Soil water vapour isotopes identify missing water source for streamside trees

Abstract

In a seminal study, Dawson and Ehleringer used deuterium measurements on water to show that boxelder (Acer negundo) and bigtooth maple (Acer grandidentatum) trees growing directly next to a perennial stream in a mountain meadow did not use the stream water for transpiration, nor could they identify any other potential water source. Since then, similar observations of plant water isotopic signatures that do not match any available water source have been reported from a range of ecosystems worldwide. Here, we use a combination of non-destructive in situ isotopic soil water sampling along with destructive soil sampling and cryogenic water extraction to demonstrate that isotopically distinct water pools can be identified in soils at the same site as the 1991 study during parts of the summer growing season. Water extracted from xylem tissues shows that streamside boxelder and bigtooth maple at the sites were predominantly using soil water with an isotopic composition similar to that measured by the in situ soil measurement technique. We introduce the use of water vapour probes that were directly attached to intact tree roots in situ to measure the isotopic composition of root xylem water and how it varies through the growing season and evaluate their potential and utility.

Spatio‐temporal heterogeneity in soil water stable isotopic composition and its ecohydrologic implications in semiarid ecosystems

“…We interpret this change as the flushing of stagnant xylem water from winter senescence and replacement with new water at the onset of leaf emergence in the spring. Similar isotopic shifts in δ 2 H liq and δ 18 O liq values of xylem water of multiple tree species in semiarid Utah montane ecosystems have been interpreted to result from this leaf flush mechanism (Oerter & Bowen, ; Oerter et al, ; Phillips & Ehleringer, ).…”

Possible Increases in Flood Frequency Due to the Loss of Eastern Hemlock in the Northeastern United States: Observational Insights and Predicted Impacts

“…Although we have designed our field collections to sample only during periods of dry weather, we acknowledge that some uncertainty is introduced by the assumption that discrete plant stem and soil samples are representative of uptake and available soil water. Previous isotopic evaluation of plant xylem water for understanding root water uptake sources has suggested potentially missing end members (Bowling et al, ; Oerter et al, ). Our findings are similar in that plant xylem measurements of EH and AB are more enriched than observed soil water isotopic composition, particularly in the fall season.…”

“…Our findings are similar in that plant xylem measurements of EH and AB are more enriched than observed soil water isotopic composition, particularly in the fall season. This may be related to missing end members as in Oerter et al (2019), or possibly related to storage and mixing dynamics within trees. Finally, we acknowledge that assuming stable water isotopic analysis of extracted stem water is a meaningful approximation of water taken up by plant roots remains a methodology with several open questions (Penna et al, 2018;Berry et al, 2018).…”

Calibration method affects the measured δ2H and δ18O in soil water by direct H2Oliquid–H2Ovapour equilibration with laser spectroscopy

“…The calibration methods were conducted based on destructively collected soil samples. However, the DVE-LS method could also be applied for in situ isotope monitoring with gas-permeable probes (Gaj et al, 2016;Oerter, Siebert, Bowling, & Bowen, 2019;Volkmann & Weiler, 2014).…”