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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.
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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 com...
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Orogenic forelands host interactions between deformation and static or migrating fluids. Given their accessibility and dimensions, these areas are not only historic landmarks for structural geology, but they are also areas of prime interest for georesource exploration and geological storage, and loci of potential geohazards. Geochemical techniques...
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... 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). ...
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.
... 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). ...
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.
... 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). ...
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.
... 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). ...
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.
... 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. ...
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.
... 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. ...
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.
... 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). ...
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.
... 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). ...
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.
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.
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.
