Examining anomalous summer carbonyl sulfide emissions in a boreal forest after thinning

Abstract Carbonyl sulfide (COS) is gaining interest as a proxy for gross primary productivity (GPP). Thinning of the Hyytiälä (Finland) forest in the winter of 2019–2020 altered the response of COS fluxes to environmental conditions in the summer of 2021. For the first time, extended periods of ecosystem-scale COS emissions were observed in a boreal forest. The warm and dry conditions in the summer of 2021 reduced the COS uptake by the canopy and elevated soil abiotic COS production. However, the reduction in canopy uptake and the increase in soil production do not fully explain the observed ecosystem-level emissions. The analysis suggests an unidentified, homogeneously distributed COS source in the eddy covariance footprint area, potentially from the photodegradation of forest floor litter and cutting residue from thinning. Such a source in a boreal forest stand warrants further source apportionment studies to effectively use COS as a proxy for GPP.