Soil water vapour isotopes identify missing water source for streamside trees

Abstract

AbstractIn 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.