Preprints
https://doi.org/10.5194/hess-2021-456
https://doi.org/10.5194/hess-2021-456

  22 Oct 2021

22 Oct 2021

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

Xylem water in riparian Willow trees (Salix alba) reveals shallow sources of root water uptake by in situ monitoring of stable water isotopes

Jessica Landgraf1,2, Dörthe Tetzlaff1,2,3, Maren Dubbert1,4, David Dubbert1, Aaron Smith1, and Chris Soulsby3 Jessica Landgraf et al.
  • 1Department of Ecohydrology and Biogeochemistry, Leibniz Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 310, 12587 Berlin, Germany
  • 2Department of Geography, Humboldt-Universität zu Berlin, Rudower Chaussee 16, 12489 Berlin, Germany
  • 3Northern Rivers Institute, University of Aberdeen, St. Mary’s Building, Kings College, Old Aberdeen, AB24 3UE, UK
  • 4Landscape Functioning, Leibniz Centre for Agricultural Landscape Research, Eberswalder Straße 84, 15374 Müncheberg

Abstract. Root water uptake is an important critical zone process, as plants can tap various water sources and transpire these back into the atmosphere. However, knowledge about the spatial and temporal dynamics of root water uptake and associated water sources at both high temporal resolution (e.g. daily) and over longer time periods (e.g. seasonal) is still limited. We used cavity ring-down spectroscopy (CRDS) for continuous in situ monitoring of stable water isotopes in soil and xylem water for two riparian willow (Salix alba) trees over the growing season (May to October) of 2020. This was complemented by isotopic sampling of local precipitation, groundwater and stream water in order to help constrain the potential sources of root water uptake. A local flux tower, together with sap flow monitoring, soil moisture measurements and dendrometry were also used to provide the hydroclimatic and ecohydrological contexts for in situ isotope monitoring. In addition, bulk samples of soil water and xylem water were collected to corroborate the continuous in situ data. The monitoring period was characterised by frequent inputs of precipitation, interspersed by warm dry periods which resulted in variable moisture storage in the upper 20 cm of the soil profile and dynamic isotope signatures. This variability was greatly damped in 40 cm and the isotopic composition of the sub-soil and groundwater was relatively stable. The isotopic composition and dynamics of xylem water was very similar to that of the upper soil and analysis using a Bayesian mixing model inferred that overall ~90 % of root water uptake was derived from the upper soil profile. Sap flow and dendrometry data indicated that soil water availability did not seriously limit transpiration during the study period, though there was a suggestion that deeper (> 40 cm) soil water might provide a higher proportion of root water uptake (~30 %) in a drier period in the late summer. The study demonstrates the utility of prolonged real time monitoring of natural stable isotope abundance in soil-vegetation systems, which has great potential for further understanding of ecohydrological partitioning under changing hydroclimatic conditions.

Jessica Landgraf et al.

Status: open (until 25 Dec 2021)

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  • RC1: 'Comment on hess-2021-456', Valentin Couvreur, 30 Nov 2021 reply

Jessica Landgraf et al.

Jessica Landgraf et al.

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Short summary
Using water stable isotopes, we studied from which water source (lake, stream, ground or soil water) two willows were taking their water. We monitored the environmental conditions (e.g., air temperature, soil moisture) and the behavior of the trees (water flow in the stem). We found that the most likely water sources of the willows were the upper soil layers, but that there were seasonal dynamics.