EGU2020-22007, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-22007
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Inferring plant physiologic parameters for root water uptake modelling from high frequency in-situ isotope measurements

Stefan Seeger, Michael Rinderer, and Markus Weiler
Stefan Seeger et al.
  • Earth and Environmental Sciences, University of Freiburg, Freiburg, Germany (stefan.seeger@hydrology.uni-freiburg.de)

In the face of global climate change, a well-informed knowledge of plant physiologic key parameters is essential to predict the behavior of ecosystems in a changing environment. Many of these parameters may be determined with lab or pot experiments, but it could prove problematic to transfer results obtained in a such experiments with small trees to fully grown trees. Therefore, new approaches to determine relevant parameters for mature trees are still required. Regarding plant water uptake, parameters related to fine root distribution (maximum depth, depth distribution and rhizosphere radius) and parameters describing the physiological limits of root water uptake are important, but usually hard or costly to assess for fully grown trees.  In-situ isotope probes (Volkmann et al. 2016a  & 2016b) are a promising recent development that offer new possibilities for the investigation of plant water uptake and associated physiological parameters.

In this study we used in-situ stable water isotope probes in soil (six depths from 10 to 100 cm) and in tree xylem of mature (140 years) European beech trees (three heights between 0 and 8 m). With those probes, we monitored soil and xylem isotope signatures after an isotopically labeled (Deutrium-Excess = 100 ‰) irrigation pulse equivalent to 150 mm of precipitation and foursubsequent natural precipitation events over a period of twelve weeks with a high temporal resolution (six or more measurements per probe per day). Those measurements were complemented with measurements of soil moisture and sap flow dynamics. We interpolated our measured soil isotope and soil moisture data in order to obtain spatially and temporally continuous data for those soil parameters. Then we used this data as an input to the Feddes-Jarvis plant water uptake model, in order to predict the isotopic signature of plant water uptake at daily time steps. With the help of our observed isotopic signatures, we were able to directly constrain the critical water potential parameter of the Feddes model as well as the underlying fine root distribution. Furthermore, the observed dampening of the breakthrough curve of our Deuterium-labeling pulse allowed us to infer information on the rhizosphere  radius and water transport velocities in the fine roots and stem between the points of root water uptake and the eight meter stem height.

With our field experiment we showed that in-situ isotope measurements in soil profiles and in tree xylem sap can help to constrain plant water uptake modelling parameters. Future experiments might use this approach to scrutinize lab-scale derived hypothesizes regarding tree water uptake and to investigate the temporal and spatial dynamics of root water uptake in the field.

 

Volkmann, T. H., Haberer, K., Gessler, A., & Weiler, M. (2016a). High‐resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. New Phytologist, 210(3), 839-849. 

Volkmann, T. H., 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 & environment, 39(9), 2055-2063. 

Jarvis, N. J. (1989). A simple empirical model of root water uptake. Journal of Hydrology, 107(1-4), 57-72. 

How to cite: Seeger, S., Rinderer, M., and Weiler, M.: Inferring plant physiologic parameters for root water uptake modelling from high frequency in-situ isotope measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22007, https://doi.org/10.5194/egusphere-egu2020-22007, 2020

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Presentation version 3 – uploaded on 05 May 2020
- updated link to one of the related displays on the second slide
  • CC1: Comment on EGU2020-22007, Valentin Couvreur, 05 May 2020

    Thanks Stefan for the very interesting display!
    I also wanted to ask you the following question:
    The Jarvis function allows a redistribution of the root water uptake profile (so-called “compensation”) without necessarily reducing transpiration rate, provided that the omega_c parameter is lower than Feddes’ average soil water stress function Beta. That seems to be what you observed, which is an exciting result. Were you able to compare the optimal parameter sets of root distribution, soil water stress and compensation to physically measurable quantities (such as the actual root length density profile), and did you find that they were consistent?
    Thanks in advance for your reply!
    Val

    • AC1: Reply to CC1, Stefan Seeger, 05 May 2020

      Dear Valentin,

      unfortunately, we are not as far as we hoped to be at this point when we wrote the abstract for this contribution. We have not yet been able to put all the pieces together and infer model parameter ranges from my observations, but we are on it.

      The biggest issue with our data, is that we only measured volumetric soil mositure and no soil water potentials. As the Feddes model is based on water potentials, we would need a reliable pF-curve to make use of our soil moisture measurements. The soil at our site is really skeleton rich, which makes it hard to infer pF-curves from lab based soil sample analysis or texture based transfer functions. Fortunatley, this summer our filed site will be equipped with additional matrix potential probes that work down to pF 7. So we hope to eventually get our much needed pF-curves within a year.

      Until then, we will have to stick to some assumed possible pF-curve which will introduce additional uncertainty. We still will be able to estimate paramter ranges for the Feddes model and might be able to quantify the uncertainty.

      Best regards,
      Stefan

      • CC2: Reply to AC1, Valentin Couvreur, 06 May 2020

        Thanks Stefan!

        Looking forward to see the results using the new matric potential curve.

        In its original form (the 1976 paper), Feddes function was using soil water content instead of soil matric potential, so I guess you could as well use it in the meantime.

        We did a similar inverse modelling exercise with Youri Rothfuss and colleagues. Will be discussed this afternoon. Feel free to stop by or post comments on the Display (https://meetingorganizer.copernicus.org/EGU2020/EGU2020-18071.html).

        Cheers,

        Val

Presentation version 2 – uploaded on 04 May 2020 , no comments
- added a second slide that contains links to related presentations at EGU2020
Presentation version 1 – uploaded on 24 Apr 2020 , no comments