Assessing the impact of drought on water cycling in urban trees via in-situ isotopic monitoring of plant xylem water

Urban trees are an integral part of sustainable cities. They regulate the local microclimate and enhance the urban water cycle. Increasing periods of drought can impair urban trees by affecting their water uptake, transpiration and growth patterns. In this study, we used a multi-proxy approach to assess how non-irrigated urban trees react to changing water supply throughout the full vegetative period of 2022 including a major drought in Berlin, Germany. Our work focused on individual mature trees in an urban green space; examining daily mean in-situ isotopes in plant xylem water (δ_xyl) while also monitoring vegetation dynamics via sap flow, stem increments, LAI, as well as groundwater, and soil moisture at different depths. The monitoring period was characterised by a spring with average precipitation inputs, followed by an extremely dry period from July until mid-August, then a gradual rewetting from the end of August until October. At the beginning of the growing period, changes in the ecohydrological dynamics of the investigated maple and birch trees were high with increases in stem size and LAI, but also decreasing soil moisture. In spring, δ_xyl signatures were high in both trees, with the effect more marked in the maple hinting at a dependence of δ_xyl on species specific-storage effects and a distinct start of transpiration. During summer, drought stress was apparent in the ecohydrological fluxes of the monitored trees in the reduction of stem growth, LAI, midday water potential and soil moisture. Yet sap flow rates were relatively stable and tree transpiration maintained. We noted a midsummer enrichment of δ_xyl in both species. Most importantly, the measured δ_xyl signatures were isotopically in the range of deep soil waters and groundwater implying that deeper sources were sustaining the trees’ water supply during the drought. We also detected fractionation of δ_xyl, which is possibly induced by heterogenous water uptake strategies and biochemical processes in the tree xylem, including CH₄ transport. Our results suggest that urban trees rely on deep water supply and internal storage during drought. We conclude urban trees and shrubs with shallow root development would be more vulnerable to dry summers with a particular threat during future accelerated summer droughts combined with insufficient autumn rewetting causing deep soil layers to dry out.