From the comfort zone to crown dieback: Sequence of physiological stress thresholds in mature European beech trees across progressive drought

Lorenz Walthert,Andrea Ganthaler,Stefan Mayr,Matthias Saurer,Peter Waldner,Marco Walser,Roman Zweifel,Georg Von Arx

doi:10.1016/j.scitotenv.2020.141792

Abstract

Drought responses of mature trees are still poorly understood making it difficult to predict species distributions under a warmer climate. Using mature European beech (Fagus sylvatica L.), a widespread and economically important tree species in Europe, we aimed at developing an empirical stress-level scheme to describe its physiological response to drought. We analysed effects of decreasing soil and leaf water potential on soil water uptake, stem radius, native embolism, early defoliation and crown dieback with comprehensive measurements from overall nine hydrologically distinct beech stands across Switzerland, including records from the exceptional 2018 drought and the 2019/2020 post-drought period. Based on the observed responses to decreasing water potential we derived the following five stress levels: I (predawn leaf water potential >−0.4 MPa): no detectable hydraulic limitations; II (−0.4 to −1.3): persistent stem shrinkage begins and growth ceases; III (−1.3 to −2.1): onset of native embolism and defoliation; IV (−2.1 to −2.8): onset of crown dieback; V (<−2.8): transpiration ceases and crown dieback is >20%. Our scheme provides, for the first time, quantitative thresholds regarding the physiological downregulation of mature European beech trees under drought and therefore synthesises relevant and fundamental information for process-based species distribution models. Moreover, our study revealed that European beech is drought vulnerable, because it still transpires considerably at high levels of embolism and because defoliation occurs rather as a result of embolism than preventing embolism. During the 2018 drought, an exposure to the stress levels III-V of only one month was long enough to trigger substantial crown dieback in beech trees on shallow soils. On deep soils with a high water holding capacity, in contrast, water reserves in deep soil layers prevented drought stress in beech trees. This emphasises the importance to include local data on soil water availability when predicting the future distribution of European beech.

Highlights

IntroductionClimate change is leading to more severe and longer drought episodes in many regions worldwide (Dai, 2013; Trenberth et al, 2014) including mountainous environments such as the European Alps (Gobiet et al, 2014; Milano et al, 2015), causing stress responses, reduced productivity, mortality and tree species range shifts in numerous forest biomes (Allen et al, 2010; Williams et al, 2013; Martinez-Vilalta and Lloret, 2016; McDowell et al, 2020)
We investigated poorly understood hydraulics of mature European beech trees with a relevant tree physiological and environmental data set from natural beech forests on a wide variety of soils, including drought intensities that triggered stress symptoms and tree damage
Our stresslevel scheme complements the knowledge of critical thresholds during drought stress in tree species

Summary

Introduction

Climate change is leading to more severe and longer drought episodes in many regions worldwide (Dai, 2013; Trenberth et al, 2014) including mountainous environments such as the European Alps (Gobiet et al, 2014; Milano et al, 2015), causing stress responses, reduced productivity, mortality and tree species range shifts in numerous forest biomes (Allen et al, 2010; Williams et al, 2013; Martinez-Vilalta and Lloret, 2016; McDowell et al, 2020). Predictions of future droughtinduced forest dynamics and species range shifts require reliable data on temporal and spatial changes of environmental factors, as well as on key ecophysiological processes The latter determine species competitiveness and mortality under drought, which represent important plant characteristics in vegetation models (Choat et al, 2018; Hartmann et al, 2018). Trees control water loss by stomatal regulation, they downregulate their physiological activity, and in many species leaf and branch shedding is a further strategy to mitigate water loss (Tyree et al, 1993; Rood et al, 2000; Wason et al, 2018) Despite these water-saving mechanisms, Ψ may decrease, leading to the formation of embolisms in the xylem (Tyree and Sperry, 1989; Zweifel and Zeugin, 2008), which disrupt water transport in the tree. As trees try to avoid embolism formation, xylem vulnerability curves may help to predict mortality thresholds of different species (e.g. Choat et al, 2012; Choat et al, 2018)

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