Insights into plant water uptake from xylem-water isotope measurements in two tropical catchments with contrasting moisture conditions

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.&#160; In-situ isotope probes (Volkmann et al. 2016a&#160; & 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 &#8240;) 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&#160; 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.&#160;Volkmann, T. H., Haberer, K., Gessler, A., & Weiler, M. (2016a). High&#8208;resolution isotope measurements resolve rapid ecohydrological dynamics at the soil&#8211;plant interface. New Phytologist, 210(3), 839-849.&#160;Volkmann, T. H., K&#252;hnhammer, 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.&#160;Jarvis, N. J. (1989). A simple empirical model of root water uptake. Journal of Hydrology, 107(1-4), 57-72.&#160;

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Monitoring tree species-specific water uptake strategies via continuous in-situ stable water isotope measurements

10.5194/egusphere-egu2020-16786 ◽

2020 ◽

Cited By ~ 1

Author(s):

Natalie Orlowski ◽

Stefan Seeger ◽

David Mennekes ◽

Hugo de Boer ◽

Markus Weiler ◽

...

Keyword(s):

Water Uptake ◽

Temporal Resolution ◽

Tree Species ◽

Isotopic Signature ◽

Plant Water ◽

Destructive Sampling ◽

Species Specific ◽

Isotope Measurements ◽

Water Isotope

Water isotope tracing techniques in combination with laser-based isotopic analyses have advanced our understanding of plant water uptake patterns providing opportunities to carry out observational studies at high spatio-temporal resolution. Studying these highly dynamic processes at the interface between soils and trees can be challenging under natural field conditions, as available water resources are difficult to control. On the other hand, the results of small pot experiments in the greenhouse using tree seedlings are often difficult to transfer to mature trees. Here, we setup a controlled outdoor large pot experiment with three different, 4-6 meter high and 20 year old trees: Pinus pinea, Alnus spaethii and Quercus suber. We took advantage of stable water isotope techniques by tracing plant water uptake from the root zone through the xylem via isotopically labelled irrigation water. We combined ecohydrological observations of sapflow, photosynthesis, soil moisture and temperature and soil matrix potential with high resolution measurements of water stable isotopes in soils and trees to understand how soil water is used by different tree species. We monitored the isotopic composition of soil and xylem water in high temporal resolution with in-situ isotope probes installed at different depths in the soil and different heights in the tree stem. We further compared the water isotopic composition of our in-situ monitoring setup with destructive sampling methods for soil and plant water (vapour equilibration method and cryogenic extraction).Our results from the continuous monitoring showed a distinct difference in the xylem sap isotopic signature between Quercus on the one hand and Alnus and Pinus on the other hand. This is likely due to different water use strategies of these tree species. The tree xylem isotopic signature of Alnus and Pinus responded to the isotopic label within one day and six days at 15 cm and 150 cm stem height, respectively. The peak isotopic signature in the tree xylem due to the label application was similar to the isotopic signature of the soil in 30 cm (for Alnus) and 15&#160;cm (for Pinus). Quercus showed a delayed and much slower increase in the xylem isotopic signature in response to the label and the highest values were significantly lower than the corresponding soil isotopic signatures. Our methodological comparison showed that the isotopic signature of the destructive samples (from both methods) had a larger spread and this spread tended to become larger with subsequent labeling. Destructive soil samples showed a wider isotopic variation than destructive xylem samples. The in-situ isotope measurements in comparison showed a relative constant small to medium spread for soil and xylem isotopic measurements. Our in-situ isotope probes therefore seem to be a potential alternative or supplement to destructive sampling offering much higher temporal resolution. The continuation of the labeling experiments in 2020 will allow us to further study tree-species specific water uptake strategies, which will become important under future climatic conditions in terms of development of adaptation strategies for sustainable forest management.

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