Methane flux measurements above a Scots Pine forest in Austria

Forests play an important role in the exchange of greenhouse gases like CO₂ and CH₄ between the biosphere and the atmosphere. Although fluxes vary depending on biotic and abiotic factors like ecosystem composition and climatic conditions, forests generally are a sink for CO₂. For CH₄, forests with well-drained soils are considered a sink due to the oxidation of CH₄ within those soils. However, recent study results indicate that CH₄ emissions from trees may offset this soil CH₄ sink, resulting in a lower sink strength or even source at the ecosystem level.

To quantify the net CH₄ ecosystem exchange of a temperate needleleaf forest, we started CH₄ flux measurements above a Scots Pine forest at the ‘FAIR’ research site in Austria in August 2022 using the eddy covariance method. The results show very small fluxes with nighttime fluxes close to zero while fluxes during the day are more variable. Overall, the site is a very small sink for CH₄ at most. This site is temporally snow-covered during winter. As snowmelt increases soil moisture, we hypothesize that snowmelt in spring may further reduce the ecosystem CH₄ sink strength. Here, besides presenting flux results, we will also discuss methodical challenges of measuring very small fluxes.