Unenriched xylem water contribution during cellulose synthesis influenced by atmospheric demand governs the intra-annual tree-ring δ18 O signature

Opposing seasonal trends in source water and sugar dampen intra-annual variability in tree rings oxygen isotopes

Variations of oxygen isotopes δ18O in tree rings provide critical insights into past climate and tree physiological processes, yet the mechanisms shaping the intra-annual δ18O signals remain incompletely understood. To address this gap, we investigated how seasonal changes in source water, leaf water, and sugars influence δ18O recorded along the tree rings of Pinus sylvestris in Finland. We conducted a seasonal analysis measuring δ18O from needle water, source water, and phloem sugars and investigated the fraction of oxygen isotope exchange during wood formation. We found that seasonal δ18O amplitudes are significantly reduced from leaf water to tree rings, driven by opposing seasonal patterns in increasing source water δ18O and decreasing evaporative enrichment as relative humidity increases. Additionally, the isotope exchange between source water and phloem sugars further dampens seasonal δ18O signals in the rings. Our findings show that oxygen isotope exchange is critical in shaping δ18O signals, influencing the role of source water and relative humidity recorded on intra-annual resolution. This refined understanding helps interpret tree physiological responses under changing conditions and improves climate reconstructions based on tree rings using intra-annual resolution.

Tracing Oxygen and Hydrogen Isotope Signals From Water Sources to Tree-Ring Compounds

Stable oxygen (δ18O) and hydrogen (δ2H) isotope compositions of tree-ring compounds preserve information about environmental waters; however, our understanding of their isotopic relationships is hampered by the lack of long-term data sets. We investigated correlations using unique 17-year (2006-2022) δ18O and δ2H time series of bi-weekly measured soil solution, modelled precipitation and xylem water, along with those of tree-ring α-cellulose and lignin methoxy groups from Norway spruce (Picea abies) across three Swiss forest sites. We show that tree-ring cellulose δ18O preserves water source information more effectively than δ2H, making it better suited for ecohydrological reconstructions. We propose δ2H of tree-ring lignin methoxy groups as an alternative proxy for soil water sources, supported by strong correlations where cellulose failed to track soil water isotopes. Significant linear isotopic relationships within and across sites enable the development of transfer functions that link tree-ring to water sources, particularly precipitation and xylem water. We exemplify how these transfer functions can be used to estimate the seasonal origin of water sourced by trees during the growth period. Our findings enhance the interpretation of environmental water isotope signals in tree rings and promote the use of tree-ring isotope-based tools for retrospective retrieval of forest water dynamics.

The aridity influence on oxygen isotopes recorded in tree rings

The stable isotopes of oxygen in wood cellulose (δ18Ocell) have been widely used to reconstruct historical source water use in trees or changes in atmospheric humidity. However, in many cases, the δ18O of source water use is assumed to reflect that of precipitation, which is often not the case in semi-arid to arid ecosystems where trees use deeper and older water from previous precipitation events (or even groundwater). Furthermore, the degree to which δ18Ocell reflects source water and atmospheric aridity depends on pex, normally defined as the proportion of oxygen atoms that exchange between isotopically enriched carbohydrates from the leaf and unenriched xylem water during cellulose synthesis. Many studies treat pex as a constant. However, pex can only be estimated with direct measurements of δ18Ocell and the δ18O of tree source water and sucrose. Additionally, other physiological mechanisms (e.g., photosynthate translocation) can alter the isotopic signal before cellulose is produced. Thus, determining this 'apparent pex' (apex; which includes those other physiological mechanisms such as photosynthate translocation plus the exchange of oxygen atoms during cellulose synthesis), can be difficult. In this study, we collected δ18O of xylem water and δ18O of wood cellulose from seven stands of Ponderosa pine situated at the northern boundary of the North American Monsoon (NAM) climate system to assess how potential variability in apex influenced how source water and aridity were recorded in δ18Ocell. We compared measured and modeled values of δ18Ocell and found that more arid sites under-represented the vapor pressure deficit (VPD) signal in cellulose while wetter sites over-represented the VPD signal in cellulose. We also found that apex varied as a function of site aridity, where low precipitation and high VPD led to high apex, while high precipitation and low VPD led to low apex. Future studies can use our emerging understanding of the aridity-apex relationship in different portions of the annual ring to better disentangle the source water and VPD signals in cellulose, particularly for regions such as the NAM region.

Tree-ring stable isotopes from the European Alps reveal long-term summer drying over the Holocene

Here, we use 7437 stable oxygen (δ18O) isotope ratios extracted from 192 living and relict Alpine trees to reconstruct trends and extremes in European summer hydroclimate from 8980 before the present to 2014 Common Era. Our continuous tree-ring δ18O record reveals a significant long-term drying trend over much of the Holocene (P < 0.001), which is in line with orbital forcing and independent evidence from proxy reconstructions and model simulations. Wetter conditions in the early-to-mid Holocene coincide with the African Humid Period, whereas the most severe summer droughts of the past 9000 years are found during the Little Ice Age in the 18th and 19th centuries Common Era. We suggest that much of Europe was not only warmer but also wetter during most of the preindustrial Holocene, which implies a close relationship between insolation changes and long-term hydroclimate trends that likely affected natural and societal systems across a wide range of spatiotemporal scales.

Growth and Assemblage Dynamics of Temperate Forest Tree Species Match Physiological Resilience to Changes in Atmospheric Chemistry

Human-induced environmental changes are altering forest productivity and species composition, significantly impacting tree physiology, growth, water uptake, and nutrient acquisition. Investigating the intricate interplay between plant physiology and environmental shifts, we analyzed tree-ring isotopes (δ13C, δ18O, and δ15N) to track long-term trends in intrinsic water-use efficiency (iWUE) and nitrogen availability for European beech, Norway spruce, and silver fir in a unique old-growth temperate mountain forest since 1501 ce. Our findings reveal that Norway spruce, a dominant species, exhibited iWUE saturation, exacerbated by acidic precipitation, resulting in growth declines during periods of high acidic air pollution and increased drought frequency. In contrast, deep-rooted, deciduous European beech demonstrated physiological resilience to acid deposition, benefiting from lower dry deposition of precipitation acidity and thriving under conditions of increased nitrogen deposition and elevated air temperatures, thereby sustaining stem growth regardless of potential climatic limitations. Silver fir showed the most dynamic response to acidic air pollution, with contemporary adaptations in leaf gas exchange allowing accelerated stem growth under cleaner air conditions. These different species responses underscore shifts in species competition, with European beech gaining dominance as Norway spruce and silver fir decline. Furthermore, the influence of ontogeny is evident, as tree-rings exhibited lower initial iWUE values and higher δ15N, reflecting changes in nitrogen uptake dynamics and the ecological role of tree age. Our study integrates tree-growth dynamics with physiological and nutrient availability trends, revealing the pivotal role of atmospheric chemistry changes in shaping the competitive dynamics and long-term growth trajectories of dominant tree species in temperate forests.

Decoupling of Tree-Ring Cellulose δ18O and δ2H Highlighted by Their Contrasting Relationships to Climate and Tree Intrinsic Variables

Oxygen (δ18O) and hydrogen (δ2H) stable isotope ratios are tightly coupled in precipitation and, albeit damped, in leaf water, but are often decoupled in tree-ring cellulose. The environmental and physiological conditions in which this decoupling occurs are not yet well understood. We investigated the relationships between δ18O and δ2H and tree-ring width (TRW), tree crown volume, tree age and climate in silver fir and Douglas-fir and found substantial differences between δ18O and δ2H. Overall, δ18O-δ2H correlations were weak to absent but became significantly negative under high summer vapour pressure deficit (VPD). δ18O and δ2H had positive and negative nonlinear relationships with TRW, respectively, with clear relationships at the site and tree levels for silver fir and, to a lesser extent, for Douglas-fir. Age trends for silver fir were weakly negative in δ18O but positive in δ2H. Tree crown volume and δ18O or δ2H had no significant relationships. Most strikingly, δ18O strongly depended on spring climate (precipitation and VPD), whereas δ2H depended on summer climate (temperature and VPD) for both species. Our study shows that the δ18O-δ2H decoupling in tree-ring cellulose in two temperate conifer species could be highlighted by their contrasting relationships to climate and tree intrinsic variables (TRW, age).

Triple-isotope analysis in tree-ring cellulose suggests only moderate effects of tree species mixture on the climate sensitivity of silver fir and Douglas-fir

Disentangling the factors influencing the climate sensitivity of trees is crucial to understanding the susceptibility of forests to climate change. Reducing tree-to-tree competition and mixing tree species are two strategies often promoted to reduce the drought sensitivity of trees, but it is unclear how effective these measures are in different ecosystems. Here, we studied the growth and physiological responses to climate and severe droughts of silver fir and Douglas-fir growing in pure and mixed conditions at three sites in Switzerland. We used tree-ring width data and carbon (δ13C), oxygen (δ18O) and hydrogen (δ2H) stable isotope ratios from tree-ring cellulose to gain novel information on water relations and the physiology of trees in response to drought and how tree species mixture and competition modulate these responses. We found significant differences in isotope ratios between trees growing in pure and mixed conditions for the two species, although these differences varied between sites, e.g. trees growing in mixed conditions had higher δ13C values and tree-ring width than trees growing in pure conditions for two of the sites. For both species, differences between trees in pure and mixed conditions regarding their sensitivity to temperature, precipitation, climatic water balance and vapor pressure deficit were minor. Furthermore, trees growing in pure and mixed conditions showed similar responses of tree-ring width and isotope ratios to the past severe droughts of 2003, 2015 and 2018. Competition had only a significantly negative effect on δ13C of silver fir, which may suggest a decrease in photosynthesis due to higher competition for light and nutrients. Our study highlights that tree species mixture may have only moderate effects on the radial growth and physiological responses of silver fir and Douglas-fir to climatic conditions and that site condition effects may dominate over mixture effects.

Contrasting water-use strategies to climate warming in white birch and larch in a boreal permafrost region

The effects of rising atmospheric CO2 concentrations (Ca) with climate warming on intrinsic water-use efficiency and radial growth in boreal forests are still poorly understood. We measured tree-ring cellulose δ13C, δ18O, and tree-ring width in Larix dahurica (larch) and Betula platyphylla (white birch), and analyzed their relationships with climate variables in a boreal permafrost region of northeast China over past 68 years covering a pre-warming period (1951-1984; base period) and a warm period (1985-2018; warm period). We found that white birch but not larch significantly increased their radial growth over the warm period. The increased intrinsic water-use efficiency in both species was mainly driven by elevated Ca but not climate warming. White birch but not larch showed significantly positive correlations between tree-ring δ13C, δ18O and summer maximum temperature as well as vapor pressure deficit in the warm period, suggesting a strong stomatal response in the broad-leaved birch to temperature changes. The climate warming-induced radial growth enhancement in white birch is primarily associated with a conservative water-use strategy. In contrast, larch exhibits a profligate water-use strategy. It implies an advantage for white birch over larch in the warming permafrost regions.

Half of the 18O enrichment of leaf sucrose is conserved in leaf cellulose of a C3 grass across atmospheric humidity and CO2 levels

The 18O enrichment (Δ18O) of cellulose (Δ18OCel) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (pex) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C3 grasses demonstrated that the Δ18O of leaf sucrose (Δ18OSuc, the parent substrate for cellulose synthesis) can be much higher than predicted from daytime Δ18O of leaf water (Δ18OLW), which could alter conclusions on photosynthetic versus post-photosynthetic effects on Δ18OCel via pex. Here, we assessed pex in leaves of perennial ryegrass (Lolium perenne) grown at different atmospheric relative humidity (RH) and CO2 levels, by determinations of Δ18OCel in leaves, Δ18OLGDZW (the Δ18O of water in the leaf growth-and-differentiation zone) and both Δ18OSuc and Δ18OLW (adjusted for εbio, the biosynthetic fractionation between water and carbohydrates) as alternative proxies for the substrate for cellulose synthesis. Δ18OLGDZW was always close to irrigation water, and pex was similar (0.53 ± 0.02 SE) across environments when determinations were based on Δ18OSuc. Conversely, pex was erroneously and variably underestimated (range 0.02-0.44) when based on Δ18OLW. The photosynthetic signal fraction in Δ18OCel is much more constant than hitherto assumed, encouraging leaf physiological reconstructions.

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.

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.