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

Boreal forests play a critical role in the global carbon cycle due to their vast coverage and ability to consistently absorb significant amounts of atmospheric carbon. In contrast, hemiboreal forests, which serve as a transitional zone between southern boreal and northern temperate forests, remain relatively understudied. Given their unique position and the rapidly changing climatic conditions in the Northern Hemisphere, hemiboreal forests are increasingly vulnerable to extreme weather events. To improve scientific understanding of long-term carbon dynamics in hemiboreal forests, we investigated annual and seasonal carbon fluxes, their connections to environmental factors, and the forest's response to an extreme weather event—the 2018 heatwave. Using the eddy covariance method, we studied an old upland coniferous hemiboreal forest in Estonia over an eight-year period (2016–2023). This forest is representative of coniferous forests in the hemiboreal zone, and our study provides one of the few long-term datasets available for this region. Our multiyear study reveals that the forest shifted between being a carbon sink and a carbon-neutral state, becoming carbon-neutral in 2020 due to Estonia's warmest autumn in 19 years and atypical weather events in June of the same year. In the following years, the forest's sink strength recovered. Moreover, air temperature was confirmed as the most significant driver of the forest's carbon dynamics. During the 2018 heatwave from mid-July to early August, we observed reductions in ecosystem respiration and gross ecosystem productivity, but by autumn, they had returned to their usual multi-year ranges with no legacy effect in 2019. While our results raise some concerns about the forest’s carbon sink stability, the absence of a legacy effect highlights its resilience to extreme weather events. This underscores the need for long-term monitoring of carbon dynamics in the hemiboreal forest zone to better understand their responses to both rapid and gradual temperature changes.