Post-fire methane fluxes from boreal forest soils depend on burn severity

Forest fires are arguably one of the most destructive natural disturbances in the boreal forest biome and can cause significant changes to the carbon balance in these ecosystems. Although the area of forests burned annually in Fennoscandia is currently small (<1000 ha), this area may increase both due to biodiversity directives to increase habitat diversity using prescribed burning and due to climate change intensifying wildfire regimes. Fire severity impacts the biological, chemical, and physical properties of soils which underlie greenhouse gas (GHG) fluxes, which can interact to cause complex dynamics in GHG emissions for decades after fire. Therefore, it is essential to understand the impact of fire on forest soil GHG fluxes. Currently, upland forest soils in the boreal biome act as a weak methane (CH₄) sink, but there are conflicting estimates about how these fluxes are impacted by forest fire. To better understand these dynamics and what the future may hold, we must quantify CH₄ fluxes after fire and identify the factors that impact this, namely soil, vegetation, and fire characteristics.

We aimed to measure CH₄ fluxes following prescribed restoration burning. Our research sites were thinned dry Scots pine (Pinus sylvestris) forest with sandy soils and sparse understorey vegetation at four locations in central Finland. We established permanent sample plots in each research site and installed collars for static chamber measurements. Plots were located in unburned, low-severity burn, and high-severity burn areas. Prescribed burning was conducted under suitable weather conditions, usually in the first half of June. Burning resulted in ground vegetation and logging debris were consumed across the site, but standing tree mortality varied between 0 and 100%. To measure CH₄ fluxes from the soils, we used a dark static chamber and a portable trace gas analyzer (Licor LI-7810). CH₄ fluxes were measured daily immediately after fire, then bi-monthly up to two growing seasons after prescribed burning.

Results indicate that CH₄ uptake increased following fire, but this was not equal on all sites and varied over time. In terms of burn severity, we found that plots with low-severity burning had greater increases in CH₄ uptake. Immediately following fire (i.e. when some active smoldering still present), we found that CH₄ fluxes were highly variable and included very high CH₄ emissions. We found no significant differences in soil CH₄ fluxes between control and treated plots prior to burning, despite different forest management histories in some cases. Increased CH₄ uptake in low severity plots is likely also linked to low microbe mortality, potential increases in microbe diversity, and soil temperature (Köster et al. 2011). However, a more complete understanding of the mechanisms and conditions that drive increases in CH₄ uptake in low-severity burns requires further research.