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

Abstract

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.

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

“…More recent work has demonstrated the feasibility of field application of spectroscopes in direct in situ monitoring to produce longer-term, reliable data of water in different compartments of the critical zone (Mennekes et al, 2021;Seeger and Weiler, 2021). Our study contributes to this evolution of such isotopic techniques to directly observe ecohydrological processes in soil and vegetation systems.…”

“…With the development of compact laser spectrometry systems, in situ monitoring of stable water isotopes in soil and tree xylem is now facilitating higher temporal and spatial resolution assessment to advance well-established destructive sampling methods (Herbstritt et al, 2012;Rothfuss et al, 2013;Volkmann and Weiler, 2014;Oerter and Bowen, 2017). However, destructive sampling followed by cryogenic vacuum extraction, is still widely used for stable water isotope analyses in plants (West et al, 2006;Yang et al, 2015;Orlowski et al, 2016;Sohel et al 2021) and is usually required to corroborate in situ measurements (see Mennekes et al, 2021). Despite the complexity, daily maintenance and resource demands, the application of in situ methods in different compartments of the critical zone including soil, trees (Kübert et al, 2020;Beyer et al, 2020;Marshall et al, 2020) and the atmospheric-interface (Braden-Behrens et al, 2019) is increasing.…”

“…Another labelling experiment by Mennekes et al (2021) investigated plot-scale xylem water isotopes in three different trees of various species (Pinus pinea, Alnus incana and Quercus suber) at two different heights (15 and 150 cm) over the course of 10 weeks using similar tree probes described by Volkmann et al (2016). They sampled xylem water isotopes at 5-hourly intervals and showed that in situ measurements delivered more consistent results compared to destructive samples.…”

“…CC BY 4.0 License.forest. More complex ecohydrological modelling that explicitly conceptualises the travel time of water in trees maybe able to provide deeper insights (e.g Mennekes et al, 2021)…”

Causes and Factors of Cryogenic Extraction Biases on Isotopes of Xylem Water

“…For example, stem water encompasses sap water, living parenchyma, phloem cell water, and the intercellular space between xylem cells (Jupa et al., 2016; Tyree & Zimmermann, 2002). While the transit time of sap water varies from hours to days in plants (Mennekes et al., 2021), tissue water may have a much longer residence time (Berghuijs & Allen, 2019). Due to the differing mobility, physiological processes, and degree of influence of external environmental conditions, the sap water and tissue water in xylem possess differing isotopic compositions (De Deurwaerder et al., 2020; Ellsworth & Sternberg, 2015).…”