Sub-daily dynamics of urban tree xylem water and ambient vapor

Abstract. Urban vegetation is vulnerable to rising temperatures and reduced rainfall, which reduces the cooling function of urban green spaces (UGS). The sub-daily dynamics of UGS water cycling and how this changes over the growing season remains largely unexplored due to measurement constraints. The monitoring of long-term in situ high-resolution water stable isotopes can provide valuable insights into how trees internally cycle water under different conditions. In this study, we analyzed a sub-daily (~3–4 hourly) dataset of atmospheric water vapor (δ_v) and tree stem xylem water (δ_xyl) in an urban tree stand in Berlin, Germany. We compared the diurnal (24 h) patterns of water cycling in δ_v, δ_xyl and ecohydrological variables during a summer drought followed by a rewetting period in 2022. Over the summer drought, water cycling was predominantly radiation driven, with highest vapor pressure deficit (VPD) rates in the afternoons and persistantly dry soils. We found systematic behaviour in both δ_v and δ_xyl signatures during the summer drought and δ_v was characterized by a daytime depletion in heavy isotopes, driven by local evaporation and atmospheric factors (entrainment). Daytime enrichment in δ_xyl, with maximal enrichment in afternoons, was consistent with diurnal hydroclimatic cycles, limited sap flow sourced from enriched soil water and stomatal regulation of transpiration. The trees showed lower twig water potential and sap flux relative to VPD in the afternoons, as well as stagnated night-time stem swelling, but the mature trees could overall sustain their physiological functioning. During rewetting, the UGS water cycle was precipitation driven, while potential evapotranspiration (PET) rates decreased. The systematic diurnal cycling of δ_v was mostly discontinued due to lower soil and canopy evaporation. Only δ_v just above the grassland surface (0.15 m) showed a significant daytime enrichment hinting for ET fluxes promoted by high moisture stored in soil and vegetation surfaces and transpiration of superficially enriched soil water. δ_xyl was still characterized by significant daytime enrichment, however, with sub-daily amplitudes more than halved compared to the drought period, when hydraulic conductance was restricted. Our continuous, sub-daily dataset of δ_v and δ_xyl have the potential to help constrain ecohydrological models towards prediction of climate change impacts on UGS. Urban planning should consider a “mosaic” of urban vegetation types together with resilient species composition to maximize cooling benefits throughout the 24-hour cycle.