Regional Mountain Conference

We present eddy-covariance measurements of CO₂ exchange (net ecosystem exchange, NEE) from 2022–2025 from the Forest-Atmosphere-Interaction-Research (FAIR) site on the Mieminger Plateau, Tyrol, Austria, in a Scots pine (Pinus sylvestris) forest. Using observations from a measurement tower above the canopy, we quantify seasonal and interannual variability in ecosystem carbon budgets. Over the study period, the forest acted as a net CO₂ sink with annual NEE on the order of -100 g C m^–2. Uptake typically peaks in spring, while frequent warm, dry spells in summer shift the system toward net carbon release, indicating that the timing of extremes is more consequential than mean climatic conditions and helps explain interannual variability.

To identify the dominant drivers of NEE dynamics, we fit Generalized Additive Models (GAMs) using air temperature, relative humidity, soil moisture, and shortwave radiation as predictors. Partial effects reveal a temperature optimum for gross primary production (GPP) around 17–20 °C. Simple scenario analyses based on the fitted GAMs suggest that, considering GPP alone, moderate warming could enhance photosynthetic uptake. However, accounting for ecosystem respiration, which is strongly temperature-sensitive, reduces sink strength and can shift the net balance towards a carbon source under warmer climates.

We further illustrate the role of event timing with simple extreme-event scenarios: a spring heatwave can increase net uptake under high light and favorable water status, whereas a comparable event in summer substantially reduces uptake or induces net release. These results emphasize how seasonal context and the timing of extreme events control alpine forest carbon dynamics.