Eddy covariance station in a pine forest (Usmansky Bor, Central Forest-Steppe): results of monitoring ecosystem carbon exchange during the autumn-winter period

Continuous eddy covariance measurements of carbon fluxes remain unknown for pine forests in the forest-steppe zone of European Russia, limiting our understanding of the seasonal dynamics of ecosystem carbon exchange. This study aimed to assess net ecosystem carbon exchange between the atmosphere and an old-growth pine forest in Usmansky Bor during the autumn–winter period of 2025–2026. Continuous measurements were conducted at the VRN-KPF Kozhevenny Cordon ecosystem flux station using the eddy covariance technique. Net ecosystem exchange (NEE), total ecosystem respiration (TER), partitioned gross primary production (GPP), air temperature, precipitation, and photosynthetically active radiation were evaluated. From September 2025 to February 2026, the pine forest acted overall as a net carbon source to the atmosphere: cumulative NEE was 197 g C m–2 and TER was 380 g C m–2. September was the only month with a slight net carbon uptake (NEE = –3 g C m–2), whereas net carbon release prevailed from October through February. The shift from a weak carbon sink to a carbon source occurred in early October and coincided with a decline in photosynthetically active radiation and changes in temperature and moisture conditions. As the ecosystem transitioned into winter, the contribution of photosynthetic carbon uptake declined, whereas ecosystem respiration persisted, resulting in positive NEE during the cold season. These results provide the first quantitative characterization of autumn–winter ecosystem carbon exchange dynamics in the old-growth pine forest of Usmansky Bor and expand the continuous flux-observation record for underrepresented forest-steppe pine ecosystems of European Russia.