Ecohydrological connectivity between trees and the capillary zone - a key driver for drought resilience of European forests?

在过去的20年里，中欧的水文干旱变得越来越普遍。这一趋势预计将在未来加剧，对森林生态系统造成严重后果。持续（直到2021年）的干旱已经被认为是中欧森林状况恶化的主要原因之一。一个普遍接受的假设是，深根树木能够更好地抵御干旱事件，然而，我们对深层水源及其利用的基于过程的理解受到有限的可及性的限制。深层土壤水（bbb10 m）常被植物生理学家和生态水文学家忽视或简化。我们项目的中心目标是探索和量化从上方（通过降水）渗透的水、地下水补给和从下方上升的毛细血管以及作为这些不同资源最终使用者的树木之间的时空动态和反馈。我们将探索具有不同用水策略和生根深度的典型欧洲森林树木（山毛榉、橡树、云杉）的吸水性。我们将区分以下几种树木：1)直接进入地下水，2)连接到位于当地地下水位以上的毛细带，3)与地下水或毛细带没有直接连接，但仍具有深根系的树木，4)浅根树木。我们假设，对某些物种来说，保持与毛细带的连通性是耐旱性的关键组成部分。然而，树木对毛管带连通性的定量影响将主要取决于1)气候和地貌条件，这些条件决定了毛管带连通性的时空动态（干旱持续时间、毛管带缓冲能力）和2)物种特异性干旱适应（生根深度、适应性根系生长、等水性程度）。为了充分了解树木与毛细带的连通性对树木和森林健康以及水文循环的影响，我们的目标是使用生态水文、植物生理学、地球物理和基于模型的方法相结合。一个特别的重点将是一个新的连续稳定水同位素观测平台，创建一个具有高时空分辨率的数据集，包括土壤、木质部和大气水蒸气，并辅以树木年轮氧同位素和韧皮部碳同位素数据。这一组合将使我们不仅可以得出不同树种当前树木连通性的结论，还可以得出其对树木年轮同位素组成的影响，从而可以分析历史干旱事件。最后，将使用同位素支持的SVAT模式MuSICA来预测极端干旱、干旱和正常年份的深水吸收与树木健康之间的关系。

Hydrologic droughts have become increasingly common in the last 20 years in central Europe. This trend is projected to intensify in the future, leading to serious consequences for forest ecosystems. Already, the ongoing (until 2021) drought was deemed one of the major causes for the worsening condition of forests in central Europe. A commonly accepted hypothesis is that deep rooted trees are able to better withstand drought events, however, our process-based understanding of deep water sources and their use has been restricted by limited accessibility. Deep soil water (> 1 m) is mostly neglected or simplified by plant physiologists and ecohydrologists.The central aim of our project is to explore and quantify spatiotemporal dynamics and feedbacks between water infiltrating from above (via precipitation), groundwater replenishment and capillary rise from below and trees as ultimate users of these different resources. We will explore water uptake by typical European forest trees (beech, oak, spruce) with distinct water-use strategies and rooting depths. We will distinguish among trees that I) directly tap into groundwater, II) connect to the capillary zone residing above the local water table, and III) those with no direct connection to the groundwater or capillary zone, but which still possess a deep root system and IV) shallow rooted trees. We hypothesize that maintaining connectivity to the capillary zone is for some species a critical component of drought tolerance. However, the quantitative impact of tree connectivity to the capillary zone will depend largely on I) climate and geomorphological conditions that define spatiotemporal dynamics of capillary zone connectivity (drought duration, capillary zone buffer capacity) and II) species-specific drought adaptations (rooting depths, adaptive root growth, degree of isohydricity). To fully understand the impact of tree connectivity to the capillary zone on tree and forest health as well as hydrological cycling we aim to use a combination of ecohydrological, plant physiological, geophysical, and model-based approaches. A particular focus will be on a novel continuous stable water isotope observation platform creating a dataset with high spatiotemporal resolution that includes soil, xylem, and atmospheric water vapor complemented by tree ring oxygen isotopic and phloem carbon isotopic data. This combination will allow us to draw conclusions not only on current tree connectivity of different tree species, but also its impacts on tree-ring isotopic composition, allowing the analysis of historic drought events. Finally, the isotope enabled SVAT model MuSICA will be used to predict the relationship between deep-water uptake and tree health over the course of extremely dry, dry and normal years.