Effects of Fire on Ecosystem Carbon Exchange in Siberian Larch Forest

Abstract:

Fire frequency and severity have been increasing across the Arctic, and fires are expected to intensify as the climate becomes warmer and dryer. Fire plays a prominent role in global carbon cycling through direct emissions of greenhouse gases from organic matter combustion as well as through indirect effects of vegetation changes and permafrost thaw, both of which can impact ecosystem carbon exchange over timescales ranging from years to centuries. We examined the indirect effects of fire (i.e., years to decades timescales) on ecosystem carbon exchange in Siberian larch (Larix cajanderi) forests underlain by continuous permafrost and carbon-rich yedoma deposits. We measured understory net ecosystem exchange (NEE) and ecosystem respiration (R_eco) from experimental burns, and from larch stands of varying stand densities occurring within a 75-yr burn scar in the vicinity of Cherskiy, Russia. The plot-level (4 m²) experimental burns were conducted in 2012 and comprise four burn treatments based on residual soil organic layer (SOL) depths: control, low severity (> 8 cm), moderate severity (5-8 cm), and high severity (2-5 cm). After three growing seasons, thaw depth was 6%, 11% and 30% deeper in the low, mid, and high severity burn plots compared to control. Immediately following the burns, R_eco declined and was related to burn severity; R_eco in the mid and high severity plots was fourfold lower than in low severity and control. In the second and third growing seasons, understory R_eco continued to be lower in the burn plots relative to control, but effects of burn severity varied across measurement years. While R_eco declined as a result of fire, there was a greater net release of CO₂ (i.e., NEE) from the burn plots compared to control because there was limited carbon uptake by the regenerating plant community. In the 75-yr burn, we found that variation in stand density, which was likely related to fire severity, significantly impacted understory CO₂ exchange through changes in light, SOL depth, and thaw depth. These results highlight the consequences of shifting fire regimes in boreal ecosystems for permafrost carbon vulnerability and the importance of understanding the responses of boreal ecosystems to fire for determining the long-term trajectory of ecosystem carbon exchange in the Arctic.