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.

Keywords: Fagus sylvatica; canopy dieback; drought legacy; leaf development; leaf to sapwood area; loss of xylem hydraulic conductance; nonstructural carbohydrates.

Fig. 1

Seasonal weather and soil moisture conditions throughout the dry year 2018 and the following year 2019. (a) Daily vapour pressure deficit (VPD; average 00:00 h to 23:00 h CET), (b) daily precipitation and (c) soil moisture at 40‐cm soil depth (solid line, average of 8 measurement positions across the site; shaded area, min./max. variation across the 8 measurement positions). All sampling dates for analysis of native loss of xylem hydraulic conductance (PLC) and nonstructural carbohydrates (NSC), as well as for xylem water transport visualization (XWV), indicated by vertical lines (PLC, 4‐Sep‐2018/5‐Jun‐2019/11‐Jul‐2019/5‐Sep‐2019; NSC, 7‐Aug‐2018/5‐Nov‐2018/28‐May‐2019/9‐Jul‐2019/5‐Sep‐2019; XWV, 9‐Jul‐2019/5‐Sep‐2019).

Fig. 2

Canopy health decline in beech trees in the year after the severe 2018 drought and native percentage loss of hydraulic conductance (PLC), foliage reduction and tree ring increment in upper canopy branches. (a) Sparse foliage development and partial branch drying in the upper canopy in late spring 2019. (b) Branches collected from the upper part of healthy and symptomatic beech canopies. (c) Degree of foliation of upper canopy branches (LA : SA; total leaf area (LA) normalized to sapwood area (SA)) in trees becoming symptomatic or remaining healthy in 2019. (d) PLC values in upper canopy branches of the studied beech trees under severe drought in 2018 (bars show the average of trees remaining healthy or becoming symptomatic in the following vegetation season 2019) and in healthy and symptomatic canopies in the following summer half‐year 2019. (e) Tree ring increment in summer half‐year 2019 in upper canopy branches (data are mean ± SE, n = 5; post hoc tests for differences between groups with: ***, P ≤ 0.001; **, P ≤ 0.01; ᵒ, P ≤ 0.1; a small subset of data shown in (c) and (d) is included in Schuldt et al., 2020).

Fig. 3

Tissue levels of nonstructural carbohydrates (NSC = sugars and starch) in upper canopy branches of beech trees during and after the severe 2018 drought. Nonstructural carbohydrate levels in (a) leaves, (b) bark and (c) wood tissue under severe drought and after drought release in 2018 (bars show the average of trees remaining healthy or becoming symptomatic in the following season 2019) and in the following summer half‐year 2019 (data are mean ± SE; n = 5 trees; post hoc tests for differences between groups with: **, P ≤ 0.01).

Fig. 4

Dependency of branch foliation and whole‐branch stomatal conductance on xylem hydraulic function in beech trees in the year after the severe 2018 drought. (a) Dye‐tracer visualizations of active xylem area in cross sections of nonsymptomatic and symptomatic branches. (b) Relationships between active xylem area and branch foliation in nonsymptomatic and symptomatic branches. (c) Relationships between active xylem area and whole‐branch stomatal conductance (g _s‐wb) in nonsymptomatic and symptomatic branches. Bar plots show stomatal conductance (g _s) in single leaves (scattered data points represent single branches, bar plots represent means ± SE, n = 10).