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

Abstract. The sub-daily dynamics of vegetation mediated 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 stable water isotopes can provide valuable insights into how trees and the surrounding atmosphere cycle water under different conditions. In this study, we analyzed a sub-daily (∼3–4 hourly) dataset of atmospheric water vapor (δv) and tree stem xylem water (δxyl) in an urban tree stand in Berlin, Germany. We compared the diurnal (24 h) patterns of water cycling in δv and δxyl values, as well as ecohydrological variables during a summer drought (1 July–14 August) followed by a rewetting period (15 August–30 September) in 2022. Over the summer drought, water cycling was predominantly radiation driven, with highest vapor pressure deficit (VPD) rates in the afternoons and persistently dry soils. We found systematic behaviour in both δv and δxyl signatures during the summer drought and δv values were characterized by a daytime depletion in heavy isotopes, driven by local evaporation and atmospheric factors (i.e., entrainment). The mean amplitude between day- and night-time values of δv during drought was 26 ‰ for δ2H. Daytime enrichment in δxyl values, with maximal isotopic enrichment in afternoons, was consistent with diurnal hydroclimatic cycles, maximum potential evaporation (PET) of ∼ 0.75 mm h−1 limited sap flow sourced from enriched soil water at the topsoil and stomatal regulation of transpiration. The mean amplitude between day- and night-time values of δxyl during drought was 38 ‰ for δ2H. 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 water cycle of the green space was precipitation driven, while maximum PET rates decreased to 0.5 mm h−1. The systematic diurnal cycling of δv values was mostly discontinued due to lower soil and canopy evaporation. Only δv values just above the grassland surface (0.15 m) showed a significant daytime enrichment in heavy isotopes (amplitude ∼36 ‰ δ2H) hinting for evapotranspiration fluxes promoted by high moisture stored in soil and vegetation surfaces and transpiration of superficially enriched soil water. δxyl values were still characterized by significant daytime enrichment, however, with sub-daily amplitudes of 20 ‰ δ2H almost halved compared to the drought period, when hydraulic conductance was restricted. Our continuous, sub-daily dataset of δv and δxyl values provides unique insights on the complex stable water isotope dynamics within trees and the surrounding atmosphere. It has the potential to help constrain ecohydrological models towards prediction of climate change impacts on water cycling within vegetated areas.