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

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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.