Vital role of hydraulic capacity uncovered by mechanistic modelling of extreme drought impacts on European forests

Climate extremes like drought are threatening forests worldwide. Record breaking forest mortality has been observed in central Europe in the past five years. Meanwhile, more and more experiments are being set up that enable measurements of the hydraulic states of dying trees under extreme drought stress. These experimental data can be exploited by mechanistic vegetation models, offering the possibility to disentangle environmental drought stressors, e.g. atmospheric and soil moisture dryness, and their effects on a plants hydraulic system, such as stomatal closure and loss of hydraulic conductivity.Here, we show how a next generation plant hydraulic modelling is able to accurately reproduce the water potential dynamics of dying trees. We apply this plant hydraulic model to European drought experimental sites, including the canopy crane experiment II in Basel, Switzerland, and the KROOF experiment in Freising, Germany. We find that soil heterogeneity, rooting depth and stem hydraulic capacity are critical in determining whether a tree survives or succumbs to drought. Furthermore, good knowledge of four parameters is crucial to accurately capture the magnitude and temporal development of observed leaf and stem water potential: (1) stem hydraulic capacitance, (2) P50 (the water potential at which 50% of a plants hydraulic conductivity is lost), (3) saturated xylem hydraulic conductivity, and (4) the reference leaf water potential associated with full stomatal closure. Finally, when implemented into the terrestrial biosphere model QUINCY, our hydraulic scheme produces a clear mortality signal associated with recent drought events, giving confidence in our capacity to project the impact of future droughts on European forests