EGU General Assembly 2020
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Where does the water come from? Variations in soil water uptake depth in a beech forest during the 2018 drought

Arthur Geßler1, Lukas Bächli1, Kerstin Treydtre1, Matthias Saurer1, Matthias Häni1, Roman Zweifel1, Andreas Rigling1, Marcus Schaub1, Stefan Seeger2, Barbara Herbsritt2, Markus Weiler2, and Katrin Meusburger1
Arthur Geßler et al.
  • 1Swiss Federal Research Institute WSL, Forest Dynamics, Birmensdorf, Switzerland
  • 2University of Freiburg, Freiburg, Germany

Water uptake under variable soil water supply is highly critical for the functioning of trees and the services provided by forests. Current climate projections predict an increasing variability of precipitation and thus a higher frequency of droughts alternating with extreme precipitation events. Reduced water availability is the most critical driver for tree mortality and impairment of trees’ functions. Under variable water supply, both the ability of a plant species to utilize remaining water under drought and to immediately capitalize on soil rewetting from subsequent rainfall events will be crucial for its survival and competitiveness. High uncertainty still exists regarding the ecohydrological belowground interactions at the soil–root interface on short to seasonal time scales.

To overcome previous limitations, we carried out high-resolution in situ observations of δ18O in soil and xylem water to track the water uptake of beech trees based on the approaches of Volkmann et al. (2016a & b) in the hot dry summer 2018. We set up a laser isotope system to continuously probe the δ18O signature in the water vapor in equilibrium with the soil water at different soil depths and with the xylem of beech trees in a forest in Switzerland and applied a Bayesian isotope mixing model (BIMM) to resolve the origin of the water taken up. Moreover, we installed xylem flow sensors, dendrometers and soil moisture sensors in the trees.

Mid of June the drought period started with extended phases of high temperature and only infrequent precipitation. At the same time, soil water content sharply decreased, especially in the upper soil layers and transpiration as well as radial growth started to decline, and this pattern became more pronounced until the end of August. In the soil water, strong 18O enrichment in the upper 5 cm and slighter enrichment in 15 cm developed during this period. The BIMM results indicated that tree xylem water was made up by > 80% of shallow soil water (0-15 cm) at the onset of the drought and that this contribution continuously dropped to < 20% by the end of August, when deeper soil water and groundwater became more important. End of August, intensive rainfall events along with decreasing temperatures terminated the drought period when shallow soil water pools became partially replenished, and transpiration increased again. Within days, the contribution of shallow soil water to tree xylem water increased and reached a share of > 70% a couple of weeks after the end of the drought.  With the in situ method applied here, real-time information of the plasticity of soil water use becomes available and we can l trace the effect of drought and drought release on root activity of trees in different soil depths.


Volkmann THM, Haberer K, Gessler A, Weiler M. 2016a.High-resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. The New phytologist210: 839-849.

Volkmann THM, Kühnhammer K, Herbstritt B, Gessler A, Weiler M. 2016b.A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy. Plant, Cell and Environment9: 2055–2063.

How to cite: Geßler, A., Bächli, L., Treydtre, K., Saurer, M., Häni, M., Zweifel, R., Rigling, A., Schaub, M., Seeger, S., Herbsritt, B., Weiler, M., and Meusburger, K.: Where does the water come from? Variations in soil water uptake depth in a beech forest during the 2018 drought, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3451,, 2020.

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Display material version 1 – uploaded on 30 Apr 2020
  • CC1: Comment on EGU2020-3451, Anke Hildebrandt, 06 May 2020

    Thank you Arthur, this is an amazing dataset!

    Do you have an idea of the total rooting depth in this location? Could the uptake have been even deeper?

    Also, was total uptake derived on sap flow ?

    • AC1: Reply to CC1, Arthur Geßler, 06 May 2020

      Dear Anke

      I could be deeper - we have also groundwater d18O and we will include it in the BIMM to see if there are any affects of deeper water

  • CC2: Comment on EGU2020-3451, Maurizio Mencuccini, 06 May 2020

    dear Arthur

    I loved the dataset. I would love to use the data with a hydraulically-based transport model to test whether we can replicate it based on hydraulic theory.

    • AC2: Reply to CC2, Arthur Geßler, 06 May 2020

      Certainly - would be cool!