Flux exchange of a near-natural temperate deciduous forest under drought stress

Understanding the exchange processes between terrestrial ecosystems and the atmosphere above is crucial for mitigating climate change and promoting ecosystem resilience. Over the past decade, I have investigated the land-atmosphere interactions with regard to energy, water vapor, and CO₂ fluxes in various ecosystems, including forests, croplands, and grasslands, at different spatial and temporal scales. In this lecture, I will present recent results of a near-natural mixed-beech forest in the National Park Hainich in central Germany. Based on a comprehensive long-term dataset of eddy covariance flux observations, we conducted statistical time series analysis to investigate the exchange processes of this diverse, near-natural ecosystem. Furthermore, in collaborations with various partner institutions additional observations are being obtained at this flux study site, such as drone imagery, terrestrial laser scans, vegetation optical depth, forest biomass inventory, phenological photos, and on tree-scale records of stem growth, sapflow and leaf water potential. Using this multi-scale dataset, we aim to improve our understanding of the link between forest exchange processes and tree response dynamics, as well as the impact of extreme weather events (e.g., droughts).

The Hainich forest represents a large carbon sink prevailing throughout the past 26 years. However, the ongoing warming trend is altering the start and duration of the growing season of trees and the herbal layer. Tree vitality is being impacted by diseases and recent drought events such as in 2018-2020 have changed the forest’s processes and dynamics. We observed an increase in the canopy gap fraction in 2021 indicating a significant increase in tree mortality. Surviving trees were affected differently by the droughts depending on their species, age, and competition. In particular, the growth of older and larger trees (mostly ash), was impaired during and after the drought period, resulting in a reduction of the overall CO₂ uptake strength of the forest ecosystem between 2018 and 2022. However, about half of the observed trees, mostly suppressed, vital beech trees, showed a positive growth trend during and after the drought period. The given structural diversity influences the responses and resilience of individual trees and the entire ecosystem. The comprehensive dataset further provides an opportunity to investigate the influence of climate and soil characteristics and of forest management on flux exchange processes within multi-site comparison studies.