No legacy effects of severe drought on carbon and water fluxes in a Mediterranean oak forest.

Severe droughts affect vegetation through several processes, such as hydraulic failure, early leaf senescence, depletion of carbon reserves, and reduced growth. These, in turn, can delay drought recovery and influence ecosystem functioning beyond the drought duration. The goal of this study is to investigate the direct response and physiological recovery of a Mediterranean oak (Quercus ilex L.) forest in southern France following the 2017 drought. We analysed eddy covariance-based observations of gross primary productivity (GPP), evapotranspiration (ET) and tree sap flow measurements. To study drought recovery, we used a random forest regression model to predict vegetation functioning in the post-drought years based on hydro-meteorological conditions. Potential legacy effects can be indicated by the difference between predicted and actual values. The 2017 drought peaked in autumn, with the lowest soil moisture of the study period 2000-2021. Concurrently, we detected the lowest GPP, ET, and sap flow for this time of the year on record. Despite severe reductions in vegetation functioning during drought, we found no legacy effects on GPP, ET, and sap flow. This suggests that the physiological functioning of Q. ilex woodlands recovers rapidly and completely. We hypothesize that this fast recovery is supported by favourable pre- and post-drought hydro-meteorological conditions, as spring 2017 was unusually sunny but not water-limited, and 2018 was the wettest year in the studied record. High drought resilience of Q. ilex forests is important in the context of anticipated increase in drought frequency and intensity under climate change. However, it remains yet to be determined to what extent the drought resilience can be sustained during potentially recurrent droughts in the future.