Drivers and impacts of combined soil and atmospheric droughts on the CO2 fluxes of a mixed deciduous forest in Switzerland

Climate change is increasing frequency and intensity of droughts across Europe, with major consequences for forest ecosystems. Often soil and atmospheric droughts occur simultaneously, resulting in combined soil atmospheric drought (CSAD) events. Which effects such CSAD events have on forest CO₂ fluxes is not clear. At the Lägeren site (CH-Lae), a mixed deciduous forest in Switzerland, we identified the three years with the lowest cumulative precipitation and the highest cumulative vapor pressure deficit (VPD) during the growing season (May – September), namely 2015, 2018 and 2022, since net ecosystem CO₂ exchange (NEE) measurements started in 2005. We then determined the CSAD events, i.e., periods in which soil and atmospheric drought occurred simultaneously. Our objectives were to (1) quantify the impacts of CSAD events in 2015, 2018 and 2022 on CO₂ fluxes against the long-term mean, (2) identify the environmental drivers of net ecosystem production (NEP) in 2015, 2018 and 2022 and forest floor respiration (Rff) in 2018 and 2022 compared to the long-term fluxes, (3) assess the temporal course of the effects of soil and atmospheric drought on NEP and Rff during the CSAD events against the long-term means. CO₂ fluxes were measured continuously with the eddy covariance technique at two distinct locations at the CH-Lae forest: above the canopy at a height of 47 meters (from 2005 to 2022) and below the canopy at 1.5 meters (from 2018 to 2022). The drivers of NEP and Rff were determined with machine learning approaches, i.e., random forest conditional variable importance and Shapley Additive exPlenations (SHAP). We found a decrease in NEP of 35%, 38% and 41% during the CSAD events in 2015, 2018 and 2022 respectively compared to the mean 2005-2022, and a decrease of 16% and 41% in the Rff during the CSAD events in 2018 and 2022 compared to the mean 2019-2021. Light is usually the main driver of NEP during the growing season, as we found in 2015, 2018 and in the mean 2005-2022. While soil water content (SWC) was the main driver of NEP for the growing season in 2022, enhancing the key effect of soil drought in the 2022 growing season. The SHAP analysis revealed the negative impacts of high temperature, high VPD, and low SWC on NEP during all CSAD events, with low SWC and high VPD in 2022 having the larger impacts on NEP. Rff was mainly decreased by low SWC in 2018 and 2022. This led to a decrease in temperature sensitivity of Rff during CSAD events compared to the mean 2019-2021. With this study we assessed the impacts CSAD events on the CO₂ fluxes of a mixed deciduous forest. Yet, the intensity, the timing, and the pre-conditions of CSAD events are crucial to explain ecosystem responses to such events. Furthermore, the increase in frequency and intensity of droughts and precipitation events with global warming call into question the predictability of forests capacity to store carbon, which is crucial for climate change mitigation through nature-based solutions.