the 75-yr burn, stand density significantly impacted both thaw depth and understory CO₂ exchange, where higher density stands, had lower thaw depths, higher understory NEE, and C loss from young C sources that have assimilated since the fire. Decades after fire, permafrost vulnerability and C accumulation are driven by stand density, where higher-density stands have higher rates of NEE, but C loss from relatively young C.