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.

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.

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.

• Climate change not only leads to higher air temperatures but also increases the vapour pressure deficit (VPD) of the air. Understanding the direct effect of VPD on leaf gas exchange is crucial for precise modelling of stomatal functioning.
• We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species across a VPD range of 0.8–3.6 kPa, while maintaining constant temperatures without soil water limitation. In addition to applying the standard assumption of saturated vapour pressure inside leaves (e_i), we inferred e_i from oxygen isotope discrimination of CO₂ and water vapour.
• e_i desaturated progressively with increasing VPD, consistently across species, resulting in an intercellular relative humidity as low as 0.73 ± 0.11 at the highest tested VPD. Assuming saturation of e_i overestimated the extent of reductions in stomatal conductance and CO₂ mole fraction inside leaves in response to increasing VPD compared with calculations that accounted for unsaturation. In addition, a significant decrease in mesophyll conductance with increasing VPD only occurred when the unsaturation of e_i was considered.
• We suggest that the possibility of unsaturated e_i should not be overlooked in measurements related to leaf gas exchange and in stomatal models, especially at high VPD.

The strong covariation of temperature and vapour pressure deficit (VPD) in nature limits our understanding of the direct effects of temperature on leaf gas exchange. Stable isotopes in CO₂ and H₂O vapour provide mechanistic insight into physiological and biochemical processes during leaf gas exchange. We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species across a leaf temperature range of 5–40°C, while maintaining a constant leaf-to-air VPD (0.8 kPa) without soil water limitation. Above the optimum temperature for photosynthesis (30°C) under the controlled environmental conditions, stomatal conductance (g_s) and net photosynthesis rate (A_n) decoupled across all tested species, with g_s increasing but A_n decreasing. During this decoupling, mesophyll conductance (cell wall, plasma membrane and chloroplast membrane conductance) consistently and significantly decreased among species; however, this reduction did not lead to reductions in CO₂ concentration at the chloroplast surface and stroma. We question the conventional understanding that diffusional limitations of CO₂ contribute to the reduction in photosynthesis at high temperatures. We suggest that stomata and mesophyll membranes could work strategically to facilitate transpiration cooling and CO₂ supply, thus alleviating heat stress on leaf photosynthetic function, albeit at the cost of reduced water-use efficiency.

Even though they share many thematical overlaps, plant metabolomics and stable isotope ecology have been rather separate fields mainly due to different mass spectrometry demands. New high-resolution bioanalytical mass spectrometers are now not only offering high-throughput metabolite identification but are also suitable for compound- and intramolecular position-specific isotope analysis in the natural isotope abundance range. In plant metabolomics, label-free metabolic pathway and metabolic flux analysis might become possible when applying this new technology. This is because changes in the commitment of substrates to particular metabolic pathways and the activation or deactivation of others alter enzyme-specific isotope effects. This leads to differences in intramolecular and compound-specific isotope compositions. In plant isotope ecology, position-specific isotope analysis in plant archives informed by metabolic pathway analysis could be used to reconstruct and separate environmental impacts on complex metabolic processes. A technology-driven linkage between the two disciplines could allow to extract information on environment–metabolism interaction from plant archives such as tree rings but also within ecosystems. This would contribute to a holistic understanding of how plants react to environmental drivers, thus also providing helpful information on the trajectories of the vegetation under the conditions to come.

We used five mature Picea abies continuously labeled with ¹³C-depleted CO₂ in a broadleaf-dominated Swiss forest to assess the spatial extent and lag time of carbon fluxes to ectomycorrhizal fungi differing in hyphal development and host association. We traced labeled carbon into ectomycorrhizal sporocarps collected for two seasons at different distances from labeled Picea. Picea-derived photosynthate reached conifer-specific sporocarps up to 6–12 m away and reached other sporocarps only 0–6 m away. At 0–6 m, genera of lesser hyphal development acquired more Picea-derived photosynthate than those of greater hyphal development, presumably from preferential fungal colonization of inner root zones by the former genera. Correlations of sporocarp δ¹³C with daily solar radiation integrated for different periods indicated that carbon fluxes from Picea to sporocarps peaked 17–21 days after photosynthesis. Thus, these results provided rough estimates of the spatial extent and temporal lags of carbon transfer from Picea to ectomycorrhizal fungi.

Background: Weather conditions and fungal morphology may influence spatial and temporal dynamics of carbon transfer to ectomycorrhizal fungi from their tree hosts but field studies of these dynamics are challenging.
Methods: Broad-leaved trees in a Swiss forest were labeled with ¹³C-depleted CO₂ from 2001–2005 and ¹³C traced into ectomycorrhizal sporocarps collected at different distances from those trees. We then correlated distance (zone), year, fungal genus, fungal morphology, and solar radiation against sporocarp δ¹³C and log_e C/N in stepwise regressions.
Results: CO₂-supplied trees contributed 71 ± 5%, 32 ± 6%, and 19 ± 7% of sporocarp carbon at 0–6, 6–12, and 12–18 m from these trees, respectively. Fungal morphology (rhizomorph presence and ectomycorrhizae hydrophobicity) was uncorrelated with fungal carbon acquisition. After drought in 2003, CO₂-supplied trees contributed less carbon in 2004 than in other years to sporocarps and sporocarp log_e C/N decreased. In contrast, sporocarp log_e C/N peaked in 2005 and contributions from CO₂-labeled trees to the 6–12 and 12–18 m zones increased. Solar radiation with a 14–21 day lag correlated negatively with log_e C/N. Sporocarp log_e C/N correlated positively with δ¹³C at 0–6 m and negatively elsewhere.
Conclusions: These patterns indicated: 1) fungal morphology did not affect carbon transfers, 2) carbohydrates rather than amino acids were transferred across zones to sporocarps (inferred from spatial correlations of log_eC/N and δ¹³C), 3) sporocarp δ¹³C and log_e C/N reflected the last 2–3 weeks of photosynthesis; 4) summer drought and plant allocation influenced the quantity and spatial extent of belowground transfer.

· Recent droughts have strongly impacted forest ecosystems and are projected to increase in frequency, intensity, and duration in the future together with continued warming. While evidence suggests that tree diversity can regulate drought impacts in natural forests, few studies examine whether mixed tree plantations are more resistant to the impacts of severe droughts.
· Using natural variations in leaf carbon (C) and nitrogen (N) isotopic ratios, that is δ¹³C and δ¹⁵N, as proxies for drought response, we analyzed the effects of tree species richness on the functional responses of tree plantations to the pan-European 2018 summer drought in seven European tree diversity experiments.
· We found that leaf δ¹³C decreased with increasing tree species richness, indicating less drought stress. This effect was not related to drought intensity, nor desiccation tolerance of the tree species. Leaf δ¹⁵N increased with drought intensity, indicating a shift toward more open N cycling as water availability diminishes. Additionally, drought intensity was observed to alter the influence of tree species richness on leaf δ¹⁵N from weakly negative under low drought intensity to weakly positive under high drought intensity.
· Overall, our findings suggest that dual leaf isotope analysis helps understand the interaction between drought, nutrients, and species richness.

Perennially frozen soil, also known as permafrost, is important for the functioning and productivity of most of the boreal forest, the world's largest terrestrial biome. A better understanding of complex vegetation-permafrost interrelationships is needed to predict changes in local- to large-scale carbon, nutrient, and water cycle dynamics under future global warming. Here, we analyze tree-ring width and tree-ring stable isotope (C and O) measurements of Gmelin larch (Larix gmelinii (Rupr.) Rupr.) from six permafrost sites in the northern taiga of central Siberia. Our multi-parameter approach shows that changes in tree growth were predominantly controlled by the air and topsoil temperature and moisture content of the active soil and upper permafrost layers. The observed patterns range from strong growth limitations by early summer temperatures at higher elevations to significant growth controls by precipitation at warmer and well-drained lower-elevation sites. Enhanced radial tree growth is mainly found at sites with fast thawing upper mineral soil layers, and the comparison of tree-ring isotopes over five-year periods with different amounts of summer precipitation indicates that trees can prevent drought stress by accessing water from melted snow and seasonally frozen soil. Identifying the active soil and upper permafrost layers as central water resources for boreal tree growth during dry summers demonstrates the complexity of ecosystem responses to climatic changes.

The Okavango wetland (Botswana) is the world's largest inland delta. A strong seasonality in water input leads to the contraction and extension of wetlands in the floodplains. The extreme evapotranspiration and little precipitation lead to a difference in the hydrogen isotope signature of rain, soil and river water. Biomarkers, such as plant waxes, are stored in the soils and preserved on geological timescales. To understand which signal is preserved in the stable isotope signatures of plant waxes, soils along a 250 m-long transect spanning waterlogged to dry soils were collected over several seasons and three years. In addition, plants, and plant and soil water were collected along this transect. First, carbon isotope ratios (δ¹³C) of plant waxes (i.e, n-fatty acids) were used to classify their metabolism. δ¹³C of bulk organic matter and individual n-fatty acids analyzed in the soils show a strong dependance on the type of vegetation found along the transect (C₃ versus C₄ plants). Hydrogen isotope ratios (δ²H) of water present in soil showed that shallow-rooted C₄ grasses use superficial soil water, whereas the xylem water δ²H content in trees growing near the flooded channel indicated the use of river water. In addition, plant hydrogen fractionation between lipids and rain showed a strong influence of carbon metabolisms with larger fractionation for C₃ plants compared with C₄ grasses. n-fatty acid δ²H ratios in surface soils followed the hydrological variation in the Delta with its floods and dry periods. Hence δ²H of long-chain fatty acids seems to track the river-level variation rather than precipitation.

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.