Xylem function and leaf physiology in European beech saplings during and after moderate and severe drought stress

The extreme drought events in the last years caused high mortality amongst European beech (Fagus sylvatica L.) across Central Europe. Thus, knowledge of the response of beech to drought and its ability to recover its xylem and leaf functions after drought release is needed to better understand beech survival. In this study, changes in xylem function, leaf gas exchange, chlorophyll fluorescence and morphology of European beech saplings during drought and recovery phases were assessed. Beech saplings were exposed to three different watering intensities-well-watered (W), moderately drought stressed (M) and severely drought stressed (S) for 25 days, and then saplings were rewatered for 14 days. Reduced irrigation caused a drop in shoot water potential to -1.1 and -3.1 MPa in M and S saplings, respectively. Stomatal conductance in M and S saplings decreased to 15 and 5 per cent of that in W saplings, respectively, and it corresponded with a decrease in the photosynthesis rate and chlorophyll fluorescence parameters. Leaf water content and shoot functional xylem area were significantly affected by drought only in S saplings. Hence, these parameters could be used as sensitive indicators that distinguish plants at no risk of mortality from those at increasing risk under drought stress. In S saplings, some leaves also dried from the edges, leaving a green active central part. During the recovery phase, the leaf gas exchange and chlorophyll fluorescence parameters had recovered 4 days after rewatering in M saplings, whereas in S saplings, they were still lower after 2 weeks. Moreover, xylem function did not fully recover in S saplings till the end of the recovery phase. Hence, subsequent drought periods, even of lower intensity, could lead to tree mortality if tree functions have not fully recovered from the previous drought.