Stress thresholds of mature European beech trees

This data set contains the data presented in the figures 1-6 in Walthert et al. (2020): From the comfort zone to crown dieback: sequence of physiological stress thresholds in mature European beech trees across progressive drought. Science of the Total Environment. DOI: 10.1016/j.scitotenv.2020.141792.
A detailed methodical description of the data can be found in the Material and Methods section of the paper.
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.

本数据集包含Walthert等人2020年发表于《整体环境科学（Science of the Total Environment）》的论文中图1至图6所呈现的相关数据，原论文题为《从舒适区到冠层枯萎：渐进干旱下成熟欧洲山毛榉的生理胁迫阈值序列》，DOI: 10.1016/j.scitotenv.2020.141792。
该数据集的详细方法学说明可参见该论文的“材料与方法”章节。
成熟树木的干旱响应机制目前仍未得到充分阐释，这使得我们难以预测气候变暖背景下的物种分布格局。本研究以欧洲广泛分布且具有重要经济价值的欧洲山毛榉（Fagus sylvatica L.）为研究对象，旨在构建一套经验性胁迫等级体系，以描述其在干旱条件下的生理响应。研究团队对瑞士境内9个水文特征各异的山毛榉林分开展了全面测量，分析了土壤与叶片水势降低对土壤水分吸收、茎干半径、自然栓塞作用（embolism）、早期落叶以及冠层枯萎的影响，测量数据涵盖了2018年极端干旱事件以及2019/2020年干旱后恢复期的观测记录。
基于观测到的水势降低响应，研究推导得到以下5级胁迫等级：I级（黎明前叶片水势（pre-dawn leaf water potential）≤-0.4 MPa）：未检测到水力限制（hydraulic limitation）；II级（-0.4 ~ -1.3 MPa）：茎干持续收缩，生长停止；III级（-1.3 ~ -2.1 MPa）：自然栓塞与落叶开始发生；IV级（-2.1 ~ -2.8 MPa）：冠层枯萎启动；V级（≤-2.8 MPa）：蒸腾作用停止，冠层枯萎程度达20%。本等级体系首次为干旱条件下成熟欧洲山毛榉的生理下调过程提供了定量阈值，从而为基于过程的物种分布模型（process-based species distribution models）整合了关键基础信息。此外，本研究揭示欧洲山毛榉对干旱较为脆弱：其一，其在栓塞程度较高时仍会进行大量蒸腾；其二，落叶更多是栓塞作用的结果，而非预防栓塞的机制。在2018年干旱事件中，仅暴露于III至V级胁迫达1个月，便足以在浅土层山毛榉林中引发显著冠层枯萎。与之相反，在持水能力较强的深土层中，深层土壤的水分储备可避免山毛榉遭受干旱胁迫。这一结果强调，在预测欧洲山毛榉的未来分布时，纳入土壤水分有效性的本地化数据至关重要。