Seasonal changes of δD and δ18O in tree-ring cellulose of Quercus crispula suggest a change in post-photosynthetic processes during earlywood growth

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.

Finding balance: Tree-ring isotopes differentiate between acclimation and stress-induced imbalance in a long-term irrigation experiment

Scots pine (Pinus sylvestris L.) is a common European tree species, and understanding its acclimation to the rapidly changing climate through physiological, biochemical or structural adjustments is vital for predicting future growth. We investigated a long-term irrigation experiment at a naturally dry forest in Switzerland, comparing Scots pine trees that have been continuously irrigated for 17 years (irrigated) with those for which irrigation was interrupted after 10 years (stop) and non-irrigated trees (control), using tree growth, xylogenesis, wood anatomy, and carbon, oxygen and hydrogen stable isotope measurements in the water, sugars and cellulose of plant tissues. The dendrochronological analyses highlighted three distinct acclimation phases to the treatments: irrigated trees experienced (i) a significant growth increase in the first 4 years of treatment, (ii) high growth rates but with a declining trend in the following 8 years and finally (iii) a regression to pre-irrigation growth rates, suggesting the development of a new growth limitation (i.e. acclimation). The introduction of the stop treatment resulted in further growth reductions to below-control levels during the third phase. Irrigated trees showed longer growth periods and lower tree-ring δ13 C values, reflecting lower stomatal restrictions than control trees. Their strong tree-ring δ18 O and δ2 H (O-H) relationship reflected the hydrological signature similarly to the control. On the contrary, the stop trees had lower growth rates, conservative wood anatomical traits, and a weak O-H relationship, indicating a physiological imbalance. Tree vitality (identified by crown transparency) significantly modulated growth, wood anatomical traits and tree-ring δ13 C, with low-vitality trees of all treatments performing similarly regardless of water availability. We thus provide quantitative indicators for assessing physiological imbalance and tree acclimation after environmental stresses. We also show that tree vitality is crucial in shaping such responses. These findings are fundamental for the early assessment of ecosystem imbalances and decline under climate change.

Covariation between oxygen and hydrogen stable isotopes declines along the path from xylem water to wood cellulose across an aridity gradient

Oxygen and hydrogen isotopes of cellulose in plant biology are commonly used to infer environmental conditions, often from time series measurements of tree rings. However, the covariation (or the lack thereof) between δ18 O and δ2 H in plant cellulose is still poorly understood. We compared plant water, and leaf and branch cellulose from dominant tree species across an aridity gradient in Northern Australia, to examine how δ18 O and δ2 H relate to each other and to mean annual precipitation (MAP). We identified a decline in covariation from xylem to leaf water, and onwards from leaf to branch wood cellulose. Covariation in leaf water isotopic enrichment (Δ) was partially preserved in leaf cellulose but not branch wood cellulose. Furthermore, whilst δ2 H was well-correlated between leaf and branch, there was an offset in δ18 O between organs that increased with decreasing MAP. Our findings strongly suggest that postphotosynthetic isotope exchange with water is more apparent for oxygen isotopes, whereas variable kinetic and nonequilibrium isotope effects add complexity to interpreting metabolic-induced δ2 H patterns. Varying oxygen isotope exchange in wood and leaf cellulose must be accounted for when δ18 O is used to reconstruct climatic scenarios. Conversely, comparing δ2 H and δ18 O patterns may reveal environmentally induced shifts in metabolism.

Tree-ring isotopes from the Swiss Alps reveal non-climatic fingerprints of cyclic insect population outbreaks over the past 700 years

Recent experiments have underlined the potential of δ2H in tree-ring cellulose as a physiological indicator of shifts in autotrophic versus heterotrophic processes (i.e., the use of fresh versus stored non-structural carbohydrates). However, the impact of these processes has not yet been quantified under natural conditions. Defoliator outbreaks disrupt tree functioning and carbon assimilation, stimulating remobilization, therefore providing a unique opportunity to improve our understanding of changes in δ2H. By exploring a 700-year tree-ring isotope chronology from Switzerland, we assessed the impact of 79 larch budmoth (LBM, Zeiraphera griseana [Hübner]) outbreaks on the growth of its host tree species, Larix decidua [Mill]. The LBM outbreaks significantly altered the tree-ring isotopic signature, creating a 2H-enrichment and an 18O- and 13C-depletion. Changes in tree physiological functioning in outbreak years are shown by the decoupling of δ2H and δ18O (O-H relationship), in contrast to the positive correlation in non-outbreak years. Across the centuries, the O-H relationship in outbreak years was not significantly affected by temperature, indicating that non-climatic physiological processes dominate over climate in determining δ2H. We conclude that the combination of these isotopic parameters can serve as a metric for assessing changes in physiological mechanisms over time.

Progress in high-resolution isotope-ratio analysis of tree rings using laser ablation

Stable isotope ratio analysis of tree rings has been widely and successfully applied in recent decades for climatic and environmental reconstructions. These studies were mostly conducted at an annual resolution, considering one measurement per tree ring, often focusing on latewood. However, much more information could be retrieved with high-resolution intra-annual isotope studies, based on the fact that the wood cells and the corresponding organic matter are continuously laid down during the growing season. Such studies are still relatively rare, but have a unique potential for reconstructing seasonal climate variations or short-term changes in physiological plant properties, like water-use efficiency. The reason for this research gap is mostly technical, as on the one hand sub-annual, manual splitting of rings is very tedious, while on the other hand automated laser ablation for high-resolution analyses is not yet well established and available. Here, we give an update on the current status of laser ablation research for analysis of the carbon isotope ratio (δ13C) of wood, describe an easy-to-use laser ablation system, its operation and discuss practical issues related to tree core preparation, including cellulose extraction. The results show that routine analysis with up to 100 laser shot-derived δ13C-values daily and good precision and accuracy (ca. 0.1‰) comparable to conventional combustion in an elemental analyzer are possible. Measurements on resin-extracted wood is recommended as most efficient, but laser ablation is also possible on cellulose extracted wood pieces. Considering the straightforward sample preparation, the technique is therefore ripe for wide-spread application. With this work, we hope to stimulate future progress in the promising field of high-resolution environmental reconstruction using laser ablation.

Foliar water uptake as a source of hydrogen and oxygen in plant biomass

Introductory biology lessons around the world typically teach that plants absorb water through their roots, but, unfortunately, absorption of water through leaves and subsequent transport and use of this water for biomass formation remains a field limited mostly to specialists. Recent studies have identified foliar water uptake as a significant net water source for terrestrial plants. The growing interest in the development of a new model that includes both foliar water uptake (in liquid form) and root water uptake to explain hydrogen and oxygen isotope ratios in leaf water and tree rings demands a method for distinguishing between these two water sources. Therefore, in this study, I have devised a new labelling method that utilizes two different water sources, one enriched in deuterium (HDO + D2O; δD = 7.0 × 10 4‰, δ18O = 4.1‰) and one enriched in oxygen-18 (H218O; δD = -85‰, δ18O = 1.1 × 104‰), to simultaneously label both foliar-absorbed and root-absorbed water and quantify their relative contributions to plant biomass. Using this new method, I here present evidence that, in the case of well-watered Cryptomeria japonica D. Don, hydrogen and oxygen incorporated into new leaf cellulose in the rainy season derives mostly from foliar-absorbed water (69% from foliar-absorbed water and 31% from root-absorbed water), while that of new root cellulose derives mostly from root-absorbed water (20% from foliar-absorbed water and 80% from root-absorbed water), and new branch xylem is somewhere in between (55% from foliar-absorbed water and 45% from root-absorbed water). The dual-labelling method first implemented in this study enables separate and simultaneous labelling of foliar-absorbed and root-absorbed water and offers a new tool to study the uptake, transport and assimilation processes of these waters in terrestrial plants.

Foliar water uptake as a source of hydrogen and oxygen in plant biomass

Introductory biology lessons around the world typically teach that plants absorb water through their roots, but, unfortunately, absorption of water through leaves and subsequent transport and use of this water for biomass formation remains a field limited mostly to specialists. Recent studies have identified foliar water uptake as a significant net water source for terrestrial plants. The growing interest in the development of a new model that includes both foliar water uptake (in liquid form) and root water uptake to explain hydrogen and oxygen isotope ratios in leaf water and tree rings demands a method for distinguishing between these two water sources. Therefore, in this study, I have devised a new labelling method that utilizes two different water sources, one enriched in deuterium (HDO + D2O; δD = 7.0 × 10 4‰, δ18O = 4.1‰) and one enriched in oxygen-18 (H218O; δD = -85‰, δ18O = 1.1 × 104‰), to simultaneously label both foliar-absorbed and root-absorbed water and quantify their relative contributions to plant biomass. Using this new method, I here present evidence that, in the case of well-watered Cryptomeria japonica D. Don, hydrogen and oxygen incorporated into new leaf cellulose in the rainy season derives mostly from foliar-absorbed water (69% from foliar-absorbed water and 31% from root-absorbed water), while that of new root cellulose derives mostly from root-absorbed water (20% from foliar-absorbed water and 80% from root-absorbed water), and new branch xylem is somewhere in between (55% from foliar-absorbed water and 45% from root-absorbed water). The dual-labelling method first implemented in this study enables separate and simultaneous labelling of foliar-absorbed and root-absorbed water and offers a new tool to study the uptake, transport and assimilation processes of these waters in terrestrial plants.

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.

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.

Recent Progresses in Stable Isotope Analysis of Cellulose Extracted from Tree Rings

In this work, the challenges and progression in stable isotope investigation, from the analytical tools and technical sample preparation procedures to the dendroclimatological experiments, were reviewed in terms of their use to assess tree physiological responses to environmental changes. Since the isotope signature of whole wood is not always a reliable tool in studying the climate changes, cellulose is often preferred as the study material in paleoclimatic studies. Nevertheless, the isotope analysis of cellulose is challenging due to the difficulty to remove the other wood components (extractives, lignin, pectin, and hemicelluloses). Additionally, in the case of hydrogen isotope analysis, about 30% of the hydrogen atoms of cellulose are exchanged with the surrounding water, which complicates the isotope analysis. In recent years, more automated isotope analysis methods were developed based on high temperature pyrolysis of cellulose, followed by the chromatographic separation of H₂ from CO and by their individual isotope analysis using isotope ratio mass spectrometry. When used to investigate climate factors, the combined isotope analysis δ¹³C and δ¹⁸O appears to be the most promising isotope tool. In contrast, the role of δ²H values is yet to be elucidated, together with the development of new methods for hydrogen isotope analysis.

Reduction in lumen area is associated with the δ18 O exchange between sugars and source water during cellulose synthesis

High temporal resolution measurements of wood anatomy and the isotopic composition in tree-rings have the potential to enhance our interpretation of climate variability, but the sources of variation within the growing season are still not well understood. Here we test the response of wood anatomical features in Pinus ponderosa and Pseudotsuga menziesii, including cell-wall thickness (CWT) and lumen area (LA), along with the oxygen isotopic composition of α-cellulose (δ¹⁸ O_cell ) to shifts in relative humidity (RH) in two treatments, one from high-low RH and the second one form low-high RH. We observed a significant decrease in LA and a small increase in CWT within the experimental growing season in both treatments. The measured δ¹⁸ O_cell along the ring was responsive to RH variations in both treatments. However, estimated δ¹⁸ O_cell did not agree with measured δ¹⁸ O_cell when the proportion of exchangeable oxygen during cellulose synthesis (P_ex ) was kept constant. We found that P_ex increased throughout the ring as LA decreased. Based on this varying P_ex within an annual ring, we propose a targeted sampling strategy for different hydroclimate signals: earlier season cellulose is a better recorder of RH while late-season cellulose is a better recorder of the source water.