High-resolution water and stable isotope dynamics in drought-stressed urban vegetation

Urban green spaces are highly valuable in supporting the climate and infrastructure of cities through rainwater retention, evaporative cooling and shading. Investigating the diurnal and seasonal ecohydrological process dynamics in the urban soil-plant-atmosphere continuum is crucial to understand what types of landcover might best balance water re-distribution for a particular urban landscape providing cooling effects whilst not compromising groundwater recharge.

Stable water isotopes are very useful tools to investigate these complex interactions between soil properties, plant physiology and atmospheric drivers. In 2022, we conducted an experimental study looking at the complex patterns of the urban soil-plant-atmosphere interface at high temporal resolution at an urban tree stand and a grassland in Berlin, Germany during an entire growing season. To assess atmospheric moisture demand and vegetation water dynamics, we performed novel in-situ and real-time sequential measurements in the field at different heights in tree xylem and in the atmosphere. This was complemented by destructive sampling of soil water isotopes from multiple depths and eddy flux measurements at an open field near the sites.

We could identify clear evaporative and drought signals in the different water cycle components, including the extensive summer drought across continental Europe in July and August 2022. Results showed faster and larger responses to precipitation inputs in the upper soil moisture signal at the grassland. Atmospheric fluxes indicated clear evaporative losses just above the grass (15 cm height). Underneath tree canopies, upper soils responded more slowly to precipitation inputs and the atmospheric profile showed more homogenous spatio-temporal distribution of water vapour signals. Xylem water dynamics revealed contrasting diurnal and seasonal variations in different tree species and tree heights. Plant water sources were mostly drawn from deeper soil (70 cm) horizons. During the extended period of water scarcity in August, drought signals came from enriched atmospheric water vapour and low sap flux.

This knowledge of the water dynamics under different drought-stressed urban vegetation is extremely useful for the development of isotope aided ecohydrological models and allows science-based evidence for sustainable urban planning tackling climate change and urban densification.