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

Abstract

European beech (Fagus sylvatica) was among the most affected tree species during the severe 2018 European drought. It not only suffered from instant physiological stress but also showed severe symptoms of defoliation and canopy decline in the following year.To explore the underlying mechanisms, we used the Swiss-Canopy-Crane II site and studied in branches of healthy and symptomatic trees the repair of hydraulic function and concentration of carbohydrates during the 2018 drought and in 2019.We found loss of hydraulic conductance in 2018, which 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 in wood starch concentration throughout 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 showed that embolized xylem does not regain function in the season following a drought and that sustained branch hydraulic dysfunction is counterbalanced by the reduction in supported leaf area. It suggests acclimation of leaf development after drought to mitigate disturbances in canopy hydraulic function.

European beech dieback after premature leaf senescence during the 2018 drought in northern Switzerland

“…They are in line with our predictions and with recent findings of Walthert et al (2021), demonstrating clear relationships between soil water potential, leaf water potential and crown dieback in years following an extreme drought. Although early leaf fall may initially have been a physiological response to reduce water loss and xylem tension with the aim of avoiding embolism (Wolfe et al 2016), our results suggest that the 2018 drought was so severe that it wasparticularly in dry regionsan inciting event causing widespread hydraulic failure (Brodribb et al 2020;Wohlgemuth et al 2020;Arend et al 2021;Walthert et al 2021;Arend et al 2022) with subsequent crown-dieback, which ultimately leads to higher tree mortality (Chakraborty et al 2017;Leuschner 2020;Schuldt et al 2020). In these regions, the observed early leaf senescence during summer 2018 was, in fact, an indicator of stress and a predisposition for eventual crown dieback and tree mortality.…”

Soil–plant interactions modulated water availability of Swiss forests during the 2015 and 2018 droughts

“…In contrast, the fitted psicr had median values of −1.4 MPa for deciduous and −1.1 MPa for coniferous trees, which was higher than the default settings indicating an earlier stomata closure. The fitted psicr values were at the higher spectrum of leaf water potential compared to minimum leaf water potential reported across Germany, Switzerland, and Austria (Arend et al, 2021, 2022; Walthert et al, 2021; Zweifel et al, 2007). These studies found minimum leaf water potential of Fagus sylvatica in the range of −1.2 to −3.3 MPa, of Quercus spp .…”

SurEau-Ecos v2.0: a trait-based plant hydraulics model for simulations of plant water status and drought-induced mortality at the ecosystem level

“…Future developments will aim at integrating SurEau-Ecos with other forest models that are designed to represent the carbon cycle and vegetation dynamics, including the forest growth models CASTANEA (Dufrêne et al, 2005) and GO+ (Moreaux et al, 2020), as well as the gap model ForCEEPS (Morin et al, 2021) under the Capsis platform (Dufour-Kowalski et al, 2012). These future research projects and developments will also be an opportunity to further evaluate the feedbacks between carbon balance, growth metabolism and hydraulic properties, including the impacts of post-drought growth on the recovery of hydraulic properties and therefore on tree vulnerability to water stress in the long run (Arend et al, 2022).…”