Long-term carbon sequestration and heatwave resilience in an old hemiboreal upland coniferous forest

• From 2016 to 2023 forest acted as a net carbon sink, with one carbon-neutral year. • R eco is the main contributor to inter-annual NEP dynamics. • Warm autumn 2020 increased R eco , shifting annual NEP to carbon neutral. • The forest showed high resilience to 2018 heatwave, with no legacy effect. Boreal forests play an important role in the global carbon cycle due to their extensive area and ability to sequester a considerable amount of atmospheric carbon dioxide (CO 2 ). They are generally stable ecosystems that function as carbon sinks. However, their sink capacity is vulnerable to the impact of extreme weather conditions. In this study, we aim to investigate the multi-year and seasonal carbon dynamics of an old upland coniferous forest in the hemiboreal zone, identify the main environmental drivers influencing annual NEP, and explore the potential legacy effects of the 2018 heatwave. Over an eight-year period (2016–2023), the forest shifted from a carbon sink (mean net ecosystem productivity (NEP) of 238 ± 52 g C m −2 year −1 ) to a carbon-neutral state in 2020 (NEP = -2 ± 5 g C m −2 year −1 ) and back to a net carbon sink (NEP = 136 ± 50 g C m −2 year −1 ). The average NEP over the eight-year period was 170 ± 42 g C m −2 year −1 . Our research showed no significant year-to-year changes in GEP during the study period, while the changes in R eco were substantial. Our results confirm that air temperature has the greatest influence on annual NEP. The warmest autumn over the past 19 years, recorded in 2020, and an atypical June together resulted in a noticeable increase in ecosystem respiration, which shifted annual NEP towards negative net values, while no significant impact on GEP was found. Additionally, our study found that the old upland hemiboreal forest showed no legacy effect in the years following the 2018 heatwave, demonstrating its resilience to extreme temperature events. Our results underscore the importance of continuous monitoring carbon dynamics variability to determine the ecosystem's resilience to seasonal temperature fluctuations and to inform management strategies for forests preservation.