Preprints
https://doi.org/10.5194/bg-2021-35
https://doi.org/10.5194/bg-2021-35

  15 Feb 2021

15 Feb 2021

Review status: a revised version of this preprint is currently under review for the journal BG.

Temporal dynamics of tree xylem water isotopes: In-situ monitoring and modelling

Stefan Seeger and Markus Weiler Stefan Seeger and Markus Weiler
  • Hydrology, Faculty of Environment and Natural Resources, University of Freiburg

Abstract. We developed a setup for a fully automated, high frequency in-situ monitoring system of the stable water isotopes Deuterium and 18O in soil water and tree xylem. The setup was tested for 12 weeks within an isotopic labelling experiment during a large artificial sprinkling experiment including three mature European beech (Fagus sylvatica) trees. Our setup allowed for one measurement every 12–20 minutes, enabling us to obtain about seven measurements per day for each of our 15 in-situ probes in the soil and tree xylem. While the labelling induced an abrupt step pulse in the soil water isotopic signature, it took seven to ten days until the isotopic signatures at the trees' stem bases reached their peak label concentrations and it took about 14 days until the isotopic signatures at 8 m stem height levelled off around the same values. During the experiment, we observed the effects of several rain events and dry periods on the xylem water isotopic signatures, which fluctuated between the measured isotopic signatures observed in the upper and lower soil horizons. In order to explain our observations, we combined an already existing root water uptake (RWU) model with a newly developed approach to simulate the propagation of isotopic signatures from the root tips to the stem base and further up along the stem. The key to a proper simulation of the observed short term dynamics of xylem water isotopes, was accounting for sap flow velocities and the flow path length distribution within the root and stem xylem. Our modelling framework allowed us to identify parameter values that relate to root depth, horizontal root distribution and wilting point. The insights gained from this study can help to improve the representation of stable water isotopes in trees within ecohydrological models and the prediction of transit time distribution and water age of transpiration fluxes.

Stefan Seeger and Markus Weiler

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-35', John Marshall, 24 Mar 2021
    • AC1: 'Reply on RC1', Stefan Seeger, 11 Apr 2021
  • RC2: 'Reviewer comment on bg-2021-35', Anonymous Referee #2, 26 Mar 2021
    • AC2: 'Reply on RC2', Stefan Seeger, 11 Apr 2021

Stefan Seeger and Markus Weiler

Model code and software

In-situ Water Isotope Sampling System Stefan Seeger https://github.com/stseeger/IsWISaS

Stefan Seeger and Markus Weiler

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Short summary
We developed a setup for fully automated in-situ measurements of stable water isotopes in soil and the stems of fully grown trees. We used this setup in a 12 week field campaign to monitor the propagation of a labelling pulse from the soil up to a stem height of 8 m. We could observe trees shifting their main water uptake depths multiple times, depending on water availability. The gained knowledge about the temporal dynamics can help to improve water uptake models and future study designs.
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