Preprints
https://doi.org/10.5194/hess-2020-674
https://doi.org/10.5194/hess-2020-674

  09 Feb 2021

09 Feb 2021

Review status: this preprint is currently under review for the journal HESS.

Ecohydrological travel times derived from in situ stable water isotope measurements in trees during a semi–controlled pot experiment

David Mennekes1,a, Michael Rinderer1, Stefan Seeger1, and Natalie Orlowski1 David Mennekes et al.
  • 1Hydrology, University of Freiburg, Freiburg im Breisgau, Germany
  • anow at: Empa − Swiss Federal Laboratories for Materials Science and Technology, Technology and Society Laboratory, St. Gallen, Switzerland

Abstract. Recent advances in in situ measurement techniques for stable water isotopes offer new opportunities to improve the understanding of tree water uptake processes and ecohydrological travel times. In our semi–controlled experiment with 20–year–old trees of three different species (Pinus pinea, Alnus incana and Quercus suber) placed in large pots, we applied in situ probes for stable water isotope measurements to monitor the isotopic signatures of soil water and tree xylem before and after two deuterium labelled irrigations. Additional destructive sampling of soil and plant material complemented the in situ measurements and allowed for a comparison between destructive (cryogenic vacuum extraction and direct water vapour equilibration) and in situ isotope measurements. For the first labelling pulse, the tracer based travel time at a stem height of 15 cm was 0.7 days for all three tree species but at 150 cm height tracer based travel times ranged between 2.4 (for Alnus incana) and 3.3 days (for Quercus suber). The tracer based travel time from the root zone to 15 cm stem height was similar to the sap flow based travel times (i. e., for all trees 0.7 days). However, sap flow based travel times were 1.3 days (for Alnus incana) longer than tracer based travel times at 150 cm stem height. In terms of different between tree species, we found similar tracer movement in Pinus pinea and Alnus incanca while in Quercus suber tracer travel times were longer which is likely due to lower water uptake rates of Quercus suber. The comparison of destructive and in situ isotope measurement techniques suggests notable differences in the sampled water pools. In situ measurements of soil and xylem water were much more consistent between the three tree pots (on average standard deviations were by 8.4 ‰ smaller for δ2H and by 1.6 ‰ for δ18O for the in situ measurements) but also among the measurements from the same tree pot in comparison to the destructive methods (on average standard deviations were by 7.8 ‰ and 1.6 ‰ smaller for δ2H and δ18O, respectively). Our study demonstrates the potential of semi-controlled large scale pot experiments and high-frequent in situ isotope measurements for monitoring tree water uptake and ecohydrological travel times. It also shows that differences in sampling techniques or sensor types need to be considered, when comparing results of different studies and within one study using different methods.

David Mennekes et al.

Status: open (until 06 Apr 2021)

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David Mennekes et al.

David Mennekes et al.

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Short summary
In situ stable water isotope measurements are a recently developed method to measure water movement from the soil through the plant to the atmosphere in high resolution and precision. Here, we present important advantages of the new method in comparison to commonly used measurement methods in an experimental setup. Overall, this method can help to answer research questions like plant responds to the climate change with potentially shifting water availability or temperatures.