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

Nocturnal sucrose does not reflect the hydrogen isotope composition of transitory starch in leaves as expected

The hydrogen isotope composition (δ2H) of cellulose is inherently linked to that of sucrose synthesized in leaves. Daytime sucrose is synthesized from triose phosphates produced by the Calvin-Benson-Bassham cycle, while nighttime sucrose is synthesized from remobilized transitory starch in leaves. Photosynthetic metabolism causes starch to be naturally 2H-depleted relative to triose phosphates. Thus, sucrose δ2H values should vary diurnally. However, this has not been tested. We made diel measurements of sucrose and starch δ2H in three species differing in their sucrose/starch dynamics (bean, radish and sunflower) under climate controlled and steady-state isotopic conditions. Leaf starch was degraded at night and 2H-depleted by around 90‰ compared with daytime sucrose. However, in all tested species we surprisingly observed no effect on nighttime sucrose δ2H and, instead, only species-specific differences. Consequently, the apparent isotope fractionation associated with sucrose biosynthesis (εsucrose) was indistinguishable between day and night and within -140‰ to -180‰ across the three species. The lack of day/night variation in εsucrose could originate from cytosolic sugar metabolism and 2H-enrichment at H-atom positions counteracting the 2H-depletion in starch. Using a simplified steady-state isotopic model of metabolism, we show that differences in day/night fluxes can reduce the expected differences between day/night εsucrose. From a practical perspective, this suggests that: (i) estimating εsucrose at a single time point might be sufficient to capture δ2H variation under steady-state conditions; and (ii) to extract more than one metabolically sensitive isotope signals from a given compound, position-specific isotope analysis will be required.

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.

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).

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.

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.