Plant and root‐zone water isotopes are difficult to measure, explain, and predict: Some practical recommendations for determining plant water sources

Abstract

1. Stable isotope ratios of water (δ 18 O, δ 2 H) have long been used to study a core question in plant ecology and ecohydrology: 'From where do plants take up water?' Indeed, decades of research has involved sampling potential plant water sources in the subsurface, classifying those sources as distinct endmembers (e.g. deep vs. shallow soil waters) and then evaluating their contributions to a xylem water sample through mixing-model analysis to identify the depths of root water uptake. 2. However, more detailed interrogations of the subsurface and plant domains have revealed under-considered transport and isotopic fractionation phenomena. These now apparent complexities raise new questions and challenge the many past assumptions inherent in endmember-mixing models that now seem overly simple. 3. Here, we introduce discussions of these recent insights and provide an overview of isotope effects that occur naturally in the root zone and in the plant, as well as artificially during sample handling. Better accounting for these complexities and their associated uncertainties can lead to more accurate and robust study designs, analytical frameworks and, ultimately, inferences. 4. Finally, to more robustly characterize plant water sources using δ 18 O and δ 2 H, we provide some practical recommendations that aim at maximizing the isotopic contrast between endmembers and/or minimizing potential uncertainties. K E Y W O R D S ecohydrology, endmember-mixing models, plant water uptake, review, root zone, stable water isotopes, uncertainties 1 | INTRODUC TION AND MOTIVATION Plant water use can represent up to 90% of terrestrial evapotranspiration (Jasechko et al., 2013), and therefore is an important driver of the global water cycle (Sellers et al., 1997). Because plant water relations are closely linked to carbon and nutrient relations (Schulze, 1982), understanding plant water supplies is also key to predict global carbon and nutrient cycles (Lange, Kappen, | 1353 Methods in Ecology and Evoluঞon von FREYBERG Et al. How to cite this article: von Freyberg J, Allen ST, Grossiord C, Dawson TE. Plant and root-zone water isotopes are difficult to measure, explain, and predict: Some practical recommendations for determining plant water sources.

On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

“…In some cases, these effects can lead to biases and possibly incorrect interpretation of the results even at small scales. Some of these issues have been discussed in recent works (Penna et al, 2018;Barbeta et al, 2019Barbeta et al, , 2020Beyer et al, 2020;Sprenger and Allen, 2020;Von Freyberg et al, 2020) but in this opinion paper we aim at summarizing them in a comprehensive way and stimulating community efforts to address uncertainty in isotope-base studies of vegetation water sources and uncertainty propagation across spatial scales.…”

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

“…Our semi-controlled outdoor pot experiment aimed to minimize potential influences of boundary conditions that typically occur in natural forest environments e.g., soil heterogeneity, subsurface flow and redistribution of soil water, rainfall input, stemflow and associated variability in isotope signatures in the soil-tree compartment (see von Freyberg et al (2020).…”

“…We argue that the accuracy is well suited to perform tracer labelling experiments and monitor the tracer arrival that is typically orders of magnitude larger than natural isotopic variability. Studies that used destructive sampling also reported variability of isotope measurements taken from one single tree (von Freyberg et al, 2020). This is due to the fact that destructive sampling relies on multiple sampling locations that can differ (naturally) in their isotopic signature.…”

“…matric potential, VWC, sap flow). High temporal resolution constitutes an important step towards better understanding of fractioning processes or mixing of different water pools in soils and trees (von Freyberg et al, 2020;McDonnell, 2014).…”

“…Isotope measurements along extended flow paths might also be subject to potential isotope fractioning processes in the root zone, during water uptake or in the plant (Ellsworth and Williams, 2007;von Freyberg et al, 2020;Poca et al, 2019;Vargas et al, 2017) which would further complicate the interpretation of tracer breakthrough signals. Others have discussed the influence of an exchange of water stored in wood cells and transported in tree xylem (Martín-Gómez et al, 2016).…”