Stem water cryogenic extraction biases estimation in deuterium isotope composition of plant source water

Partitioning urban forest evapotranspiration based on integrating eddy covariance of water vapor and carbon dioxide fluxes

Partitioning of evapotranspiration (ET) in urban forest lands plays a vital role in mitigating ambient temperature and evaluating the effects of urbanization on the urban hydrological cycle. While ET partitioning has been extensively studied in diverse natural ecosystems, there remains a significant paucity of research on urban ecosystems. The flux variance similarity (FVS) theory is used to partition urban forest ET into soil evaporation (E) and vegetation transpiration (T). This involves measurements from eddy covariance of water vapor and carbon dioxide fluxes, along with an estimated leaf-level water use efficiency (WUE) algorithm. The study compares five WUE algorithms in partitioning the average transpiration fraction (T/ET) and validates the results using two years of oxygen isotope observations. Although all five FVS-based WUE algorithms effectively capture the dynamic changes in hourly scale T and E across the four seasons, the algorithm that assumes a constant ratio of intercellular CO2 concentration (ci) to ambient CO2 concentration (ca) provides the most accurate simulation results for the ratio of T/ET. The performance metrics for this specific algorithm include the RMSE of 0.06, R2 of 0.88, the bias of 0.02, and MAPE of 8.9 %, respectively. Comparing urban forests to natural forests, the T/ET in urban areas is approximately 2.4-25.3 % higher, possibly due to the elevated air temperature (Ta), greater leaf area index (LAI), and increased soil water availability. Correlation analysis reveals that the T/ET dynamic is primarily controlled by Ta, LAI, net radiation, ca, and soil water content at half-hourly, daily, and monthly scales. This research provides valuable insights into the performance and applicability of various WUE algorithms in urban forests, contributing significantly to understanding the impact of urbanization on energy, water, and carbon cycles within ecosystems.

Biochemical and biophysical drivers of the hydrogen isotopic composition of carbohydrates and acetogenic lipids

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.

Plant water source effects on plant-soil feedback for primary succession of terrestrial ecosystems in a glacier region in China

Despite the extensive research conducted on plant-soil-water interactions, the understanding of the role of plant water sources in different plant successional stages remains limited. In this study, we employed a combination of water isotopes (δ2H and δ18O) and leaf δ13C to investigate water use patterns and leaf water use efficiency (WUE) during the growing season (May to September 2021) in Hailuogou glacier forefronts in China. Our findings revealed that surface soil water and soil nutrient gradually increased during primary succession. Dominant plant species exhibited a preference for upper soil water uptake during the peak leaf out period (June to August), while they relied more on lower soil water sources during the post-leaf out period (May) or senescence (September to October). Furthermore, plants in late successional stages showed higher rates of water uptake from uppermost soil layers. Notably, there was a significant positive correlation between the percentage of water uptake by plants and available soil water content in middle and late stages. Additionally, our results indicated a gradual decrease in WUE with progression through succession, with shallow soil moisture utilization negatively impacting overall WUE across all succession stages. Path analysis further highlighted that surface soil moisture (0- 20 cm) and middle layer nutrient availability (20- 50 cm) played crucial roles in determining WUE. Overall, this research emphasizes the critical influence of water source selection on plant succession dynamics while elucidating underlying mechanisms linking succession with plant water consumption.

Different responses of oxygen and hydrogen isotopes in leaf and tree-ring organic matter to lethal soil drought

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 to disentangle the reasons for tree mortality. However, how both elements respond to soil drought conditions before death has 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 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, carbon-to-nitrogen ratios). Across all species, lethal soil drought generally caused a homogenous 2H-enrichment in leaf and tree-ring organic matter, but a low and heterogenous δ18O response in the same tissues. Unlike δ18O values, δ2H values of tree-ring organic matter were correlated with those of leaf and twig water and with plant physiological traits across treatments and species. The 2H-enrichment in plant organic matter also went along with a decrease in stem starch concentrations under soil drought compared with 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.

Quantifying isotope parameters associated with carbonyl-water oxygen exchange during sucrose translocation in tree phloem

Stable oxygen isotope ratio of tree-ring α-cellulose (δ18Ocel) yields valuable information on many aspects of tree-climate interactions. However, our current understanding of the mechanistic controls on δ18Ocel is incomplete, with a knowledge gap existent regarding the fractionation effect characterizing carbonyl-water oxygen exchange during sucrose translocation from leaf to phloem. To address this insufficiency, we set up an experimental system integrating a vapor 18O-labeling feature to manipulate leaf-level isotopic signatures in tree saplings enclosed within whole-canopy gas-exchange cuvettes. We applied this experimental system to three different tree species to determine their respective relationships between 18O enrichment of sucrose in leaf lamina (Δ18Ol_suc) and petiole phloem (Δ18Ophl_suc) under environmentally/physiologically stable conditions. Based on the determined Δ18Ophl_suc-Δ18Ol_suc relationships, we estimated that on average, at least 25% of the oxygen atoms in sucrose undergo isotopic exchange with water along the leaf-to-phloem translocation path and that the biochemical fractionation factor accounting for such exchange is c. 34‰, markedly higher than the conventionally assumed value of 27‰. Our study represents a significant step toward quantitative elucidation of the oxygen isotope dynamics during sucrose translocation in trees. This has important implications with respect to improving the δ18Ocel model and its related applications in paleoclimatic and ecophysiological contexts.

Two common pitfalls in the analysis of water-stable isotopologues with cryogenic vacuum extraction and cavity ring-down spectroscopy

Water stable isotopologue analysis is widely used to disentangle ecohydrological processes. Yet, there are increasing reports of measurement uncertainties for established and emerging methods, such as cryogenic vacuum extraction (CVE) or cavity ring-down spectroscopy (CRDS). With this study, we investigate two pitfalls, that potentially contribute to uncertainties in water-stable isotopologue research. To investigate fractionation sources in CVE, we extracted pure water of known isotopic composition with cotton, glass wool or without cover and compared the isotopologue results with non-extracted reference samples. To characterise the dependency of δ2H and δ18O on the water mixing ratio in CRDS, which is of high importance for in-situ applications with large natural variations in mixing ratios, we chose samples with a large range of isotopic compositions and determined δ2H and δ18O for different water mixing ratios with two CRDS analysers (Picarro, Inc.). Cotton wool had a strong fractionation effect on δ2H values, which increased with more 2H-enriched samples. δ2H and δ18O values showed a strong dependency on the water mixing ratio analysed with CRDS with differences of up to 34.5‰ (δ2H) and 3.9‰ (δ18O) for the same sample at different mixing ratios. CVE and CRDS, now routinely applied in water stable isotopologue research, come with pitfalls, namely fractionation effects of cover materials and water mixing ratio dependencies of δ2H and δ18O, which can lead to erroneous isotopologue results and thus, invalid conclusions about (ecohydrological) processes. These practical issues identified here should be reported and addressed adequately in water-stable isotopologue research.

The determining factors of hydrogen isotope offsets between plants and their source waters

A fundamental assumption when using hydrogen and oxygen stable isotopes to understand ecohydrological processes is that no isotope fractionation occurs during plant water uptake/transport/redistribution. A growing body of evidence has indicated that hydrogen isotope fractionation occurs in certain environments or for certain plant species. However, whether the plant water source hydrogen isotope offset (δ2 H offset) is a common phenomenon and how it varies among different climates and plant functional types remains unclear. Here, we demonstrated the presence of positive, negative, and zero offsets based on extensive observations of 12 plant species of 635 paired stable isotopic compositions along a strong climate gradient within an inland river basin. Both temperature and relative humidity affected δ2 H offsets. In cool and moist environments, temperature mainly affected δ2 H offsets negatively due to its role in physiological activity. In warm and dry environments, relative humidity mainly affected δ2 H offsets, likely by impacting plant leaf stomatal conductance. These δ2 H offsets also showed substantial linkages with leaf water 18 O enrichment, an indicator of transpiration and evaporative demand. Further studies focusing on the ecophysiological and biochemical understanding of plant δ2 H dynamics under specific environments are essential for understanding regional ecohydrological processes and for conducting paleoclimate reconstructions.

Determination of the water-use patterns for two xerophyte shrubs by hydrogen isotope offset correction

The stable hydrogen and oxygen isotope technique is typically used to explore plant water uptake; however, the accuracy of the technique has been challenged by hydrogen isotope offsets between plant xylem water and its potential source water. In this study, the soil hydrogen and oxygen isotope waterline was used to correct the hydrogen isotope offsets for Salix psammophila and Caragana korshinskii, two typical shrub species on the Chinese Loess Plateau. Five different types of isotopic data [(i) δ18O, (ii) δ2H, (iii) combination δ18O with δ2H, (iv) corrected δ2H and (v) combination δ18O with corrected δ2H] were separately used to determine the water-use patterns of the two shrubs. The δ2H offset values of S. psammophila and C. korshinskii did not show significant temporal variation among the sampling months (May, July and September) but showed notable differences between the two shrubs (-0.4 ± 0.5‰ in S. psammophila vs -4.3 ± 0.9‰ in C. korshinskii). The obtained water absorption proportion (WAP) of S. psammophila in the different soil layers (0-20, 20-60 and 60-200 cm) did not differ significantly among the five different input data types. However, compared with the input data types (iii) and (v), the data types (i), (ii) and (iv) overestimated the WAP of C. korshinskii in the 0-20 cm soil layer and underestimated that in the 60-200 cm layer. The data type (iii) overestimated the WAP of C. korshinskii in 0-20 cm soil layer (25.9 ± 0.8%) in July in comparison with the WAP calculated based on data type (v) (19.1 ± 1.1%). The combination of δ18O and corrected δ2H, i.e., data type (v), was identified as the best data type to determine the water use patterns of C. korshinskii due to the strong correlation between the calculated WAP and soil water content and soil sand content. In general, S. psammophila mainly used (57.9-62.1%) shallow soil water (0-60 cm), whereas C. korshinskii mainly absorbed (52.7-63.5%) deep soil water (60-200 cm). We confirm that the hydrogen isotope offsets can cause significant errors in determining plant water uptake of C. korshinskii, and provide valuable insights for accurately quantifying plant water uptake in the presence of hydrogen isotope offsets between xylem and source water. This study is significant for facilitating the application of the stable hydrogen and oxygen isotope technique worldwide, and for revealing the response mechanism of shrub key ecohydrological and physiological processes to the drought environment in similar climate regions.

Water sources for street trees in mesic urban environments

Street trees support climate resiliency through a variety of pathways, such as offsetting urban heat and attenuating storm water runoff. While urban trees in arid and semiarid ecosystems have been shown to take up water from irrigation, it is unknown where street trees in mesic cities obtain their water. In this study, we use natural abundance stable isotopes to estimate the proportional sources of water taken up by Acer platanoides street trees in Boston, Massachusetts, United States, including precipitation, irrigation, groundwater, and wastewater. We use Bayesian multisource mixing models to estimate water sources by comparing the natural abundance isotopic ratios of hydrogen and oxygen across potential water sources with water extracted from tree stem samples. We find that during the summer of 2021, characterized by anomalously high rainfall, street trees predominantly utilized water from precipitation. Precipitation accounted for 72.3 % of water extracted from trees sampled in August and 65.6 % from trees sampled in September. Of the precipitation taken up by street trees, most water was traced back to large storm events in July, with July rainfall alone accounting for up to 84.0 % of water found within street trees. We find strong relationships between canopy cover fractions and the proportion of precipitation lost to evapotranspiration across the study domain, supporting the conclusion that tree planting initiatives result in storm water mitigation benefits due to utilization of water from precipitation by urban vegetation. However, while the mature trees studied here currently support their water demand from precipitation, the dependency of street trees on precipitation in mesic cities may lead to increased water stress in a changing climate characterized by a higher frequency and severity of drought.

Belowground niche partitioning is maintained under extreme drought

Belowground niche partitioning presents a key mechanism for maintaining species coexistence and diversity. Its importance is currently reinforced by climate change that alters soil hydrological conditions. However, experimental tests examining the magnitude of its change under climate change are scarce. We combined measurements of oxygen stable isotopes to infer plant water-uptake depths and extreme drought manipulation in grasslands. Belowground niche partitioning was evidenced by different water-uptake depths of co-occurring species under ambient and extreme drought conditions despite an increased overlap among species due to a shift to shallower soil layers under drought. A co-occurrence of contrasting strategies related to the change of species water-uptake depth distribution was likely to be key for species to maintain some extent of belowground niche partitioning and could contribute to stabilizing coexistence under drought. Our results suggest that belowground niche partitioning could mitigate negative effects on diversity imposed by extreme drought under future climate.

Coordination of available soil water content and root distribution modifies water source apportionment of the shrub plant Caragana korshinskii

Stable isotopes have been widely used to identify root water uptake (RWU) by classifying potential water sources as distinct endmembers and evaluating their contributions to xylem water. However, the estimated contributions of endmembers (mainly soil layers) are usually based on variations in soil water isotopes alone. Available soil water and root distributions are key limiting factors of RWU but are rarely considered in water source apportionment. Thus, we have compared the relative contributions of distinct soil layers based on mean soil water isotope values, and values weighted by both available soil water content (AWC) and root weight density (RWD), to RWU of Caragana korshinskii. We derived these values (hereafter mean and weighted contributions, respectively) using three Bayesian mixing models (SIAR, simmr and MixSIAR) at three sites with different water conditions. We calculated the differences between the mean and weighted contributions (DC) and the accumulation of the absolute value of DC (AADC) to analyse the differences between them and their relationships with AWC and RWD. Both the weighted and mean contributions varied with sites and models. We obtained the following AADC values: 27, 8 and 11 % for Sites 1-3, respectively, using SIAR; 39, 13 and 14 %, respectively, using simmr; 68, 40 and 25 %, respectively, using MixSIAR. We detected a significant correlation between DC and RWD when AWC ≤ 6 %, as well as a significant correlation between DC and AWC when AWC > 6 %, indicating that the influence of RWD on DC depended on soil water conditions. Based on our findings, endmembers weighted by AWC and RWD altered the proportion of water source allocation relative to non-weighted endmembers, while the magnitude of the effect was related to the model used. Thus, we suggest careful consideration of the characterisation of endmember isotopes and model selection when partitioning plant water sources using δ2H and δ18O.

Increased uptake of deep soil water promotes drought resistance in mixed forests

The intensity and frequency of droughts are projected to rise in recent years and adversely affect forests. Thus, information on plant water use and acclimation during and after droughts is crucial. This study used the stable isotope and thermal dissipation probes to detect the water-use adaptation of mixed forests to drought using a precipitation gradient control experiment in the field. The results showed that Platycladus orientalis and Quercus variabilis mainly absorbed stable water from deep soil layers during the drought (32.05% and 28.2%, respectively). The synergetic nocturnal sap flow in both species replenished the water loss, but P. orientalis experienced a greater decline in transpiration acclimation to drought. The transpiration of Q. variabilis remained high since it was mainly induced by radiation. After short-term exposure to drought, P. orientalis majorly obtained shallow soil water, confirming its sensitivity to shallow water. Contrarily, Q. variabilis mainly absorbed stable water from deep soil layers regardless of the soil water content. Therefore, these findings suggest that Q. variabilis cannot physiologically adjust to extreme drought events, possibly limiting their future distributions and altering the composition of boreal forests.

Hydrogen and Oxygen Isotope Fractionation Effects in Different Organ Tissues of Grapes under Drought Conditions

A study of different grapevine tissues and organs (root, stem, leaf, fruit) water isotope fractionation models from high-quality wine grapes produced in the Helan Mountains, a key wine-producing area in northwestern China, was undertaken. Results showed that δ2H values of local groundwater sources were more negative than rivers and precipitation. Soil water δ2H and δ18O values were significantly higher than those of other environmental water sources. Water from the soil surface layer (0-30 cm, δ2H and δ18O values) was more positive than the deeper layer (30-60 cm), indicating that soil water has undergone a positive fractionation effect. δ2H and δ18O values of tissues and organs from different grape varieties followed a similar pattern but were more negative than the local atmospheric precipitation line (slope between 4.1 to 5.2). The 2H and 18O fractionation relationship in grapevine organs was similar, and 18O has a higher fractionation effect than 2H. δ2H and δ18O values showed a strong fractionation effect during the transportation of water to different grape organs (trend of stem > fruit > leaf). This study showed that 18/16O fractionation in grapes is more likely to occur under drought conditions and provides a theoretical basis to improve traceability accuracy and origin protection of wine production areas.

Water use strategies of Nitraria tangutorum in the lake-basin region of the Badain Jaran Desert

Information regarding plant water-use strategies is essential for understanding the hydrological processes and plant survival adaptation mechanisms in desert lake basin regions. To examine the water use strategies of plants in desert lake basin areas, water uptake patterns, water use efficiency, and water potential of Nitraria tangutorum were investigated at different distances from the lake duringhe growing seasons in the lake basin regions of the Badain Jaran Desert. The results indicate that N. tangutorum primarily absorbed groundwater in May (63.8%) and August (53.5%), relied on deep soil water in June (75.1%), and uniformly absorbed soil water from different layers in July. These observations could be explained by periodic fluctuations in the groundwater level and the consequent decrease in soil water availability, as well as plant root adjustments. As soil water availability decreases, N. tangutorum adapts to water variation by increasing its water use efficiency (WUE) and reducing its leaf water potential (Ψ). With intensified water stress, N. tangutorum gradually shifted from adventurous anisohydric regulation to conservative isohydric regulation. Thus, N. tangutorum responds to diverse degrees of environmental changes by altering its water-use strategy. A better understanding of the adaptive water use strategies developed by desert plants under varying water availability conditions provides insight into the diversity of species' reactions to long-term drought and quantifies the hydrological cycle of desert ecosystems against the background of worldwide climate warming.

Challenges in studying water fluxes within the soil-plant-atmosphere continuum: A tracer-based perspective on pathways to progress

Tracing and quantifying water fluxes in the hydrological cycle is crucial for understanding the current state of ecohydrological systems and their vulnerability to environmental change. Especially the interface between ecosystems and the atmosphere that is strongly mediated by plants is important to meaningfully describe ecohydrological system functioning. Many of the dynamic interactions generated by water fluxes between soil, plant and the atmosphere are not well understood, which is partly due to a lack of interdisciplinary research. This opinion paper reflects the outcome of a discussion among hydrologists, plant ecophysiologists and soil scientists on open questions and new opportunities for collaborative research on the topic "water fluxes in the soil-plant-atmosphere continuum" especially focusing on environmental and artificial tracers. We emphasize the need for a multi-scale experimental approach, where a hypothesis is tested at multiple spatial scales and under diverse environmental conditions to better describe the small-scale processes (i.e., causes) that lead to large-scale patterns of ecosystem functioning (i.e., consequences). Novel in-situ, high-frequency measurement techniques offer the opportunity to sample data at a high spatial and temporal resolution needed to understand the underlying processes. We advocate for a combination of long-term natural abundance measurements and event-based approaches. Multiple environmental and artificial tracers, such as stable isotopes, and a suite of experimental and analytical approaches should be combined to complement information gained by different methods. Virtual experiments using process-based models should be used to inform sampling campaigns and field experiments, e.g., to improve experimental designs and to simulate experimental outcomes. On the other hand, experimental data are a pre-requisite to improve our currently incomplete models. Interdisciplinary collaboration will help to overcome research gaps that overlap across different earth system science fields and help to generate a more holistic view of water fluxes between soil, plant and atmosphere in diverse ecosystems.

Stable isotope signatures of deuterium, oxygen 18, and carbon 13 (δ2H, δ 18O, δ 13C) in imported apples available in the markets of Vietnam

This study examines whether fresh apples from the United States, New Zealand, and China sold in the markets of Vietnam can be distinguished based on the stable isotopic signatures of their water and carbon (δ²H, δ¹⁸O, and δ¹³C). The δ²H and δ¹⁸O in apples from the United States were found to average - 100.1 ‰ and - 10.5 ‰ vs VSMOW, respectively, lighter than those from New Zealand and China. In apples from China, the δ¹³C_VBDP averaged - 25.8 ‰, which is more enriched than apples from the United States and New Zealand. A clear distinction was shown in the δ²H, δ¹⁸O, and δ¹³C values in apple samples from the three regions based on the statistical treatment at a 95 % confidence level and p < 0.05. This method is reliable enough to control the import and export of agricultural products.

Tracing plant source water dynamics during drought by continuous transpiration measurements: An in-situ stable isotope approach

The isotopic composition of xylem water (δ_X ) is of considerable interest for plant source water studies. In-situ monitored isotopic composition of transpired water (δ_T ) could provide a nondestructive proxy for δ_X -values. Using flow-through leaf chambers, we monitored 2-hourly δ_T -dynamics in two tropical plant species, one canopy-forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δ_T -dynamics in response to an experimental severe drought, followed by a ² H deep-water pulse applied belowground before starting regular rain. We also sampled branches to obtain δ_X -values from cryogenic vacuum extraction (CVE). Daily flux-weighted δ¹⁸ O_T -values were a good proxy for δ¹⁸ O_X -values under well-watered and drought conditions that matched the rainforest's water source. Transpiration-derived δ¹⁸ O_X -values were mostly lower than CVE-derived values. Transpiration-derived δ² H_X -values were relatively high compared to source water and consistently higher than CVE-derived values during drought. Tracing the ² H deep-water pulse in real-time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in-situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species.

Age-specific and species-specific tree response to seasonal drought in tropical dry forests

Millions of people depend on ecosystem services provided by Tropical Dry Forests (TDFs), yet their proximity to population centers, seasonally dry climate, and the ease at which they are converted to agriculture has left only 10 % of their original extent globally. As more TDFs become protected, basic information relating TDF age to subsurface water resources will help guide forest recovery. Severe deforestation and recent reforestation around Bahía de Caráquez, Ecuador produced a mosaic of different successional stages ideal for exploring relationships between TDF age, subsurface water availability and species-specific responses to seasonal drought. Over one year, we measured gravimetric water content, predawn and midday leaf water potential, and the stable isotope composition of xylem and source waters in two regenerating and one primary forest. Over the transition from wet to dry season, we discovered a sharper decrease in predawn water potential in younger successional forests than in the primary forest. Growing in degraded subsurface environments under increased competition, successional forest trees accessed deeper sources of moisture from unsaturated weathered bedrock and groundwater through the dry season; however, different species employed distinct water use strategies. Ceiba trichistandra maintained midday water potentials above -1.27 MPa through a drought avoidance strategy dependent on groundwater. Sideroxylon celastrinum tolerated drought by lowering predawn and midday water potential through the early dry season but took up greater proportions of saprolite moisture and groundwater as the dry season progressed. Contrastingly, Handroanthus chrysanthus maintained access to shallow soil and saprolite moisture by dropping midday water potential to -4.30 MPa, reflecting drought tolerance. Our results show that limited subsurface water resources in regenerating TDF's lead to species-specific adaptations reliant on deeper sources of moisture. The recovery of soil and saprolite hydrologic properties following disturbances is likely to exceed 100 years, highlighting the importance of forest conservation.

Contrasting water sources used by a coniferous forest in the high-altitude, southeastern Tibetan Plateau

Forest trees use various water sources to adapt to environmental conditions in mountainous regions. However, water resources variances along elevational gradients are not clearly understood. This limits the assessment of the ecosystem responses to climate change. In this study, stable oxygen and hydrogen isotopes were used to investigate the spatiotemporal patterns of water sources for Faber's fir in a humid high-altitude elevational gradient (ranging between 2800 m.a.s.l. and 3700 m.a.s.l.) on the southeastern Tibetan Plateau. The results indicated that 27 ± 8.3 % of the xylem water was from previous winter snowmelt between May and June. In contrast, almost all xylem water was from current summer precipitation between July and October. Faber's fir at the lower elevation (2800 m.a.s.l.) primarily relied on water derived from winter precipitation during May and June. Yet, trees located near the tree line (3700 m.a.s.l.) were mostly dependent on current precipitation over the entire growing season. However, when statistically analyzing data from all seven different elevation gradients in this study, the contribution of winter precipitation to xylem water was not elevation dependent. Precipitation contributed to a large proportion (59.86 % ± 33.43 %) of xylem water between May and October. Meanwhile, no linear contribution ratio of precipitation to trees was identified in this high-altitude elevational gradient. The replenishment of soil water and the soil water storage determine the spatiotemporal patterns of water sources. Climate change has the possibility of reducing winter precipitation at high altitudes on the Tibetan Plateau. Thus, tree water use at different altitude gradients will play varied roles in influencing the evolution of forest composition under ongoing climate change.

Comparative cryogenic extraction rehydration experiments reveal isotope fractionation during root water uptake in Gramineae

Determining whether isotope fractionation occurs during root water uptake is a prerequisite for using stem or xylem water isotopes to trace water sources. However, it is unclear whether isotope fractionation occurs during root water uptake in gramineous crops. We conducted prevalidation experiments to estimate the isotope measurement bias associated with cryogenic vacuum distillation (CVD). Next, we assessed isotope fractionation during root water uptake in two common agronomic crops, wheat (Triticum aestivum L.) and maize (Zea mays L.), under flooding after postdrought stress conditions. Cryogenic vacuum distillation caused significant depletion of ² H but negligible effects on ¹⁸ O for both soil and stem water. Surprisingly CVD caused depletion of ² H and enrichment of ¹⁸ O in root water. Stem and root water δ¹⁸ O were more than soil water δ¹⁸ O, even considering the uncertainty of CVD. Soil water ¹⁸ O was depleted compared with irrigation water ¹⁸ O in the pots with plants but enriched relative to irrigation water ¹⁸ O in the pots without plants. These results indicate that isotope fractionation occurred during wheat and maize root water uptake after full irrigation and led to a heavy isotope enrichment in stem water. Therefore, the xylem/stem water isotope approach widely used to trace water sources should be carefully evaluated.

The extent to which soil hydraulics can explain ecohydrological separation

Field measurements of hydrologic tracers indicate varying magnitudes of geochemical separation between subsurface pore waters. The potential for conventional soil physics alone to explain isotopic differences between preferential flow and tightly-bound water remains unclear. Here, we explore physical drivers of isotopic separations using 650 different model configurations of soil, climate, and mobile/immobile soil-water domain characteristics, without confounding fractionation or plant uptake effects. We find simulations with coarser soils and less precipitation led to reduced separation between pore spaces and drainage. Amplified separations are found with larger immobile domains and, to a lesser extent, higher mobile-immobile transfer rates. Nonetheless, isotopic separations remained small (<4‰ for δ²H) across simulations, indicating that contrasting transport dynamics generate limited geochemical differences. Therefore, conventional soil physics alone are unlikely to explain large ecohydrological separations observed elsewhere, and further efforts aimed at reducing methodological artifacts, refining understanding of fractionation processes, and investigating new physiochemical mechanisms are needed.

Species variation in the hydrogen isotope composition of leaf cellulose is mostly driven by isotopic variation in leaf sucrose

Experimental approaches to isolate drivers of variation in the carbon-bound hydrogen isotope composition (δ² H) 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 ² H-fractionations in carbohydrates differ between species. We analysed, for the first time, the δ² H of leaf sucrose along with the δ² H and δ¹⁸ O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ² H of sucrose explained 66% of the δ² H variation in cellulose (R²  = 0.66), which was associated with species differences in the ² H enrichment of sucrose above leaf water ( ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0001" wiley:location="equation/pce14362-math-0001.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mtext>\unicode{x003B5}</mtext><mtext>sucrose</mtext></msub></mrow></math> : -126% to -192‰) rather than by variation in leaf water δ² H itself. ε sucrose <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0002" wiley:location="equation/pce14362-math-0002.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mtext>\unicode{x003B5}</mtext><mtext>sucrose</mtext></msub></mrow></math> 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 <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0003" wiley:location="equation/pce14362-math-0003.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mi mathvariant="normal">\unicode{x003B5}</mi><mtext>sucrose</mtext></msub></mrow></mrow></math> was fixed to the literature accepted value of - 171 <math altimg="urn:x-wiley:01407791:media:pce14362:pce14362-math-0004" wiley:location="equation/pce14362-math-0004.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mo>\unicode{x02212}</mo><mn>171</mn></mrow></mrow></math> ‰. 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 δ² H of sugars is needed.

Do 2 H and 18 O in leaf water reflect environmental drivers differently?

We compiled hydrogen and oxygen stable isotope compositions (δ² H and δ¹⁸ O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ² H was more closely correlated with δ² H of xylem water or atmospheric vapour, whereas leaf water δ¹⁸ O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ² H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ¹⁸ O. We next expressed leaf water as isotopic enrichment above xylem water (Δ² H and Δ¹⁸ O) to remove the impact of xylem water isotopic variation. For Δ² H, leaf water still correlated with atmospheric vapour, whereas Δ¹⁸ O showed no such correlation. This was explained by covariance between air relative humidity and the Δ¹⁸ O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that ² H and ¹⁸ O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

How do precipitation events modify the stable isotope ratios in leaf water at Lhasa on the southern Tibetan Plateau?

Serving as a medium between source water and cellulose, leaf water contributes to the isotope ratios (δ¹⁸O, δ²H) of plant organic matter, which can be used for paleoclimate reconstruction. This study is the first to examine the diurnal variations in the δ¹⁸O and δ²H of leaf water on the southern Tibetan Plateau. The δ¹⁸O and δ²H of leaf water were relatively low when precipitation events occurred. In particular, ¹⁸O and ²H of leaf water became extremely depleted 5 h after the precipitation event. Our findings demonstrate that precipitation can modify the isotope ratios of leaf water from external and internal causes. First, precipitation events affect meteorological elements, lead to decreases in leaf transpiration, and immediately weaken the isotope enrichment of leaf water ('rapid effect' of precipitation). Second, precipitation events affect the internal plant-soil water cycle process, causing the plant to preferentially use deeper soil water, and the corresponding isotope ratios of leaf water exhibit extremely low values 5 h after precipitation events ('delay effect' of precipitation). This study suggests that researchers need to be cautious in separating the signals of precipitation and hydrological processes when interpreting isotope records preserved in tree-ring cellulose archives from the Tibetan Plateau.

Metabolism is a major driver of hydrogen isotope fractionation recorded in tree-ring glucose of Pinus nigra

Stable isotope abundances convey valuable information about plant physiological processes and underlying environmental controls. Central gaps in our mechanistic understanding of hydrogen isotope abundances impede their widespread application within the plant and biogeosciences. To address these gaps, we analysed intramolecular deuterium abundances in glucose of Pinus nigra extracted from an annually resolved tree-ring series (1961-1995). We found fractionation signals (i.e. temporal variability in deuterium abundance) at glucose H¹ and H² introduced by closely related metabolic processes. Regression analysis indicates that these signals (and thus metabolism) respond to drought and atmospheric CO₂ concentration beyond a response change point. They explain ≈ 60% of the whole-molecule deuterium variability. Altered metabolism is associated with below-average yet not exceptionally low growth. We propose the signals are introduced at the leaf level by changes in sucrose-to-starch carbon partitioning and anaplerotic carbon flux into the Calvin-Benson cycle. In conclusion, metabolism can be the main driver of hydrogen isotope variation in plant glucose.

The unknown third - Hydrogen isotopes in tree-ring cellulose across Europe

This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ²H_c) 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 δ²H_c chronologies were weaker than those recorded in carbon (δ¹³C_c) and oxygen isotope signals (δ¹⁸O_c) but similar to the tree-ring width ones (TRW). The δ²H_c 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 δ²H_c 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 δ²H_c, we investigated its relationships with δ¹⁸O_c and TRW. We found significant negative relationships between δ²H_c and TRW (7 sites), and positive ones between δ²H_c and δ¹⁸O_c (10 sites). The strength of these relationships was nonlinearly related to temperature and precipitation. Mechanistic δ²H_c models performed well for both genera at continental scale simulating average values, but they failed on capturing year-to-year δ²H_c variations. Our results suggest that the information recorded by δ²H_c is significantly different from that of δ¹⁸O_c, 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 δ²H_c. Such advancements could lead to new insights into trees' carbon allocation mechanisms, and responses to abiotic and biotic stress conditions.

Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval

RATIONALE

New methods to measure stable isotopes of soil and tree water directly in the field enable us to increase the temporal resolution of obtained data and advance our knowledge on the dynamics of soil and plant water fluxes. Only few field applications exist. However, these are needed to further improve novel methods and hence exploit their full potential.

METHODS

We tested the borehole equilibration method in the field and collected in situ and destructive samples of stable isotopes of soil, trunk and root xylem water over a 2.5-month experiment in a tropical dry forest under natural abundance conditions and following labelled irrigation. Water from destructive samples was extracted using cryogenic vacuum extraction. Isotope ratios were determined with IRIS instruments using cavity ring-down spectroscopy both in the field and in the laboratory.

RESULTS

In general, timelines of both methods agreed well for both soil and xylem samples. Irrigation labelled with heavy hydrogen isotopes clearly impacted the isotope composition of soil water and one of the two studied tree species. Inter-method deviations increased in consequence of labelling, which revealed their different capabilities to cover spatial and temporal heterogeneities.

CONCLUSIONS

We applied the novel borehole equilibration method in a remote field location. Our experiment reinforced the potential of this in situ method for measuring xylem water isotopes in both tree trunks and roots and confirmed the reliability of gas permeable soil probes. However, in situ xylem measurements should be further developed to reduce the uncertainty within the range of natural abundance and hence enable their full potential.

Evidence for distinct isotopic compositions of sap and tissue water in tree stems: consequences for plant water source identification

The long-standing hypothesis that the isotopic composition of plant stem water reflects that of source water is being challenged by studies reporting bulk water from woody stems with an isotopic composition that cannot be attributed to any potential water source. The mechanism behind such source-stem water isotopic offsets is still poorly understood. Using a novel technique to extract selectively sap water from xylem conduits, we show that, in cut stems and potted plants, the isotopic composition of sap water reflects that of irrigation water, demonstrating unambiguously that no isotopic fractionation occurs during root water uptake or sap water extraction. By contrast, water in nonconductive xylem tissues is always depleted in deuterium compared with sap water, irrespective of wood anatomy. Previous studies have shown that isotopic heterogeneity also exists in soils at the pore scale in which water adsorbed onto soil particles is more depleted in deuterium than unbound water. Data collected at a riparian forest indicated that sap water matches best unbound soil water from depth below -70 cm, while bulk stem and soil water differ markedly. We conclude that source-stem isotopic offsets can be explained by micrometre-scale heterogeneity in the isotope ratios of water within woody stems and soil micro-pores.

Drought reduces water uptake in beech from the drying topsoil, but no compensatory uptake occurs from deeper soil layers

The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial. We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use. Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting. These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Spatiotemporal origin of soil water taken up by vegetation

Vegetation modulates Earth's water, energy and carbon cycles. How its functions might change in the future largely depends on how it copes with droughts^1-4. There is evidence that, in places and times of drought, vegetation shifts water uptake to deeper soil^5-7 and rock^8,9 moisture as well as groundwater^10-12. Here we differentiate and assess plant use of four types of water sources: precipitation in the current month (source 1), past precipitation stored in deeper unsaturated soils and/or rocks (source 2), past precipitation stored in groundwater (source 3, locally recharged) and groundwater from precipitation fallen on uplands via river-groundwater convergence toward lowlands (source 4, remotely recharged). We examine global and seasonal patterns and drivers in plant uptake of the four sources using inverse modelling and isotope-based estimates. We find that (1), globally and annually, 70% of plant transpiration relies on source 1, 18% relies on source 2, only 1% relies on source 3 and 10% relies on source 4; (2) regionally and seasonally, source 1 is only 19% in semi-arid, 32% in Mediterranean and 17% in winter-dry tropics in the driest months; and (3) at landscape scales, source 2, taken up by deep roots in the deep vadose zone, is critical in uplands in dry months, but source 4 is up to 47% in valleys where riparian forests and desert oases are found. Because the four sources originate from different places and times, move at different spatiotemporal scales and respond with different sensitivity to climate and anthropogenic forces, understanding the space and time origins of plant water sources can inform ecosystem management and Earth system models on the critical hydrological pathways linking precipitation to vegetation.