Dr. Matthias Saurer

• Reliable insights from key δ²H bioindicators, n-alkanes and carbohydrates, are hindered by our limited understanding of isotope fractionation processes related to leaf water and primary assimilates. We addressed this with the first study to investigate time-integrated intra-annual δ²H signals of n-alkanes and carbohydrates in a natural forest.
• We sampled 1-yr-old needles (1N) and current-year needles (0N) from five Scots Pine trees in a coniferous forest in Hyytiälä, Finland, biweekly during 2019. The δ²H of their n-alkanes (δ²H_alkane), water-soluble carbohydrates (δ²H_WSC) and starch (δ²H_starch) were evaluated for time-integrated physiological (gas-exchange) and environmental (hydrological) signals.
Time integration was critical for interpreting δ²H_WSC and δ²H_alkane.
• Time-integrated net assimilation rate (T:A_n), the strongest signal in δ²H_WSC, correlated negatively with 1N δ²H_WSC (early season) and positively with 0N δ²H_WSC (late season). Unexpectedly, δ²H_alkane exhibited stronger environmental signals in 1N than in 0N, with the most pronounced being physiologically mediated hydrological signals.
• T:A_n is a major signal in intra-annual δ²H_WSC in a boreal forest, subject to seasonal interactions with needle age and δ²H_starch. There could be enough de novo n-alkane synthesis in situ to enhance the reliability of δ²H_alkane as a hydrological indicator, bringing promise to interpretations of the n-alkane palaeohydrological record.

While evidence supports the idea that a portion of the many raindrops that fall onto a forest canopy may be directly absorbed by the twigs they land on, we do not know how much is absorbed, how it enters the twig, or what internal path it might take on its way to the xylem. Using a diverse series of five experiments encompassing isotopic labelling, fluorescent tracers, rehydration kinetics, synchrotron-based X-ray tomographic microscopy, and thermal imaging, we follow the fate of rainwater from initial contact with the twig to its distribution to adjacent tissues. We provide conclusive, multi-pronged evidence of surface water-absorption into the xylem of 1-year-old conifer twigs with incomplete bark development. Additionally, we demonstrate a surface capillary phase, mixed apoplastic and symplastic internal routes, and the strong influence of intercellular airspace as a hydraulic capacitor across multiple tissues. We show that twigs are capable of rapid, large-volume water absorption, which may help trees take advantage of crown-wetting events and support the repair of hydraulic damage from frost and drought. Forecasting the impacts of climatic stress on different tree species will benefit from understanding the importance, and tissue-level specifics, of this critical water-acquisition pathway. Our work tells a detailed story of rain absorption by twigs and lays a foundation for future trait-based research into among-species differences in absorption capacity.

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.

Carbon (C) allocation among different plant tissues is crucial for maintaining C balance in forest ecosystems, especially under changing climate conditions. The partitioning of newly assimilated C among plant tissues, interconnected ramets and soil in forests dominated by giant clonal plants, such as moso bamboo (Phyllostachys edulis), and the influence of drought on this partitioning remain poorly understood.
In August 2019, we performed in situ labelling of the entire crown of R0 (ramets that emerged in 2019) of moso bamboo with ¹³CO₂ in plots subjected to a 5-year drought or left untreated (ambient control) in subtropical China. We then traced the ¹³C signatures in the leaves, twigs and fine roots of R0, R1 (ramets that emerged in 2018 and are connected with R0) and R2 (ramets that emerged in 2017 and are connected with R1), as well as in soil organic C (SOC) and soil respiration over the course of 1-year post-labelling.
Drought reduced leaf ¹³C assimilation and its allocation to sink tissues but did not alter the velocity of C transport from source to sink compared to controls. The peak ¹³C signal was observed on day 15 for SOC and on day 5 for respired CO₂ in both drought and ambient control forests. Labelled ¹³C was detected in R1 ramets on day 3 and in R2 on day 7 post-labelling. This study reveals that new assimilates produced by the ‘younger’ R0 ramets are preferentially retained within their own tissues to meet their own demands rather than being allocated to interconnected neighbouring R1 and R2 ramets.
Synthesis. In forests dominated by large clonal plants, such as giant moso bamboo, drought can alter the allocation of newly assimilated C within the tissues of source ramets but may not affect its allocation among interconnected ramets or within plant–soil systems. Our findings highlight the complexity of newly assimilated C partitioning in these forests and suggest that clonal integration may mitigate drought-induced dieback in older ramets through resource sharing under climate change.

Aims Drought effects on the carbon balance are considered as the major factor of tree mortality and are as-sumed to be regulated by soil nutrient (e.g., nitrogen) availability. However, the effects of nitrogen addition on trees carbon and nitrogen utilization between aboveground and belowground and on coupling relationship of carbon and nitrogen in various organs in response to drought are still unclear in trees.
Methods A two-year full factorial microcosm experiment was set up with sessile oak (Quercus petraea). Nitrogen addition was performed in the first year, and drought was conducted in the second year. The isotope ¹⁵N labeling and ¹³C labeling were carried out before drought and during drought, respectively. Three consecutive samplings were conducted after dual labeling of ¹³C and ¹⁵N labelling in the second year, and the effects of ni-trogen addition on carbon and nitrogen allocation dynamics during progressive drought were tested.
Important findings Our results showed that previous nitrogen addition promoted aboveground photosyn-thetic carbon fixation and nitrogen allocation, increased root nitrogen uptake, reduced the non-structural carbo-hydrates (NSC) contents in all organs and changed the relationships of carbon and nitrogen in aboveground and belowground organs in sessile oak. However, drought had minor effects on nitrogen and carbon allocation be-tween aboveground and belowground, and had minor effects on the relationship of carbon with nitrogen in all organs (represented by the ratio of ¹³C to ¹⁵N in all organs). Drought only significantly reduced the content of NSC in sessile oak. During drought (from day 40 to 73), previous nitrogen addition led sessile oak to prioritize belowground carbon and nitrogen allocation. Our results indicate that sessile oak can change its carbon and ni-trogen allocation strategies to adapt to drought well, while previous nitrogen addition may increase its drought sensitivity.

To investigate which physiological predispositions led to the drought-induced vitality decline in beech (Fagus sylvatica L.) following the severe 2018 drought, we studied trees with premature leaf discoloration and shedding (early-browning trees) and trees showing no symptoms (vital trees) in a forest in northern Switzerland. We analyzed annual tree-ring width (TRW) and applied a triple isotope approach (i.e., carbon (δ¹³C), oxygen (δ¹⁸O), and hydrogen (δ²H) isotopes) in tree-ring cellulose for the period 1960–2020. To retrieve tree physiological responses, we normalized the tree-ring δ values to temporal isotopic variations in CO₂ or precipitation (Δ). Δ¹³C and Δ¹⁸O values suggest that the early-browning trees had a more conservative water-use strategy and lower stomatal conductance than the vital trees in the initial decades of the measurement period. However, several decades before the onset of crown dieback in 2018, the early-browning trees showed a decrease in TRW and an increase in Δ²H, suggesting a higher use of carbon reserves for the early-browning trees. These long-term trends may be the first signs of a progressive deterioration of the physiology of the early-browning trees. During and after the 2018 drought, changes in Δ²H suggested high carbon investments into drought damage repair for the early-browning trees. Moreover, a higher TRW and isotope sensitivity to previous year’s summer climate in early-browning than vital trees suggests stronger negative carry-over effects. Our findings highlight that the early-browning trees may have already been weakened before the 2018 drought, eventually pushing them beyond their physiological tipping points and inducing dieback.

The world's longest tree-ring chronology comprises thousands of oak and pine series from Germany and continuously covers the Holocene back to 12,325 cal BP. A lack of relict wood from the Younger Dryas cold reversal ca. 12,900-11,700 cal BP, however, challenges the extension of this absolutely dated ring width record further back in time. Here, we combine 646 high-resolution stable oxygen isotope and 795 radiocarbon measurements from subfossil pines that grew during the Younger Dryas at three different sites near Zurich, Switzerland, to extend the record. Coherency of the oxygen isotope variations secures internal crossdating, and radiocarbon wiggle-matching places the final 425-year-long ring-width chronology between 12,716 and 12,292 cal BP with an uncertainty of ±8 years. Our study describes an important step towards annual dating precision further into the Late Glacial period.

Rationale: determining several isotope ratios in one analysis multiplies the information that can be retrieved from a sample in a cost-efficient way. The stable isotope ratios of hydrogen (δ²H), carbon (δ¹³C), and oxygen (δ¹⁸O) in organic compounds are highly relevant due to their complimentary hydroclimatic and physiological signals. Different types of organic material reflect different processes and integration times, like short term in leaf sugars and long term in tree ring cellulose, but currently, no simple method exists for their triple isotope analysis.
Methods: here, we present a method that enables the isotopic analyses of the three elements H, C, and O in one run and is applicable to different types of carbohydrates and bulk organic matter. We discuss all steps required from water vapor equilibration necessary for obtaining reliable δ²H values of carbon-bound H to high-temperature conversion (HTC) of the sample to CO and H₂ and to the mass-spectrometric isotope-ratio analysis.
Results: we show that reliable triple isotope analysis is possible for a large range of samples, although it results in some reduction of precision compared to individual isotope analysis. Important considerations are the equilibration procedure, the type of autosampler, selection of HTC reactor, the influence of nitrogen in the sample, the verification of δ¹³C values obtained by HTC versus combustion, and the selection of reference materials.
Conclusions: by presenting a relatively simple triple-isotope method, we promote the use of multi-isotope studies in environmental sciences, which helps in addressing many important climate and ecological research challenges that we face today.

• Die Häufung von warmen und trockenen Jahren wird zunehmend zu einer Belastung für den Wald. Selbst Baumarten, die bisher als trockenresistent galten, leiden darunter.
• Zwischenergebnisse des fünften Landesforstinventars zeigen eine Zunahme von geschädigten Bäumen. Hauptursachen sind Insekten und Krankheitserreger sowie Windwurf und Vitalitätsverlust nach Trockenheit.
• Das Bundesamt für Umwelt (BAFU) und die Eidg. Forschungsanstalt für Wald, Schnee und Landschaft (WSL) haben Werkzeuge zur Baumartenwahl im Klimawandel entwickelt. Wichtige Massnahmen sind die Förderung von Mischwäldern und die Erhöhung der genetischen Vielfalt der Wälder.

• As hot, dry years become more frequent, forests are placed under stress. Even tree species previously considered drought-resistant are suffering as a result.
• Interim results of the fifth National Forest Inventory  show an increase in damaged trees. The main causes are insects and pathogens as well as windthrow and loss of vitality after drought.
• The Federal Office for the Environment (FOEN) and the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) have developed tools for selecting tree species in a changing climate. Key measures include promoting mixed forests and increasing the genetic diversity of forests.

• La multiplication d'années chaudes et sèches pèse de plus en plus sur la forêt. Même des essences réputées jusqu'ici résistantes à la sécheresse en souffrent.
• Les résultats intermédiaires du cinquième inventaire forestier national montrent une hausse des arbres endommagés, principalement en raison d'insectes, d'agents pathogènes, de chablis et de pertes de vitalité dues à la sécheresse.
• L'Office fédéral de l'environnement (OFEV) et l'Institut fédéral de recherches sur la forêt, la neige et le paysage (WSL) ont développé des outils pour un choix d'essences adaptées aux changements climatiques. La promotion de forêts mélangées et l'augmentation de la diversité génétique des forêts sont des mesures importantes.

• La frequenza di anni caldi e secchi sta diventando sempre più problematica per il bosco. Ne stanno risentendo anche le specie un tempo considerate resistenti alla siccità.
• I risultati intermedi del quinto Inventario forestale nazionale indicano un aumento degli alberi danneggiati,
soprattutto a causa di insetti e agenti patogeni ma anche per schianti da vento e perdita di vitalità in seguito alla siccità.
• L'Ufficio federale dell'ambiente (UFAM) e l'Istituto federale di ricerca per la foresta, la neve e il paesaggio
(WSL) hanno sviluppato degli strumenti per selezionare specie arboree adatte al cambiamento climatico. Le principali misure consistono nella promozione dei boschi misti e nell’incremento della loro diversità genetica.

• 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.

Stable carbon and hydrogen isotopes of wood methoxy groups (δ²H_meth, δ¹³C_meth), mainly sourced by structural biomolecules like lignin and pectin, provide important insights into climatic, hydrological and physiological conditions. This study systematically investigated species-specific δ²H_meth and δ¹³C_meth variations in leaves and woody twigs of 65 different tree species grown in a common garden. Significant phylogenetic patterns were observed in δ²H_meth and δ¹³C_meth of both tissues, with stronger signals in leaves and the most pronounced differences between angiosperms and gymnosperms. δ¹³C_meth variations are likely explained by anatomical and physiological differences between seed types, while δ²H_meth variations were attributed to temporal differences in water uptake or isotope fractionation processes. Notably, δ¹³C_meth values were more negative in leaves than in twigs, while δ²H_meth values showed no tissue-specific difference. This suggests that serine, a methoxy precursor, is differently synthesised in autotrophic than in heterotrophic tissues. Hydrogen isotope fractionation between xylem water and twig methoxy groups averaged at −197 mUr, with mean isotope fractionation of gymnosperms −209 mUr being significantly different to that of angiosperms −184 mUr. Weak relationships between δ²H_meth and δ²H values of carbohydrates indicated that distinct signals are preserved within the two compounds. This study highlights the importance of phylogenetic considerations when using methoxy group isotopes as proxies and provides new insights into methoxy group biosynthesis.

• In most tree species, roots serve as major carbon (C) sinks, where C is depleted first when C assimilation is limited. Recent methodological advancements in sugar infusion allow for a better understanding of physiological processes alleviating root C limitation.
• We conducted a glasshouse experiment with maple (Acer pseudoplatanus L.) and pine (Pinus sylvestris L.) saplings that underwent defoliation followed by either slow, fast, or no ¹³C-labeled glucose infusion. We measured photosynthetic parameters, nonstructural carbohydrate (NSC) concentrations, and δ¹³C in cellulose of leaves, twigs, and fine roots, as well as the isotopic composition of dark-respired CO₂.
• Sugar infusion induced photosynthetic downregulation and leaf senescence in maple but not in pine. Leaf photosynthesis was negatively correlated with leaf NSC concentration in maple. These responses exacerbated root C limitation in maple. Conversely, pine maintained stable photosynthetic rates and needle NSC concentrations across treatments, showing the potential of sugar infusion to mitigate root C limitation.
• Our study suggests that exogenous sugar supply reduces the root C availability when it impairs a plant's photosynthetic performance. Species-specific differences influence infused sugar transport and overall source–sink responses. Alleviating C limitation in roots via exogenous sugar addition is feasible only if photosynthesis is not impeded.

Plants’ non-structural carbohydrates (NSCs) serve as their capital for growth, reproduction, defense, and survival. To increase the NSC availability of carbon-limited trees, a recent study revealed the possibility of adding exogenous soluble sugars to carbon-starved trees. This provides an opportunity to investigate carbon allocation between source and sink, as well as the growth and physiological responses to external sugars. Using this method, we infused ¹³C-labeled glucose solution into the stem xylem of sycamore maple (Acer pseudoplatanus L.) trees (Experiment 1) and immersed branch cuttings of various tree species in a ¹³C-labeled glucose solution (Experiment 2). Our aim was to study whether infused sugars contribute to structural growth and how they influence photosynthesis. Specifically, we focused on whether trees can transport and utilize exogenous sugars for growth, and if sugar addition might trigger negative feedback mechanisms on carbon gain. We then traced the ¹³C label in bulk tissue and cellulose, and measured tissue NSC concentrations and leaf photosynthesis. Glucose addition consistently increased leaf NSC concentrations (Experiments 1 and 2), and exogenous sugars added were transported and incorporated into biomass formation in Experiment 1. However, excessive sugar addition triggered a negative feedback response, leading to leaf senescence (Experiments 1 and 2) and reduced photosynthesis (Experiment 2). Our findings validate the recently developed sugar addition method but emphasize the importance of carefully controlling the amount and rate of sugar addition to avoid negative feedback responses. This study will contribute to carbon physiological research, particularly in understanding carbon balance and source-sink relationships at the whole-plant level.

Introduction: The Global Meteoric Water Line (GMWL) describes the linear relationship between stable hydrogen (δ²H) and oxygen (δ¹⁸O) isotopes in precipitation over large spatial scales and therefore represents a unique reference for water isotopic values. Although trees have the potential to capture the isotopic composition of precipitation, it remains unclear if the GMWL can be reconstructed from tree-ring stable isotopes, since δ¹⁸O and δ²H undergo in vivo physiological fractionation.

Methods: We analyze the tree rings δ¹⁸O and δ²H values from six regions along a latitudinal gradient from Spain to Greenland.

Results: The data show that the covariance between δ¹⁸O and δ²H closely follows the GMWL, which reflects the isotopic signature of large-scale precipitation patterns. We show that changes in regional tree-ring δ¹⁸O and δ²H values along wide latitudinal ranges are influenced by the isotopic composition of precipitation with temperature and latitude being the most significant drivers of spatial variation across the studied regions. In contrast, local tree-ring δ¹⁸O and δ²H values are mainly controlled by plant physiological fractionation processes that mask the isotopic signature of precipitation.

Conclusion: We conclude that covariance in tree-ring δ¹⁸O and δ²H reflects the GMWL at larger spatial scales, but not when evaluating them at individual sites.

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).

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.

The current lack of information on past summer sunshine duration variability from annually resolved palaeoclimatological archives is hindering progress in the understanding and modelling of the earth climate system. We show that a combination of tree-ring carbon and oxygen isotopes from Siberia provides robust information on summer sunshine duration, which we use for an annual 1505-year reconstruction of July sunshine duration variability (1,5K-SIB-JSDR). We found that the Medieval maximum is 56 % higher than the average over 1505 years. Rapid and drastic decreases in sunshine duration up to 60 % correspond to major stratospheric volcanic eruptions. Grand Solar Minima and total sunspot numbers are also well preserved in the 1,5K-SIB-JSDR. Coherency with a global air temperature composite and spring Arctic Oscillation indicate that a large-scale climate signal is retained in our sunshine reconstruction.