Evidence for Vegetation Acclimation to Atmospheric Dryness During European Droughts From a Land Surface Model

Accurately characterizing the response of Gross Primary Production (GPP) and plant transpiration to soil moisture stress is crucial for reliable climate simulations as we expect an increase in drought events. In this study, we used the ORCHIDEE land surface model to investigate how the physiological response of vegetation to soil moisture stress can be refined, focusing on Europe, which has experienced significant droughts. We used in situ GPP and latent heat flux (LE) data from more than 40 sites across various biomes, along with data assimilation techniques, to improve GPP and plant transpiration representations. This work shows that the speed of stomatal closure under soil moisture stress can be refined by incorporating vegetation acclimation to long-term vapor pressure deficit (VPD) conditions. This new drought response results in a greater reduction in GPP root mean square deviation than a response based solely on biome type, achieving an 18% improvement at the site scale, whereas the biome-type-only version shows no improvement. Then, the two model versions show similar performance in simulating LE, with an 8%–9% improvement at the site scale. Projections up to 2100 show that, over Europe, incorporating VPD acclimation results in a vegetation sensitivity to drought with stomatal closure that is 22% lower than a response that only depends on the vegetation type. This study underscores the importance of better understanding and accounting for potential acclimation mechanisms in vegetation response to climate change and extreme events.