Impact of recurrent droughts on ecohydrological feedbacks and drought legacies of key European tree species

• Applicants: Dr. Simon Haberstroh; Professor Dr. Markus Weiler; Professorin Dr. Christiane Werner
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548817456

In recent years, Central European forests have been exposed to exceptional droughts. However, our understanding of legacy effects, i.e. deviations from the pre-drought state due to delayed recovery or overcompensation is still limited. Drought impact on trees and their recovery are shaped by a complex interplay between species-specific drought adaptations and the edaphic and hydrological conditions. These ecohydrological feedbacks at the soil-plant interface may be key factors to enhance or buffer severe drought impacts and rate of recovery. In particular, below-ground processes and the influence of species-specific ecohydrological feedbacks under drought remain a black box. Here, stable isotopes of water are ideal tracers to study ecohydrological feedbacks and processes. With our novel system for continuous in-situ measurement of stable isotopes in xylem and soil water, validated in beech, spruce and mixed forests, we are able to quantify important below-ground processes, such as root water uptake, and their importance for water relations on a daily basis. To study the drought response and drought legacies of five key tree species (beech, spruce, fir, sessile oak, Douglas fir), we will apply drought regimes of varying timing and frequency with rain out shelters. Information from post-drought isotopic tracer applications in combination with ecohydrological response parameters will be used to quantify drought legacies of a) summer, b) spring and c) recurrent droughts and the impact of ecohydrological feedbacks. Our main objective is to quantify species-specific drought legacies and vulnerability to recurrent drought. Furthermore, we hypothesize that trees generate species-specific ecohydrological feedbacks at the soil-plant interface, which are critical for species-specific recovery under recurrent drought. Work package 1 (WP1) will advance our continuous water stable isotope measurement system to enable measurements at a higher temporal and spatial resolution. After sensor installation, such as sap flow, water isotope probes, dendrometers, or stem water potential (WP2), we will impose different drought regimes with dynamic rain out shelters (WP3). Half of the trees will be exposed to a spring drought in the first growing season, while all trees will experience a severe summer drought in the second year. This experimental design will allow for the investigation of drought timing (spring vs. summer) and recurrence on drought legacy effects. Pulse labeling with an isotopic tracer in post-drought periods will elucidate ecohydrological feedbacks at the tree-soil interface and uncover their importance for drought legacies. WP4 will quantify and model drought legacies and ecohydrological feedbacks and aim to adapt model structures to include ecohydrological feedbacks (WP4). Elucidating drought vulnerability, legacy and the role of ecohydrological feedback will provide important insights to aid climate-resistant forests adaptations.

• DFG Programme: Research Grants
• Major Instrumentation: Wasserisotopenlaser
• Instrumentation Group: 1890 Optische Spektrometer (außer 180-186)