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Identification of Source‐Water Oxygen Isotopes in Trees Toolkit (ISO‐Tool) for Deciphering Historical Water Use by Forest Trees

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Water Resources Research
Authors:
Christopher Sargeant at Swiss Federal Institute for Forest, Snow and Landscape Research
Christopher Sargeant
Michael Bliss Singer at University of California, Santa Barbara
Michael Bliss Singer
Christine Vallet-coulomb at Aix-Marseille University
Christine Vallet-coulomb
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Abstract and Figures

Hydrological regimes are being perturbed under climate change due to the regional expression of the water cycle across the globe, leading to alterations in the spatial and temporal distribution of water near the Earth's surface. Water is a critical resource for plant ecosystems, and hydrological limitations on vegetative health are particularly complex. To anticipate how subsurface water availability may evolve in the future and affect the dynamics of plant water source usage, as well as the health and functioning of vegetation in various biomes, we need a robust, quantitative framework for linking water availability to past plant water use, which is constrained by historical data. Here, we outline the Identification of Source‐water Oxygen isotopes in trees Toolkit (ISO‐Tool), designed to retrospectively investigate the dynamics of tree water uptake. ISO‐Tool utilizes tree‐ring isotopes (δ¹⁸O) combined with a biomechanistic fractionation model to retrodict the δ¹⁸O of water utilized during any period of growth. Through comparisons with measured δ¹⁸O in local water sources, climatic, and hydrological variables, ISO‐Tool can reconstruct and inform on past ecohydrological interactions. We provide an overview of the modeling components and data requirements necessary to constrain the retrodictions of source‐water δ¹⁸O. We demonstrate the utility and efficacy of ISO‐Tool for three riparian field sites characterized by differences in climatic, geomorphic, and hydrologic complexity. We also state that ISO‐Tool can be applied to a range of vegetated environments where distinct isotopic endmembers exist. We present a set of tool groups, which can be applied adaptively, ensuring that scientific progress in understanding retrospective ecohydrology can be made, even under varying degrees of data availability.
Overview of ISO‐Tool and the primary components.
Overview of ISO‐Tool and the primary components.
… 
Key model components and how the required user input values are utilized (a–d), with associated terminology, and schematics of each process (e).
Key model components and how the required user input values are utilized (a–d), with associated terminology, and schematics of each process (e).
… 
Results of sensitivity analyses of independently (a) and co‐varied (b–g) input variables on the modeled δ¹⁸Osw value. Sensitivity analyses of co‐varied model inputs of (b) T and RH, (c) RH and δ¹⁸Owv, (d) T and δ¹⁸Owv, (e) δ¹⁸Owv with gs/E pairing, (f) RH with gs/E pairing, and (g) T with gs/E pairing. Gray shading in (b–d) indicates the range of δ¹⁸Omsw results produced by varying the respective input variables over their observed range of variance for the DM site in SE France (May–September: 2000–2010).
Results of sensitivity analyses of independently (a) and co‐varied (b–g) input variables on the modeled δ¹⁸Osw value. Sensitivity analyses of co‐varied model inputs of (b) T and RH, (c) RH and δ¹⁸Owv, (d) T and δ¹⁸Owv, (e) δ¹⁸Owv with gs/E pairing, (f) RH with gs/E pairing, and (g) T with gs/E pairing. Gray shading in (b–d) indicates the range of δ¹⁸Omsw results produced by varying the respective input variables over their observed range of variance for the DM site in SE France (May–September: 2000–2010).
… 
The ISO‐Toolkit components and interpretative techniques, divided into tool groups based on data availability and resolution, with Tool Group A being the most desirable set of data and interpretative techniques. A review of the mentioned data sets and interpretative techniques is given in Texts S1–S3.
The ISO‐Toolkit components and interpretative techniques, divided into tool groups based on data availability and resolution, with Tool Group A being the most desirable set of data and interpretative techniques. A review of the mentioned data sets and interpretative techniques is given in Texts S1–S3.
… 
The mean annual δ¹⁸Omsw for Populus nigra trees at site PB in relation to potential endmember water sources mean δ¹⁸O (a), sub‐annual δ¹⁸Omsw for Fraxinus excelsior and Populus nigra at site DM shown in relation to potential endmember water source mean δ¹⁸O (b), and the annual and sub‐annual δ¹⁸Omsw for Fraxinus excelsior at site MT (c). Colored banding around all reported δ¹⁸Omsw is ±1 SD (based on the output of 1,000 Monte Carlo simulations), and endmember water sources are shown as mean δ¹⁸O ± 1 SD.
The mean annual δ¹⁸Omsw for Populus nigra trees at site PB in relation to potential endmember water sources mean δ¹⁸O (a), sub‐annual δ¹⁸Omsw for Fraxinus excelsior and Populus nigra at site DM shown in relation to potential endmember water source mean δ¹⁸O (b), and the annual and sub‐annual δ¹⁸Omsw for Fraxinus excelsior at site MT (c). Colored banding around all reported δ¹⁸Omsw is ±1 SD (based on the output of 1,000 Monte Carlo simulations), and endmember water sources are shown as mean δ¹⁸O ± 1 SD.
… 
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Identification of Source‐Water Oxygen Isotopes in Trees
Toolkit (ISO‐Tool) for Deciphering Historical
Water Use by Forest Trees
Christopher I. Sargeant
1,2
, Michael Bliss Singer
3,4,5
, and Christine Vallet‐Coulomb
6
1
Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland,
2
Previously at the
School of Earth & Environmental Sciences, University of St Andrews, St Andrews, UK,
3
School of Earth and Ocean
Sciences, Cardiff University, Cardiff, UK,
4
Water Research Institute, Cardiff University, Cardiff, UK,
5
Earth Research
Institute, University of California Santa Barbara, Santa Barbara, CA, USA,
6
Aix Marseille Université, CNRS, IRD, CDF,
CEREGE UM 34, Aix‐en‐Provence, France
Abstract Hydrological regimes are being perturbed under climate change due to the regional
expression of the water cycle across the globe, leading to alterations in the spatial and temporal
distribution of water near the Earth's surface. Water is a critical resource for plant ecosystems, and
hydrological limitations on vegetative health are particularly complex. To anticipate how subsurface
water availability may evolve in the future and affect the dynamics of plant water source usage, as well
as the health and functioning of vegetation in various biomes, we need a robust, quantitative framework
for linking water availability to past plant water use, which is constrained by historical data. Here, we
outline the Identification of Source‐water Oxygen isotopes in trees Toolkit (ISO‐Tool), designed to
retrospectively investigate the dynamics of tree water uptake. ISO‐Tool utilizes tree‐ring isotopes (δ
18
O)
combined with a biomechanistic fractionation model to retrodict the δ
18
O of water utilized during any
period of growth. Through comparisons with measured δ
18
O in local water sources, climatic, and
hydrological variables, ISO‐Tool can reconstruct and inform on past ecohydrological interactions. We
provide an overview of the modeling components and data requirements necessary to constrain the
retrodictions of source‐water δ
18
O. We demonstrate the utility and efficacy of ISO‐Tool for three
riparian field sites characterized by differences in climatic, geomorphic, and hydrologic complexity. We
also state that ISO‐Tool can be applied to a range of vegetated environments where distinct isotopic
endmembers exist. We present a set of tool groups, which can be applied adaptively, ensuring that
scientific progress in understanding retrospective ecohydrology can be made, even under varying
degrees of data availability.
1. Introduction
1.1. Background
Forests worldwide are becoming increasingly vulnerable to variations in water availability as hydrological
regimes respond to climate change (Allen et al., 2010, 2015; Choat et al., 2012; Clark et al., 2011;
Hartmann et al., 2013). Yet despite the fundamental role water plays in the health, productivity, and distri-
bution of tree species (Currie & Paquin, 1987; Hsiao, 1973; Schulze et al., 1987; Stephenson, 1990), there
remain considerable uncertainties in how terrestrial water availability to forests will evolve under future cli-
mate (Allen & Ingram, 2002; Donat et al., 2016; IPCC, 2014; Sippel et al., 2016; Trenberth et al., 2014). Such
shortcomings result from the incomplete characterization of moisture sources over a range of timescales,
which is further complicated by the contribution of several potential sources to tree‐available water.
These sources include infiltrated precipitation in the vadose zone and shallow groundwater in the phreatic
zone, where the latter can be derived from hyporheic streamflow contributions to shallow water tables
(Busch et al., 1992; Evans et al., 2018; Singer et al., 2014; White & Smith, 2013). Therefore, for any interval
of time, the particular water source used by a tree is a function of specific tree rooting depths as well as by the
time‐varying availability of root‐zone water, which varies in response to climatic trends and fluctuations
(Dawson & Pate, 1996; Snyder & Williams, 2000). The details surrounding these dynamic ecohydrological
relationships are poorly understood, a knowledge gap which restricts our ability to anticipate how forests
will respond to alterations in hydrological regimes that may affect one or both soil hydrological reservoirs
©2019. American Geophysical Union.
All Rights Reserved.
RESEARCH ARTICLE
10.1029/2018WR024519
Key Points:
•A toolkit for determining the oxygen
isotopic signature of historical
source water to trees is presented
•The parameterization of ISO‐Tool is
subdivided based on data availability
and output resolution
•The tool is designed to
retrospectively assess water source
usage by plants at annual and
sub‐annual timescales
Supporting Information:
•Supporting Information S1
Correspondence to:
C. I. Sargeant and M. B. Singer,
christopher.sargeant@wsl.ch;
bliss@eri.ucsb.edu
Citation:
Sargeant, C. I., Singer, M. B., & Vallet‐
Coulomb, C. (2019). Identification of
source‐water oxygen isotopes in trees
toolkit (ISO‐Tool) for deciphering
historical water use by forest trees.
Water Resources Research,55
Received 10 DEC 2018
Accepted 24 OCT 2019
Accepted article online 8 NOV 2019
SARGEANT ET AL. 10,954
,
Published online 2 DEC 20190
1–
1,.
https://doi.org/10.1029/
2018WR024519
0,954 0975
... Carbon isotope discrimination (∆ 13 C) derived from tree rings provides a retrospective measure of canopyintegrated leaf gas-exchange (Cernusak et al., 2013;Farquhar et al., 1989;Francey & Farquhar, 1982), making it a useful tool to evaluate past drought responses of plants (Klein et al., 2013). Within the same tree-ring series, stable oxygen isotope ratios (δ 18 O) can be used to infer changes in source water use (Ehleringer & Dawson, 1992;Sargeant et al., 2019) and utilized in coordination with ∆ 13 C to investigate plant ecohydrologic responses (Altieri et al., 2015;Battipaglia & Cherubini, 2022;Gessler et al., 2018;Moreno-Gutiérrez et al., 2012). ...
... Additional influences on tree-ring δ 18 O include diffusion of water vapor from the air into leaves during humid conditions, mixing of leaf water with unenriched stem water (the Péclet effect), and post-photosynthetic processes that may complicate interpretations of leaf-level processes (Barbour, 2007;Gessler et al., 2014;Roden et al., 2000). Recently developed mechanistic models can account for many of these influences and allow for the estimation of source water δ 18 O based on tree-ring δ 18 O, facilitating direct comparisons with endmember δ 18 O composition to better infer plant water sources (Sargeant et al., 2019). ...
Seasonal and Species‐Level Water‐Use Strategies and Groundwater Dependence in Dryland Riparian Woodlands During Extreme Drought
Article
Full-text available
Drought‐induced groundwater decline and warming associated with climate change are primary threats to dryland riparian woodlands. We used the extreme 2012–2019 drought in southern California as a natural experiment to assess how differences in water‐use strategies and groundwater dependence may influence the drought susceptibility of dryland riparian tree species with overlapping distributions. We analyzed tree‐ring stable carbon and oxygen isotopes collected from two cottonwood species (Populus trichocarpa and P. fremontii) along the semi‐arid Santa Clara River. We also modeled tree source water δ¹⁸O composition to compare with observed source water δ¹⁸O within the floodplain to infer patterns of groundwater reliance. Our results suggest that both species functioned as facultative phreatophytes that used shallow soil moisture when available but ultimately relied on groundwater to maintain physiological function during drought. We also observed apparent species differences in water‐use strategies and groundwater dependence related to their regional distributions. P. fremontii was constrained to more arid river segments and ostensibly used a greater proportion of groundwater to satisfy higher evaporative demand. P. fremontii maintained ∆¹³C at pre‐drought levels up until the peak of the drought, when trees experienced a precipitous decline in ∆¹³C. This response pattern suggests that trees prioritized maintaining photosynthetic processes over hydraulic safety, until a critical point. In contrast, P. trichocarpa showed a more gradual and sustained reduction in ∆¹³C, indicating that drought conditions induced stomatal closure and higher water use efficiency. This strategy may confer drought avoidance for P. trichocarpa while increasing its susceptibility to anticipated climate warming.
View
... Another application of tree-ring δ 18 O ratios is identifying the water sources used by trees. Often questions pertain to inferring changes in sources of water to trees (e.g., by depth, or between soil water, stream water and groundwater), across species, climates, landscapes, or time (e.g., (Sargeant et al. 2020;Miller et al. 2006;Sarris et al. 2013;Saurer et al. 2016;Sargeant and Singer 2016). All of these questions require first accounting for the fractionation undergone from source-water to cellulose, a sequence known to vary with meteorological conditions, species, and canopy position (this is the subject of Chaps. ...
... These processes can be uncertain and their effect size can be similar or greater in magnitude than the effects of δ Source variation (Song et al. 2014). As a starting point to constrain those processes, interannual variations in temperature and relative humidity are key data, which can be applied in models to constrain expected inter-annual variability attributable to leaf fractionation processes (Sargeant et al. 2020;Cernusak et al. 2016). These processes (described in Chap. ...
Spatial and Temporal Variations in Plant Source Water: O and H Isotope Ratios from Precipitation to Xylem Water
Chapter
Full-text available
  • Jan 2022
The water present within trees when sugars and cellulose are formed is the source of hydrogen and oxygen atoms that are incorporated into tree-ring cellulose (see Chaps. 10.1007/978-3-030-92698-4_10 and 10.1007/978-3-030-92698-4_11 ). However, the isotope composition of relevant water pools is often unknown when trying to interpret δ ¹⁸ O and δ ² H isotopic records in tree rings. This chapter focuses on the factors that can influence the O and H isotope ratios of source waters for trees. Trees generally use water that originated as precipitation, but this does not mean that the isotope ratios of water used by trees—predominantly taken up by roots from soils—and incorporated in cellulose exactly matches precipitation isotope ratios. Precipitation isotope ratios vary in space and time, and only a fraction of all precipitation infiltrates soils, reaches roots, and is ultimately taken up by trees. Considering species, soils, and climates may allow for predicting which fraction of water resides in the root-zone during the growing seasons, and how its isotope ratios deviate from that of average precipitation. Here we provide an overview of the terrestrial water cycle and the associated transport and fractionation processes that influence the stable isotope ratios of water used by trees. We highlight obstacles and opportunities to be considered, towards more accurately interpreting the records of O and H isotope ratios in tree cellulose.
View
... They have enabled improved understanding of the evolution of seasonally available water sources at critical periods of growth, and to identify the dominant water source usage over an entire season of growth [48,49]. Recent developments in the study of δ 18 O from tree ring cellulose have enabled the analysis of water source variability at sub-annual resolution in the same reference frame as the potential contributing endmember sources for co-occurring riparian tree species that typically use different water sources [48,50]. This sub-annual information about water source usage is critical for predicting how riparian forests may respond to climatic changes that affect either local or non-local controls on seasonal water availability. ...
... The investigation utilizes an 11-year dataset of sub-annual tree-ring cellulose (δ 18 O cell ) (n = 792) from individual, co-located Fraxinus-Populus pairs from both near channel (NC) and interior floodplain (IF) locations, within each study plot along the Rhône hydroclimatic gradient (figure 2(a)). We then employ an inverse biomechanistic model [50] to determine the δ 18 O of source water (δ 18 O msw ) utilized by each tree during the formation of each respective sub-annual δ 18 O cell sample (supplementary material, figure S2). ...
Local and non-local controls on seasonal variations in water availability and use by riparian trees along a hydroclimatic gradient
Article
Full-text available
As global climate change continues to impact regional water cycles, we may expect further shifts in water availability to forests that create challenges for certain species and biomes. Lowland deciduous riparian forests are particularly vulnerable because tree species cannot migrate out of the stream corridor, and they rely on root zone water availability that is controlled by variations in both local climate conditions (e.g. precipitation, evaporation, and infiltration) and non-local hydroclimatic forcing (e.g. streamflow, snowmelt, recharge). To determine how the seasonal water source usage of riparian trees is controlled by local versus non-local variability in hydroclimatic regime, we reconstructed the seasonal oxygen isotope (δ ¹⁸ O) signature of water used by two riparian tree species with contrasting rooting depths, comprising ∼800 δ ¹⁸ O tree-ring cellulose measurements from 12 tree-level decadal time-series at sub-annual resolution (six samples per year), along a strong hydroclimatic gradient within the Rhône River basin, SE France. These results were evaluated alongside δ ¹⁸ O measurements made from potential endmember water sources and independent hydroclimatic metrics. Thus we characterize the seasonal evolution of both potential water availability at distinct rooting depths and tree water source use and investigate the generalized riparian tree response to seasonal variations in local versus non-local hydroclimatic forcing over a decade. We show: (a) distinct seasonal water use between species, based on differential access to groundwater; (b) substantial source switching in both species based on evolving water availability; and (c) that riparian trees are more dependent on locally controlled soil moisture with distance downstream, creating increased vulnerability to locally increasing temperatures. We also find that deeply rooted trees in lowland riparian floodplains are potentially vulnerable to climate change because of their high dependence on water supply from mountains. This effect is more pronounced downstream, where seasonal water table decline may lead to loss of water required for deeply rooted trees.
View
... Certainly, our study has implications for hydrogen isotope models, as we show that isotopic variations in leaf and source water are not sufficient to explain variations in cellulose δ 2 H values under all conditions. Considering the influence of biophysical and biochemical drivers on 2 H-fractionation shaping plant carbohydrates will be key for improving tree-ring model outcomes (24) and will lead to more accurate predictions of climatic conditions (e.g., RH or temperature) and water sources (2,67). ...
Biochemical and biophysical drivers of the hydrogen isotopic composition of carbohydrates and acetogenic lipids
Article
Full-text available
  • Jul 2024
The hydrogen isotopic composition (δ2H) of plant compounds is increasingly used as a hydroclimatic proxy; however, the interpretation of δ2H values is hampered by potential coeffecting biochemical and biophysical processes. Here, we studied δ2H values of water and carbohydrates in leaves and roots, and of leaf n-alkanes, in two distinct tobacco (Nicotiana sylvestris) experiments. Large differences in plant performance and biochemistry resulted from (a) soil fertilization with varying nitrogen (N) species ratios and (b) knockout-induced starch deficiency. We observed a strong 2H-enrichment in sugars and starch with a decreasing performance induced by increasing NO3−/NH4+ ratios and starch deficiency, as well as from leaves to roots. However, δ2H values of cellulose and n-alkanes were less affected. We show that relative concentrations of sugars and starch, interlinked with leaf gas exchange, shape δ2H values of carbohydrates. We thus provide insights into drivers of hydrogen isotopic composition of plant compounds and into the mechanistic modeling of plant cellulose δ2H values.
View
... The rate of triose phosphate cycling is related to the exchange rate of carbonyl oxygen atoms during cellulose biosynthesis (Barbour & Farquhar, 2000;Song, Farquhar, et al., 2014). Although the environmental and physiological factors contributing to δ 18 O variability in trees can be complex to unravel empirically, δ 18 O in cellulose can provide information about the water sources trees utilize over time (Sargeant et al., 2019). Thus, combining tree-ring isotopes with radial growth can aid in distinguishing between leaf-level and belowground physiological adjustments experienced by forests in response to climate change Castruita-Esparza et al., 2019). ...
Ecophysiological Responses of Nothofagus obliqua Forests to Recent Climate Drying Across the Mediterranean‐Temperate Biome Transition in South‐Central Chile
Article
Full-text available
  • Apr 2023
The forests of south‐central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the Mediterranean‐Temperate gradient (35.9°−40.3°S) using carbon isotope discrimination (Δ¹³ C) and intrinsic water use efficiency (iWUE) in tree rings during 1967–2017. Moreover, tree ring δ¹⁸O was evaluated in the northernmost site to better understand the effects of the megadrought in this drier location. These forests have become more efficient in their use of water. However, trees from the densest stand are discriminating more against ¹³C, probably due to reduced photosynthetic rates associated with increasing light competition. The strongest associations between climate and Δ¹³C were found in the northernmost stand, suggesting that warmer and drier conditions could have reduced ¹³C discrimination. Tree growth in this site has not decreased, and δ¹⁸O was negatively related to annual rainfall. However, a shift in this relationship was found since 2007, when both precipitation and δ¹⁸O decreased, while correlations between δ¹⁸O and growth increased. This implies that tree growth and δ¹⁸O are coupled in recent years, but precipitation is not the cause, suggesting that trees probably changed their water source to deeper and more depleted pools. Our research demonstrates that forests are not reducing their growth in central Chile, mainly due to a shift toward the use of deeper water sources. Despite a common climate trend across the gradient, there is a non‐uniform response of N. obliqua forests to climate drying, being their response site‐specific.
View
... One of the main purposes of developing semi-mechanistic models of cellulose isotopic variation, is to be able to invert the model to estimate parameters of interest, which are retrospectively difficult to assess. For example, the cellulose isotope models have been used to infer leaf temperature (Helliker and Richter, 2008); deuterium deviations (as a proxy for relative humidity) (Voelker et al., 2014); stomatal conductance induced changes in water-use efficiency (Guerrieri et al., 2019); origin of plant material (Cueni et al., 2021); and, source of water for trees (Sargeant et al., 2019); as well as estimate the fraction of isotopic exchange in sink cell water (e.g. Song et al., 2014;Cheesman and Cernusak, 2016). ...
Species variation in the hydrogen isotope composition of leaf cellulose is mostly driven by isotopic variation in leaf sucrose
Article
Full-text available
Experimental approaches to isolate drivers of variation in the carbon‐bound hydrogen isotope composition (δ²H) of plant cellulose are rare and current models are limited in their application. This is in part due to a lack in understanding of how ²H‐fractionations in carbohydrates differ between species. We analysed, for the first time, the δ²H of leaf sucrose along with the δ²H and δ¹⁸O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ²H of sucrose explained 66% of the δ²H variation in cellulose (R² = 0.66), which was associated with species differences in the ²H enrichment of sucrose above leaf water ( ε sucrose \unicode{x003B5} sucrose : −126% to −192‰) rather than by variation in leaf water δ²H itself. ε sucrose \unicode{x003B5} sucrose was positively related to dark respiration (R² = 0.27), and isotopic exchange of hydrogen in sugars was positively related to the turnover time of carbohydrates (R² = 0.38), but only when ε sucrose \unicode{x003B5} sucrose was fixed to the literature accepted value of − 171 \unicode{x02212} 171 ‰. No relation was found between isotopic exchange of hydrogen and oxygen, suggesting large differences in the processes shaping post‐photosynthetic fractionation between elements. Our results strongly advocate that for robust applications of the leaf cellulose hydrogen isotope model, parameterization utilizing δ²H of sugars is needed.
View
... Mechanistic models are an important tool for interpreting of tree-ring isotope data. They have been developed and applied for the simulation of δ 18 Oc records in relation to hydrological (Sargeant et al., 2019), climatological signals (Saurer et al., 2016), as well as for δ 13 Cc simulations that provided physiological information (Guerrieri et al., 2019;Lavergne et al., 2020), while it has been seldomly applied for the modelling of δ 2 Hc in tree rings (Nabeshima et al., 2018;Nakatsuka et al., 2020aNakatsuka et al., , 2020bVoelker et al., 6 2014). Specifically, the model of ; hereafter RE-model, is currently the applied model used to estimate δ 2 Hc in tree rings by taking into account the isotopic variation from hydrological sources (i.e. ...
The unknown third – Hydrogen isotopes in tree-ring cellulose across Europe
Article
  • Mar 2022
  • SCI TOTAL ENVIRON
This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ²Hc) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36°N to 68°N). We observed that the high-frequency climate signals in the δ²Hc chronologies were weaker than those recorded in carbon (δ¹³Cc) and oxygen isotope signals (δ¹⁸Oc) but similar to the tree-ring width ones (TRW). The δ²Hc climate signal strength varied across the continent and was stronger and more consistent for Pinus than for Quercus. For both genera, years with extremely dry summer conditions caused a significant ²H-enrichment in tree-ring cellulose. The δ²Hc inter-annual variability was strongly site-specific, as a result of the imprinting of climate and hydrology, but also physiological mechanisms and tree growth. To differentiate between environmental and physiological signals in δ²Hc, we investigated its relationships with δ¹⁸Oc and TRW. We found significant negative relationships between δ²Hc and TRW (7 sites), and positive ones between δ²Hc and δ¹⁸Oc (10 sites). The strength of these relationships was nonlinearly related to temperature and precipitation. Mechanistic δ²Hc models performed well for both genera at continental scale simulating average values, but they failed on capturing year-to-year δ²Hc variations. Our results suggest that the information recorded by δ²Hc is significantly different from that of δ¹⁸Oc, and has a stronger physiological component independent from climate, possibly related to the use of carbohydrate reserves for growth. Advancements in the understanding of ²H-fractionations and their relationships with climate, physiology, and species-specific traits are needed to improve the modelling and interpretation accuracy of δ²Hc. Such advancements could lead to new insights into trees' carbon allocation mechanisms, and responses to abiotic and biotic stress conditions.
View
... Depending on regional climate, the slope between treering width and δ 18 O R (b1 in Fig. 1d) can capture the seasonally integrated response of trees to air temperature and environmental moisture (Rebetez et al., 2003;Liu et al., 2017). However, the shape and nature of the relationship alone might be difficult to interpret without separating the effects of oxygen isotopic fractionation in leaves and source water variability on δ 18 O R (Barnard et al., 2012;Roden and Siegwolf, 2012;Sargeant et al., 2019). But when ring width variations are simultaneously negatively related to δ 18 O R and positively to 13 C R variations, as observed in Fontainebleau (Fig. 1d), it can be established with confidence that tree growth is modulated by leaf physiology through the stomatal limitation of CO 2 supply to photosynthesis as a response to air and soil humidity constraints (see axes in Fig. 1d). ...
A triple tree-ring constraint for tree growth and physiology in a global land surface model
Article
Full-text available
  • Jun 2021
  • BG
Annually resolved tree-ring records extending back to pre-industrial conditions have the potential to constrain the responses of global land surface models at interannual to centennial timescales. Here, we demonstrate a framework to simultaneously constrain the representation of tree growth and physiology in the ORCHIDEE global land surface model using the simulated variability of tree-ring width and carbon (Δ13C) and oxygen (δ18O) stable isotopes in six sites in boreal and temperate Europe. We exploit the resulting tree-ring triplet to derive integrative constraints for leaf physiology and growth from well-known mechanistic relationships among the variables. ORCHIDEE simulates Δ13C (r=0.31–0.80) and δ18O (r=0.36–0.74) better than tree-ring width (r<0.55), with an overall skill similar to that of a tree-ring model (MAIDENiso) and another isotope-enabled global vegetation model (LPX-Bern). The comparison with tree-ring data showed that growth variability is not well represented in ORCHIDEE and that the parameterization of leaf-level physiological responses (stomatal control) to drought stress in the temperate region can be constrained using the interannual variability of tree-ring stable isotopes. The representation of carbon storage and remobilization dynamics emerged as a critical process to improve the realism of simulated growth variability, temporal carryover, and recovery of forest ecosystems after climate extremes. Simulated forest gross primary productivity (GPP) correlates with simulated tree-ring Δ13C and δ18O variability, but the origin of the correlations with tree-ring δ18O is not entirely physiological. The integration of tree-ring data and land surface models as demonstrated here should guide model improvements and contribute towards reducing current uncertainties in forest carbon and water cycling.
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Different responses of oxygen and hydrogen isotopes in leaf and tree-ring organic matter to lethal soil drought
Article
Full-text available
  • Apr 2024
  • TREE PHYSIOL
The oxygen and hydrogen isotopic composition (δ18O, δ2H) of plant tissues are key tools for the reconstruction of hydrological and plant physiological processes and may therefore be used for disentangling reasons of tree mortality. However, how both elements respond to soil drought conditions before death have rarely been investigated. To test this, we performed a greenhouse study and determined predisposing fertilization and lethal soil drought effects on δ18O and δ2H values of organic matter (OM) in leaves and tree rings of living and dead saplings of five European tree species. For mechanistic insights, we additionally measured isotopic (i.e., δ18O and δ2H values of leaf and twig water), physiological (i.e., leaf water potential and gas-exchange) and metabolic traits (i.e., leaf and stem non-structural carbohydrate concentration, C:N ratios). Across all species, lethal soil drought generally caused a homogenous 2H-enrichment in leaf and tree-ring OM, but a low and heterogenous δ18O response in the same tissues. Unlike δ18O values, δ2H values of tree-ring OM were correlated with those of leaf and twig water and with plant physiological traits across treatments and species. The 2H-enrichment in plant OM also went along with a decrease in stem starch concentrations under soil drought compared to well-watered conditions. In contrast, the predisposing fertilization had generally no significant effect on any tested isotopic, physiological, and metabolic traits. We propose that the 2H-enrichment in the dead trees is related to (i) the plant water isotopic composition, (ii) metabolic processes shaping leaf non-structural carbohydrates, (iii) the use of carbon reserves for growth, and (iv) species-specific physiological adjustments. The homogenous stress imprint on δ2H but not on δ18O suggests that the former could be used as a proxy to reconstruct soil droughts and underlying processes of tree mortality.
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The drought‒dieback‒death conundrum in trees and forests
Article
  • Sep 2021
Background: Climate warming is amplifying and exacerbating drought stress worldwide. Long-term trends of increasing evaporative demand and decreasing soil moisture availability occur superimposed on severe spells of drought. These rare, extreme droughts have triggered episodes of forest dieback that have led to reduced productivity and rising mortality rates, usually at small scales (dieback hotspots), but affecting biomes worldwide. Aims: This review summarizes and discusses the drivers, patterns and mechanisms of forest dieback caused by drought. Methods: I review studies on forest dieback and tree death linked to dry spells with a focus on tools to forecast dieback. Results: Several mechanisms have been described as physiological drivers of dieback, including hydraulic failure and carbon starvation, however hydraulics-based models have shown little predictive power of dieback and mortality. Field proxies of tree vigour, including changes in canopy defoliation and water content, combined with surrogates of tree functioning (tree-ring growth, wood anatomy, tree-ring δ¹³C or δ¹⁸O composition) may improve predictions of forest dieback or at least render early-warning signals of impending tree death. Conclusions: Drought-induced dieback and mortality are concerning phenomena which lack forecasting tools with sufficient predictive power. Surrogates of tree vigour, growth and functioning should be used to build more accurate models of tree death in response to extreme climate events linked to drought. Here, I argue for combining and comparing those surrogates to better forecast forest dieback.
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How important is groundwater availability and stream perenniality to riparian and floodplain tree growth?
Article
Full-text available
  • Mar 2018
Riparian vegetation is important for stream functioning and as a major landscape feature. For many riparian plants shallow groundwater is an important source of water, particularly in areas where rainfall is low, either annually or seasonally, and when extended dry conditions prevail for all or part of the year. The nature of tree water relationships is highly complex. Therefore we used multiple lines of evidence to determine the water sources used by the dominant tree species Eucalyptus camaldulensis (river red gum), growing in riparian and floodplain areas with varying depth to groundwater and stream perenniality. Dendrometer bands were used to measure diel, seasonal and annual patterns of tree water use and growth. Water stable isotopes (δ²H and δ¹⁸O) in plant xylem, soil water and groundwater were measured to determine spatial and temporal patterns in plant water source use. Our results indicated riparian trees located on relatively shallow groundwater had greater growth rates, larger diel responses in stem diameter and were less reactive to extended dry periods, than trees in areas of deep groundwater. These results were supported by isotope analysis that suggested all trees used groundwater when soil water stores were depleted at the end of the dry season and this was most pronounced for trees with shallow groundwater. Trees may experience more frequent periods of water deficit stress and undergo reduced productivity in scenarios where water table accessibility is reduced, such as drawdown from groundwater pumping activities or periods of reduced rainfall recharge. The ability of trees to adapt to changing groundwater conditions may depend on the speed of change, the local hydrologic and soil conditions as well as the species involved. Our results suggest that E. camaldulesis growing at our study site is capable of utilising groundwater even to depths >10 m and stream perenniality is likely to be a useful indicator of riparian tree use of groundwater.
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Modeling Subsurface Hydrology in Floodplains
Article
Full-text available
  • Jan 2018
Soil moisture patterns in floodplains are highly dynamic, owing to the complex relationships between soil properties, climatic conditions at the surface, and the position of the water table. Given this complexity, along with climate change scenarios in many regions, there is a need for a model to investigate the implications of different conditions on water availability to riparian vegetation. We present a model, HaughFlow, which is able to predict coupled water movement in the vadose and phreatic zones of hydraulically connected floodplains. Model output was calibrated and evaluated at 6 sites in Australia to identify key patterns in subsurface hydrology. This study identifies the importance of the capillary fringe in vadose zone hydrology due to its water storage capacity and creation of conductive pathways. Following peaks in water table elevation, water can be stored in the capillary fringe for up to months (depending on the soil properties). This water can provide a critical resource for vegetation that is unable to access the water table. When water table peaks coincide with heavy rainfall events, the capillary fringe can support saturation of the entire soil profile. HaughFlow is used to investigate the water availability to riparian vegetation, producing daily output of water content in the soil over decadal time periods within different depth ranges. These outputs can be summarised to support scientific investigations of plant-water relations, as well as in management applications.
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Satellite Soil Moisture: Review of Theory and Applications in Water Resources
Article
Full-text available
  • Aug 2017
  • WATER RESOUR MANAG
Soil moisture (SM) plays an important role in the water and energy exchanges that occur in the terrestrial surface. Soil moisture can be retrieved at a larger scale by using the visible and InfraRed (IR) bands as well as through the microwave remote sensing. Because of very high importance of SM for variety of applications, there are now two dedicated microwave satellites in the Earth’s orbit for soil moisture retrieval from space. The first, Soil Moisture and Ocean Salinity (SMOS) satellite has been launched by the European Space Agency in November 2009 and second is Soil Moisture Active and Passive (SMAP) launched by the National Aeronautics and Space Administration (NASA) in January 2015. In this review, brief background of soil moisture retrieval algorithms are presented with different applications in the area of water resources. The first section provides the introduction of the soil moisture, presents several in situ techniques for measurement of soil moisture and soil moisture retrieval algorithms from visible/IR and microwave remote sensing. Section 2 describes the satellite soil moisture applications in water resources.
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Testing plant use of mobile vs immobile soil water sources using stable isotope experiments
Article
Full-text available
We tested for isotope exchange between bound (immobile) and mobile soil water, and whether there is isotope fractionation during plant water uptake. These are critical assumptions to the formulation of the ‘two water worlds’ hypothesis based on isotope profiles of soil water. In two different soil types, soil‐bound water in two sets of 19‐l pots, each with a 2‐yr‐old avocado plant ( Persea americana ), were identically labeled with tap water. After which, one set received isotopically enriched water whereas the other set received tap water as the mobile phase water. After a dry down period, we analyzed plant stem water as a proxy for soil‐bound water as well as total soil water by cryogenic distillation. Seventy‐five to 95% of the bound water isotopically exchanged with the mobile water phase. In addition, plants discriminated against ¹⁸ O and ² H during water uptake, and this discrimination is a function of the soil water loss and soil type. The present experiment shows that the assumptions for the ‘two water worlds’ hypothesis are not supported. We propose a novel explanation for the discrepancy between isotope ratios of the soil water profile and other water compartments in the hydrological cycle.
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Trees, forests and water: Cool insights for a hot world
Article
Full-text available
  • Feb 2017
Forest-driven water and energy cycles are poorly integrated into regional, national, continental and global decision-making on climate change adaptation, mitigation, land use and water management. This constrains humanity's ability to protect our planet's climate and life-sustaining functions. The substantial body of research we review reveals that forest, water and energy interactions provide the foundations for carbon storage, for cooling terrestrial surfaces and for distributing water resources. Forests and trees must be recognized as prime regulators within the water, energy and carbon cycles. If these functions are ignored, planners will be unable to assess, adapt to or mitigate the impacts of changing land cover and climate. Our call to action targets a reversal of paradigms, from a carbon-centric model to one that treats the hydrologic and climate-cooling effects of trees and forests as the first order of priority. For reasons of sustainability, carbon storage must remain a secondary, though valuable, by-product. The effects of tree cover on climate at local, regional and continental scales offer benefits that demand wider recognition. The forest-and tree-centered research insights we review and analyze provide a knowledge-base for improving plans, policies and actions. Our understanding of how trees and forests influence water, energy Adaptation Sustainability and carbon cycles has important implications, both for the structure of planning, management and governance institutions, as well as for how trees and forests might be used to improve sustainability, adaptation and mitigation efforts.
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Drought causes reduced growth of trembling aspen in western Canada
Article
Full-text available
Adequate and advance knowledge of the response of forest ecosystems to temperature-induced drought is critical for a comprehensive understanding of the impacts of global climate change on forest ecosystem structure and function. Recent massive decline in aspen-dominated forests and an increased aspen mortality in boreal forests have been associated with global warming, but it is still uncertain whether the decline and mortality are driven by drought. We used a series of ring-width chronologies from 40 trembling aspen (Populus tremuloides Michx.) sites along a latitudinal gradient (from 52° to 58°N) in western Canada, in an attempt to clarify the impacts of drought on aspen growth by using Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI). Results indicated that prolonged and large-scale droughts had a strong negative impact on trembling aspen growth. Furthermore, the spatiotemporal variability of drought indices is useful for explaining the spatial heterogeneity in the radial growth of trembling aspen. Due to ongoing global warming and rising temperatures, it is likely that severer droughts with a higher frequency will occur in western Canada. As trembling aspen is sensitive to drought, we suggest that drought indices could be applied to monitor the potential effects of increased drought stress on aspen trees growth, achieve classification of eco-regions and develop effective mitigation strategies to maintain western Canadian boreal forests.
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Stable-Isotope Techniques to Investigate Sources of Plant Water
Chapter
  • Aug 2018
Stable isotopologues of water (mainly 1H2 16O, HD16O and 1H2 18O) have been used for decades as tracers of the Earth’s water cycle. In this chapter, we briefly describe the theoretical background and state-of-the-art techniques of the use of water stable isotopes to investigate the sources of plant water. We aim to provide the basic understanding of stable isotope fractionation within the Earth’s critical zone that is relevant for studies of plant water sources. We then present a practical guide of their most common applications in field studies and the most common and up-to-date laboratory procedures. We finally introduce the existing statistical approaches for estimating the relative contributions of water sources to plant transpiration.By acknowledging the advantages and limitations of each approach, we aim to provide an overview of the current techniques to researchers in the fields of plant ecophysiology, ecohydrology and forest ecology, so that they can make informed decisions when designing their experiments.KeywordsPlant Water SourcesCryogenic ExtractionGlobal Network Of Isotopes In Precipitation (GNIP)Local Meteoric Water Line (LMWL)Laser-based InstrumentsThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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