Lack of hydraulic recovery as cause of post-drought foliage reduction and canopy decline in European beech

Background/Question/Methods

An exceptional global change type drought hut central Europe in the 2018 summer. Several tree species, in particular European beech (Fagus sylvatica), did not only suffer from instant physiological stress and mortality but showed severe symptoms of defoliation and canopy decline in the following years. To explore the underlying mechanisms of post-drought canopy decline, we used the Swiss-Canopy-Crane II site and studied the repair of hydraulic function and concentration of carbohydrates in branches of healthy and symptomatic trees during the 2018 drought and the following year 2019.

Results/Conclusions

We found losses of hydraulic conductance in 2018, that did not recover in 2019 in trees that developed defoliation symptoms in the year after drought. Reduced branch foliation in symptomatic trees was associated with a gradual decline of wood starch concentration throughout the summer 2019. Visualization of water transport in healthy and symptomatic branches in the year after the drought confirmed the close relationship between xylem functionality and supported branch leaf area. Our findings show that embolized xylem does not regain function in the season following a drought and that sustained branch hydraulic dysfunction leads to the reduction of supported leaf area. It suggests that disturbances in canopy hydraulic function and the inability to recover from this causes a reduction in canopy leaf area and canopy dieback after drought.