Drought impacts on the carbon uptake of an old-growth deciduous forest in Central Germany

In recent years, Europe has seen hot summers and drought conditions occur with increasing intensity and frequency. Drought and soil water limitations impact on the carbon uptake and release of forests. This study investigates the effect of recent drought events on carbon dioxide exchange of the unmanaged deciduous old-growth forest at the Fluxnet site Hainich (DE-Hai) in the years 2018 and 2019 and compares them to the previous century drought of 2003. During the 2018 event the Hainich site was at the intensity maximum of the Middle European drought event. In combination with shallow soils with low water holding capacity, this lead to severe limitations of available soil water and therefore a to a reduction in carbon fluxes. Comparing the 2003, 2018 and 2019 drought years, we find that anomalies in the annual carbon balances are not only affected by the intensity of the drought events itself but most importantly by the seasonal timing and the balance between anomalies in the carbon uptake and release. 2018 saw a significant reduction in the annual carbon uptake of the forest due to a drought starting early in Spring and limiting fluxes from May and June onwards. Contrary, 2019 experienced a less severe drought, however, the reduction in the annual carbon uptake of about 40% in 2019 was even more extreme than in the previous year. We are able to explain differences between years by two factors: firstly, the uptake deficit during the Summer and Autumn of 2018 was partially compensated by a positive uptake anomaly in Spring and early Summer, and secondly, the severe soil water limitation during the summer of 2018 lead to a decrease of ecosystem respiration, likely dominated by a decrease in soil respiration. Contrary, 2019 saw neither of the two compensating effects and therefore experienced the strongest reduction in net annual carbon uptake of the three drought years investigated. The further development of the carbon sequestration potential of the forest will remain a relevant question given the likely frequent occurence of droughts in the future and the consequences of significant forest damage already observed in 2019.