Observational evidence of legacy effects of the 2018 drought on a mixed deciduous forest in Germany

Forests play a major role in the global carbon cycle, and droughts have been shown to explain much of the interannual variability in the terrestrial carbon sink capacity. The quantification of drought legacy effects on ecosystem carbon fluxes is a challenging task, and research on the ecosystem scale remains sparse. In this study we investigate the delayed response of an extreme drought event on the carbon cycle in the mixed deciduous forest site 'Hohes Holz' (DE-HoH) located in Central Germany, using the measurements taken between 2015 and 2020. Our analysis demonstrates that the extreme drought and heat event in 2018 had strong legacy effects on the carbon cycle in 2019, but not in 2020. On an annual basis, net ecosystem productivity was similar to 16% higher in 2018 (similar to 424 g(C)m(-2)) and similar to 25% lower in 2019 (similar to 274 g(C)m(-2)) compared to pre-drought years (similar to 367 g(C)m(-2)). Using spline regression, we show that while current hydrometeorological conditions can explain forest productivity in 2020, they do not fully explain the decrease in productivity in 2019. Including long-term drought information in the statistical model reduces overestimation error of productivity in 2019 by nearly 50%. We also found that short-term drought events have positive impacts on the carbon cycle at the beginning of the vegetation season, but negative impacts in later summer, while long-term drought events have generally negative impacts throughout the growing season. Overall, our findings highlight the importance of considering the diverse and complex impacts of extreme events on ecosystem fluxes, including the timing, temporal scale, and magnitude of the events, and the need to use consistent definitions of drought to clearly convey immediate and delayed responses.

Automated summary

Forests have a significant role in the global carbon cycle, and droughts contribute to variability in the terrestrial carbon sink capacity. This study examines the delayed effects of an extreme drought event on the carbon cycle in a deciduous forest in Central Germany. The analysis shows that the drought in 2018 had a lasting impact on the carbon cycle in 2019, but not in 2020. The findings emphasize the importance of considering the complex impacts of extreme events on ecosystem fluxes and the need for consistent definitions of drought.