Analysis of a 23-years Long Eddy-covariance Fluxes Dataset from a Mixed Deciduous Forest in Germany

In this study, we present the 23-years long dataset (2000-2022) provided by the flux tower site (DE-Hai) in the National Park Hainich, Thuringia, in Central Germany. Next to eddy-covariance flux measurements comprehensive datasets of meteorological and environmental conditions (e.g., soil respiration, leaf area index) were obtained over the last two decades. These long-term datasets represent the land-atmosphere interactions and their feedback with average and several extreme meteorological conditions, and thus provide the opportunity to investigate the resilience of this old-growth deciduous forest against extreme events.

The diel, daily and annual cycles of net ecosystem CO₂ exchange, latent and sensible heat fluxes are presented. Further ecosystem functions, such as Bowen ratio and water use efficiency, are analyzed. Footprint analysis revealed that the fluxes originated for > 95% from the mixed deciduous forest and that the main source region is located within 0.5-1 km around the tower. However, the source area differs slightly between day- and nighttime. The deciduous forest was a large and persistent net carbon sink, with an annual net ecosystem exchange between -339 and -670 g C m^-2. Environmental drivers of the ecosystem flux exchange were identified based on statistical analysis. A large uncertainty was introduced to flux estimates due to the applied post-processing methods (e.g., gap-filling), and strong impacts of recent drought events were observed for the flux exchange during and after the events. Moreover, the interrelationship between tree growth estimates based on circumferences of individual trees and eddy-covariance fluxes on ecosystem-level were investigated. Discrepancies between the carbon sink estimates on tree- and ecosystem-level are discussed.