Compound soil and atmospheric drought events and CO₂ fluxes of a mixed deciduous forest: Occurrence, impact, and temporal contribution of main drivers

Abstract. With global warming, forests are facing an increased exposure to compound soil and atmospheric drought (CSAD) events, characterized by low soil water content (SWC) and high vapor pressure deficit (VPD). Such CSAD events trigger responses in both ecosystem and forest floor CO₂ fluxes, of which we know little about. In this study, we used multi-year daily and daytime above canopy (18 years; 2005–2022) and daily forest floor (five years; 2018–2022) eddy-covariance CO₂ fluxes of a Swiss forest site (montane mixed deciduous forest; CH-Lae). The objectives were (1) to characterize CSAD events at CH-Lae; (2) to quantify the impact of CSAD events on ecosystem and forest floor daily CO₂ fluxes; and (3) to identify the major drivers and their temporal contributions to changing ecosystem and forest floor CO₂ fluxes during CSAD events and CSAD growing seasons. Our results showed that the growing seasons of 2015, 2018, and 2022, were the top three driest (referred as CSAD years) at CH-Lae since 2005, with similar intensity and duration of the respective CSAD events, but considerably different pre-drought conditions. The CSAD events reduced daily mean net ecosystem productivity (NEP) in all three CSAD years, with highest reduction during 2022 (30 % decrease). This reduction in daily mean NEP was largely due to decreased gross primary productivity (GPP; >15 % decrease) rather than increased ecosystem respiration (Reco) during CSAD events. Furthermore, forest floor respiration (Rff) decreased during the CSAD events in 2018 and 2022 (no measurements in 2015), with a larger reduction in 2022 (>40 %) than in 2018 (<25 %) compared to the long-term mean (2019–2021). Using data-driven machine learning methods, we identified the major drivers of NEP and Rff during CSAD events. While daytime mean NEP during 2015 and 2018 CSAD events was limited by VPD or SWC, respectively, daytime mean NEP during the 2022 CSAD event was strongly limited by both SWC and VPD. Air temperature always had always negative effects, net radiation positive effects on daytime mean NEP during all CSAD events. Daily mean Rff during the 2018 CSAD event was driven by soil temperature and SWC, but severely limited by SWC during the 2022 CSAD event. We found that a multi-layer analysis of CO₂ fluxes in forests is necessary to better understand forest responses to CSAD events, particularly if the first signs we saw of acclimation to such CSAD events for our forest are found elsewhere as well. We conclude that such events have multiple drivers with different temporal contributions, making prediction of site-specific CSADs and forest long-term responses to such conditions more challenging.