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Increase of nitrogen depositions, as oxidized (NOx) and reduced (NHx) compounds, has important implications on ecosystem nitrogen cycle. NOx comes predominantly from fossil fuel combustion in car engines and industrial processes, while agricultural activities (cattle farming, fertilizations) are the main sources for NHx emissions. Such fertilisations could stimulate growth and, therefore, productivity of forests, especially in temperate forests, where nitrogen is a limiting factor. On the other hand an excess of nitrogen can lead to an acidification of the soil and have a negative impact on the microbial fauna and structure of plants. NOx and NHx depositions can be separated with the help of stable isotopes with different 15N-values reported for NOx originating from combustion sources. Consequently it was observed that the nitrogen isotopic composition of the vegetation reflects the isotopic signature of nitrogen sources. Our preliminary results on needles of Norway spruce trees exposed to NOx emissions along a transect perpendicular to a highway close to Faido and Brittnau show a clear isotopic enrichment in 15N in trees growing closer to traffic pollution, indicating an uptake of atmospheric nitrogen by stomatal pathway. Carbon and oxygen isotopic composition measured in tree rings reveals the physiological response in plants. Trees more exposed to air pollution from traffic show an increase in δ13C and δ18O in tree rings, since mid sixties next to Erstfeld. This could mean a higher photosynthetic activity, enhanced by NOx traffic emissions, under low or not changed stomatal conductance. Our results confirm that stable isotopes of carbon, oxygen and nitrogen measured in needles and tree rings represent an important tool to monitor the impact of NOx load on tree physiology.

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 CO 2 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 CO 2 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., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

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., Competing Interests: Declaration of competing interest The authors declare no competing interests., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

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 CO 2 and H 2 O 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 CO 2 concentration at the chloroplast surface and stroma. We question the conventional understanding that diffusional limitations of CO 2 contribute to the reduction in photosynthesis at high temperatures. We suggest that stomata and mesophyll membranes could work strategically to facilitate transpiration cooling and CO 2 supply, thus alleviating heat stress on leaf photosynthetic function, albeit at the cost of reduced water-use efficiency., (© 2024 The Authors. New Phytologist © 2024 New Phytologist Foundation.)

The combined study of carbon (C) and oxygen (O) isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional responses to environmental change. The approach relies on established relationships between leaf gas exchange and isotopic fractionation to derive a series of model scenarios that can be used to infer changes in photosynthetic assimilation and stomatal conductance driven by changes in environmental parameters (CO 2 , water availability, air humidity, temperature, nutrients). We review the mechanistic basis for a conceptual model, in light of recently published research, and discuss where isotopic observations do not match our current understanding of plant physiological response to the environment. We demonstrate that (1) the model was applied successfully in many, but not all studies; (2) although originally conceived for leaf isotopes, the model has been applied extensively to tree-ring isotopes in the context of tree physiology and dendrochronology. Where isotopic observations deviate from physiologically plausible conclusions, this mismatch between gas exchange and isotope response provides valuable insights into underlying physiological processes. Overall, we found that isotope responses can be grouped into situations of increasing resource limitation versus higher resource availability. The dual-isotope model helps to interpret plant responses to a multitude of environmental factors., (© 2023 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Boreal regions are changing rapidly with anthropogenic global warming. In order to assess risks and impacts of this process, it is crucial to put these observed changes into a long-term perspective. Summer air temperature variability can be well reconstructed from conifer tree rings. While the application of stable isotopes can potentially provide complementary climatic information over different seasons. In this study, we developed new triple stable isotope chronologies in tree-ring cellulose (δ 13 C trc , δ 18 O trc , δ 2 H trc ) from a study site in Canada. Additionally, we performed regional aggregated analysis of available stable isotope chronologies from 6 conifers' tree species across high-latitudinal (HL) and - altitudinal (HA) as well as Siberian (SIB) transects of the Northern Hemispheric boreal zone. Our results show that summer air temperature still plays an important role in determining tree-ring isotope variability at 11 out of 24 sites for δ 13 C trc , 6 out of 18 sites for δ 18 O trc and 1 out of 6 sites for δ 2 H trc . Precipitation, relative humidity and vapor pressure deficit are significantly and consistently recorded in both δ 13 C trc and δ 18 O trc along HL. Summer sunshine duration is captured by all isotopes, mainly for HL and HA transects, indicating an indirect link with an increase in air and leaf temperature. A mixed temperature-precipitation signal is preserved in δ 13 C trc and δ 18 O trc along SIB transect. The δ 2 H trc data obtained for HL-transect provide information not only about growing seasonal moisture and temperature, but also capture autumn, winter and spring sunshine duration signals. We conclude that a combination of triple stable isotopes in tree-ring studies can provide a comprehensive description of climate variability across the boreal forest zone and improve ecohydrological reconstructions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

Temperature and precipitation changes are crucial for larch trees growing at high-elevation sites covered by permafrost in the Altai-Sayan mountain range (ASMR). To contextualize the amplitude of recent climate fluctuations, we have to look into the past by analyzing millennial paleoclimatic archives recording both temperature and precipitation. We developed annually resolved 1500-year tree-ring cellulose chronologies (δ 13 C cell , δ 18 O cell ), and used these new records to reconstruct the variability in local summer precipitation and air temperature. We combined our new local reconstructions with existing paleoclimatic archives available for the Altai. The data show a strong decreasing trend by ca. 49% in regional summer precipitation, along with a regional summer temperature increase towards the twenty-first century, relative to the preceding 1500 years. Modern dry conditions (1966-2016 CE) in the ASMR are the result of simultaneous summer warming and decreased precipitation. Our new reconstructions also demonstrate that climate change in the ASMR is much stronger compared to the global average., (© 2022. The Author(s).)

Picea abies (L.) Karst. and Fagus sylvatica (L.) are important tree species in Europe, and the foreseen increase in temperature and vapour pressure deficit (VPD) could increase the vulnerability of these species. However, their physiological performance under climate change at temperate and productive sites is not yet fully understood, especially in uneven-aged stands. Therefore, we investigated tree-ring width and stable isotope chronologies (δ13C/δ18O) of these two species at 10 sites along a climate gradient in Central Europe. In these uneven-aged stands, we compared the year-to-year variability of dominant and suppressed trees for the last 80 years in relation to the sites' spatial distribution and climate. δ18O and δ13C were generally consistent across sites and species, showing high sensitivity to summer VPD, whereas climate correlations with radial growth varied much more and depended on mean local climate. We found no significant differences between dominant and suppressed trees in the response of stable isotope ratios to climate variability, especially within the annual high-frequency signals. In addition, we observed a strikingly high coherence of the high-frequency δ18O variations across long distances with significant correlations above 1500 km, whereas the spatial agreement of δ13C variations was weaker (~700 km). We applied a dual-isotope approach that is based on known theoretical understanding of isotope fractionations to translate the observed changes into physiological components, mainly photosynthetic assimilation rate and stomatal conductance. When separating the chronologies in two time windows and investigating the shifts in isotopes ratios, a significant enrichment of either or both isotope ratios over the last decades can be observed. These results, translated by the dual-isotope approach, indicate a general climate-driven decrease in stomatal conductance. This improved understanding of the physiological mechanisms controlling the short-term variation of the isotopic signature will help to define the performance of these tree species under future climate., (© The Author(s) 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Rapid changes in the hydrological and temperature regimes over the past decades at the northern latitudes enhance significantly permafrost degradation accelerating carbon release, increase the frequency of drought events and extensive wildfires. However, the mechanisms and dynamics driving drought events and their influence on Siberian forests are currently the subject of numerous research activities. Newly developed and annually resolved stable carbon and oxygen isotope chronologies of larch tree-ring cellulose (δ 13 C cell and δ 18 O cell ) for the period 516-2009 CE allowed the reconstruction of July precipitation and Arctic Oscillation (AO) in May, respectively. Unprecedented drought events occurred towards twentieth-twenty-first centuries as indicated by the July precipitation reconstruction. Positive AO phases in May were most pronounced during the second part of the first millennium, but also increased in frequency in the modern period of the twentieth-twenty-first centuries. Negative AO phases are associated with cold anomalies and show a remarkable decrease in the nineteenth century caused by a series of major volcanic eruptions. Our findings help explaining the increased frequency of Siberian forest fires over the past decades in Central Siberia consistent with a reduction of summer precipitation, triggered by a positive phase of the Arctic Oscillation in May., (© 2021. The Author(s).)

The carbon isotopic composition (δ 13 C) of foliage is often used as proxy for plant performance. However, the effect of N O 3 - vs. N H 4 + supply on δ 13 C of leaf metabolites and respired CO 2 is largely unknown. We supplied tobacco plants with a gradient of N O 3 - to N H 4 + concentration ratios and determined gas exchange variables, concentrations and δ 13 C of tricarboxylic acid (TCA) cycle intermediates, δ 13 C of dark-respired CO 2 , and activities of key enzymes nitrate reductase, malic enzyme and phosphoenolpyruvate carboxylase. Net assimilation rate, dry biomass and concentrations of organic acids and starch decreased along the gradient. In contrast, respiration rates, concentrations of intercellular CO 2 , soluble sugars and amino acids increased. As N O 3 - decreased, activities of all measured enzymes decreased. δ 13 C of CO 2 and organic acids closely co-varied and were more positive under N O 3 - supply, suggesting organic acids as potential substrates for respiration. Together with estimates of intra-molecular 13 C enrichment in malate, we conclude that a change in the anaplerotic reaction of the TCA cycle possibly contributes to 13 C enrichment in organic acids and respired CO 2 under N O 3 - supply. Thus, the effect of N O 3 - vs. N H 4 + on δ 13 C is highly relevant, particularly if δ 13 C of leaf metabolites or respiration is used as proxy for plant performance.

The carbon and oxygen isotopic composition of water and assimilates in plants reveals valuable information on plant responses to climatic conditions. Yet, the carbon and oxygen uptake, incorporation and allocation processes determining isotopic compositions are not fully understood. We carried out a dual-isotope labeling experiment at high humidity with 18O-enriched water (H218O) and 13C-enriched CO2 (13CO2) with attached Scots pine (Pinus sylvestris L.) branches and detached twigs of hemiparasitic mistletoes (Viscum album ssp. austriacum) in a naturally dry coniferous forest, where also a long-term irrigation takes place. After 4 h of label exposure, we sampled previous- and recent-year leaves, twig phloem and twig xylem over 192 h for the analysis of isotope ratios in water and assimilates. For both species, the uptake into leaf water and the incorporation of the 18O-label into leaf assimilates was not influenced by soil moisture, while the 13C-label incorporation into assimilates was significantly higher under irrigation compared with control dry conditions. Species-specific differences in leaf morphology or needle age did not affect 18O-label uptake into leaf water, but the incorporation of both tracers into assimilates was two times lower in mistletoe than in pine. The 18O-label allocation in water from pine needles to twig tissues was two times higher for phloem than for xylem under both soil moisture conditions. In contrast, the allocation of both tracers in pine assimilates were similar and not affected by soil moisture, twig tissue or needle age. Soil moisture effects on 13C-label but not on 18O-label incorporation into assimilates can be explained by the stomatal responses at high humidity, non-stomatal pathways for water and isotope exchange reactions. Our results suggest that non-photosynthetic 18O-incorporation processes may have masked prevalent photosynthetic processes. Thus, isotopic variation in leaf water could also be imprinted on assimilates when photosynthetic assimilation rates are low., (© The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Newly developed millennial δ 13 C larch tree-ring chronology from Siberia allows reconstruction of summer (July) vapor pressure deficit (VPD) changes in a temperature-limited environment. VPD increased recently, but does not yet exceed the maximum values reconstructed during the Medieval Warm Anomaly. The most humid conditions in the Siberian North were recorded in the Early Medieval Period and during the Little Ice Age. Increasing VPD under elevated air temperature affects the hydrology of these sensitive ecosystems by greater evapotranspiration rates. Further VPD increases will significantly affect Siberian forests most likely leading to drought and forest mortality even under additional access of thawed permafrost water. Adaptation strategies are needed for Siberian forest ecosystems to protect them in a warming world.

Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD). VPD has been identified as an increasingly important driver of plant functioning in terrestrial biomes and has been established as a major contributor in recent drought-induced plant mortality independent of other drivers associated with climate change. Despite this, few studies have isolated the physiological response of plant functioning to high VPD, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. An abundance of evidence suggests that stomatal conductance declines under high VPD and transpiration increases in most species up until a given VPD threshold, leading to a cascade of subsequent impacts including reduced photosynthesis and growth, and higher risks of carbon starvation and hydraulic failure. Incorporation of photosynthetic and hydraulic traits in 'next-generation' land-surface models has the greatest potential for improved prediction of VPD responses at the plant- and global-scale, and will yield more mechanistic simulations of plant responses to a changing climate. By providing a fully integrated framework and evaluation of the impacts of high VPD on plant function, improvements in forecasting and long-term projections of climate impacts can be made., (© 2020 The Authors. New Phytologist © 2020 New Phytologist Trust.)

The 18 O signature of atmospheric water vapour (δ 18 O V ) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the 18 O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with 18 O-depleted water vapour (-106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the 18 O-signal transfer by calculating the mean residence time of O in leaf water (MRT LW ) and sugars (MRT Sugars ) and related it to leaf traits and physiological drivers. MRT LW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRT Sugars was shorter in C 3 and C 4 plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRT Sugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ 18 O V variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ 18 O V variations, which is important for the interpretation of δ 18 O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses., (© 2019 John Wiley & Sons Ltd.)

Stomata control the gas exchange of terrestrial plant leaves, and are therefore essential to plant growth and survival. We investigated gas exchange responses to vapor pressure deficit (VPD) in two gray poplar ( Populus × canescens ) lines: wild type and abscisic acid-insensitive ( abi1 ) with functionally impaired stomata. Transpiration rate in abi1 increased linearly with VPD, up to about 2 kPa. Above this, sharply declining transpiration was followed by leaf death. In contrast, wild type showed a steady or slightly declining transpiration rate up to VPD of nearly 7 kPa, and fully recovered photosynthetic function afterward. There were marked differences in discrimination against 13 CO 2 (Δ 13 C) and C 18 OO (Δ 18 O) between abi1 and wild-type plants. The Δ 13 C indicated that intercellular CO 2 concentrations decreased with VPD in wild-type plants, but not in abi1 plants. The Δ 18 O reflected progressive stomatal closure in wild type in response to increasing VPD; however, in abi1 , stomata remained open and oxygen atoms of CO 2 continued to exchange with 18 O enriched leaf water. Coupled measurements of Δ 18 O and gas exchange were used to estimate intercellular vapor pressure, e i In wild-type leaves, there was no evidence of unsaturation of e i , even at VPD above 6 kPa. In abi1 leaves, e i approached 0.6 times saturation vapor pressure before the precipitous decline in transpiration rate. For wild type, a sensitive stomatal response to increasing VPD was pivotal in preventing unsaturation of e i In abi1 , after taking unsaturation into account, stomatal conductance increased with increasing VPD, consistent with a disabled active response of guard cell osmotic pressure., (© 2019 American Society of Plant Biologists. All Rights Reserved.)

Elevated CO 2 along with rising temperature and water deficits can lead to changes in tree physiology and leaf biochemistry. These changes can increase heat- and drought-induced tree mortality. We aim to reveal the impacts of climatic drivers on individual compounds at the leaf level among European larch (Larix decidua) and mountain pine (Pinus mugo) trees, which are widely distributed at high elevations. We investigated seasonal carbon isotope composition (δ 13 C) and concentration patterns of carbohydrates and organic acids in needles of these two different species from a case study in the Swiss National Park (SNP). We found that average and minimum air temperatures were the main climatic drivers of seasonal variation of δ 13 C in sucrose and glucose as well as in concentrations of carbohydrates and citric acid/citrate in needles of both tree species. The impact of seasonal climatic drivers on larch and mountain pine trees at the needle level is in line with our earlier study in this region for long-term changes at the tree-ring level. We conclude that the species-specific changes in δ 13 C and concentrations of carbohydrates and organic acids are sensitive indicators of changes in the metabolic pathways occurring as a result of climatic changes., (Copyright © 2019 Elsevier Masson SAS. All rights reserved.)

Measurements of methane ( CH 4 ) mole fractions and δ 13 C - CH 4 that resolve the diel cycle in the agriculturally dominated Reuss Valley, Switzerland, were used to quantify the contributions of different CH 4 sources to the atmospheric CH 4 source mix. Both a nocturnal (NBL) and a diurnal convective boundary layer (CBL) approach were employed. A diel course of CH 4 mole fractions was found with a daytime minimum (background around 1900 ppb) and a nocturnal maximum (up to 3500 ppb). The δ 13 C value in CH 4 only showed small variations during the day (9-21 hours CET, - 45.0 ± 0.2 ‰ mean ± SE ) when the atmosphere was well mixed, but decreased by - 4.8 ± 0.1 ‰ during the night. Biogenic emissions dominated in both approaches (ranging from 60 to 94%), but non-biogenic sources were rather important (42.2% and 46.0% with CBL, 5.8% and 40% with NBL approach in 2011 and 2012, respectively, of total emissions). The CH 4 sink, dominated by tropospheric OH oxidation and only to a minor extend by soil surface uptake, was quantified at roughly 4% of local emissions.

Stable isotope ratios in tree rings have become an important proxy for palaeoclimatology, particularly in temperate regions. Yet temperate forests are often characterized by heterogeneous stand structures, and the effects of stand dynamics on carbon (δ 13 C) and oxygen isotope ratios (δ 18 O) in tree rings are not well explored. In this study, we investigated long-term trends and offsets in δ 18 O and δ 13 C of Picea abies and Fagus sylvatica in relation to tree age, size, and distance to the upper canopy at seven temperate sites across Europe. We observed strong positive trends in δ 13 C that are best explained by the reconstructed dynamics of individual trees below the upper canopy, highlighting the influence of light attenuation on δ 13 C in shade-tolerant species. We also detected positive trends in δ 18 O with increasing tree size. However, the observed slopes are less steep and consistent between trees of different ages and thus can be more easily addressed. We recommend restricting the use of δ 13 C to years when trees are in a dominant canopy position to infer long-term climate signals in δ 13 C when relying on material from shade-tolerant species, such as beech and spruce. For such species, δ 18 O should be in principle the superior proxy for climate reconstructions., (© 2018 John Wiley & Sons Ltd.)

Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two- and four-species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L. glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant-soil-atmosphere continuum. We measured gas-exchange variables (C assimilation, transpiration and above- and belowground respiration) and δ 13 C in leaf biomass, phloem, soil microbial biomass, leaf- and soil-respired CO 2 as well as δ 18 O in leaf and xylem water. The 13 C signal in phloem and respired CO 2 in L. glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf-water 18 O enrichment above the xylem water in L. glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water-use efficiency. However, these increased C resources did not translate in higher growth of L. glaber in monoculture suggesting the existence of larger nongrowth-related C sinks in monoculture. These ecophysiological responses of L. glaber , in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L. glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture.

Time series of tree-ring growth show significant increases since the early 1970s at the alpine tree line, with simultaneously increasing temperatures and atmospheric CO2 concentration. For a comprehensive understanding of this growth change, the physiological response patterns at both the leaf and stem level need to be separately analyzed and identified, and can be retrieved from tree-ring growth and isotope (δ13C, δ18O) series. In this study, we assessed the relative contribution of environmental factors to interannual tree-ring variability by multivariate linear mixed-effects models and the dual isotope approach on a dataset of tree-ring records of ~400-year-old larch (Larix decidua Mill.) from a non-water-limited high-elevation site in the Swiss Alps. The models suggest that summer temperatures and the recent lack of larch budmoth outbreaks were most important for explaining growth variations and trends, while a significant direct effect of the continuously increasing CO2 concentration could not be confirmed. In contrast, δ13C and δ18O, which are strongly influenced by fractionation changes in the leaf, clearly reflected the impact of air humidity (precipitation and vapor pressure deficit) and CO2 concentration: the increase in (δ13C-derived) intrinsic water-use efficiency over the second half of the 20th century suggests an increase in carbon assimilation as a result of enhanced CO2 concentration. The tree-ring δ18O largely reflected recent precipitation as source water, thus indicating a low variability in stomatal conductance, which was confirmed by the dual isotope approach. These leaf-level effects were not reflected in stem growth as they may have been masked by the temperature-caused growth limitation controlling the allocation of increased amounts of photosynthates into wood growth. Our approach demonstrates that the identification of different roles of environmental factors on leaf and stem processes helps to improve the assessment of site-specific changes of carbon fluxes and growth performance under future environmental conditions.

Drought is a major environmental constraint to trees, causing severe stress and thus adversely affecting their functional integrity. European beech ( Fagus sylvatica L.) is a key species in mesic forests that is commonly expected to suffer in a future climate with more intense and frequent droughts. Here, we assessed the seasonal response of leaf physiological characteristics of beech saplings to drought and drought release to investigate their potential to recover from the imposed stress and overcome previous limitations. Saplings were transplanted to model ecosystems and exposed to a simulated summer drought. Pre-dawn water potentials (ψ pd ), stomatal conductance ( g S ), intercellular CO 2 concentration ( c i ), net-photosynthesis ( A N ), PSII chlorophyll fluorescence ( PI tot ), non-structural carbohydrate concentrations ( NSC ; soluble sugars, starch) and carbon isotope signatures were measured in leaves throughout the growing season. Pre-dawn water potentials (ψ pd ), g S , c i , A N , and PI tot decreased as drought progressed, and the concentration of soluble sugars increased at the expense of starch. Carbon isotopes in soluble sugars (δ 13 C S ) showed a distinct increase under drought, suggesting, together with decreased c i , stomatal limitation of A N . Drought effects on ψ pd , c i , and NSC disappeared shortly after re-watering, while full recovery of g S , A N , and PI tot was delayed by 1 week. The fast recovery of NSC was reflected by a rapid decay of the drought signal in δ 13 C values, indicating a rapid turnover of assimilates and a reactivation of carbon metabolism. After recovery, the previously drought-exposed saplings showed a stimulation of A N and a trend toward elevated starch concentrations, which counteracted the previous drought limitations. Overall, our results suggest that the internal water relations of beech saplings and the physiological activity of leaves are restored rapidly after drought release. In the case of A N , stimulation after drought may partially compensate for limitations on photosynthetic activity during drought. Our observations suggest high resilience of beech to drought, contradicting the general belief that beech is particularly sensitive to environmental stressors.

Our understanding of how temporal variations of atmospheric water vapour and its isotopic composition (δ 18 O V ) influence water and assimilates in plants remains limited, restricting our ability to use δ 18 O as a tracer of ecophysiological processes. We exposed oak (Quercus robur) saplings under wet and dry soil moisture conditions to 18 O-depleted water vapour (c. - 200‰) at high relative humidity (c. 93%) for 5 h, simulating a fog event. We then traced the step change in δ 18 O V into water and assimilates (e.g. sucrose, hexoses, quercitol and starch) in the leaf lamina, main veins and twigs over 24 h. The immediate δ 18 O V effect was highest for δ 18 O of leaf lamina water, but 40% lower on δ 18 O of main vein water. To a smaller extent, we also observed changes in δ 18 O of twig xylem water. Depending on the individual assimilation rate of each plant, the 18 O-label was partitioned among different assimilates, with highest changes in δ 18 O of starch/sucrose and lowest in δ 18 O of quercitol. Additionally, 18 O-label partitioning and allocation towards leaf starch and twig phloem sugars was influenced by the plant water status. Our results have important implications for water isotope heterogeneity in plants and for our understanding of how the δ 18 O signal is incorporated into biomarkers., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Rationale: Levoglucosan is formed from cellulose during biomass burning. It is therefore often used as a specific tracer to quantify the contribution of wood burning to the aerosol loading. The stable oxygen isotope composition (δ 18 O value) of biomass is determined by the water cycle and varies regionally, and hence the δ 18 O value of levoglucosan could help to identify source regions of organic aerosols., Methods: After solvent extraction of the organic fraction and concentration steps, a recently developed methylation derivatisation technique was applied on experimental (i.e. controlled wood-burning experiments) and on ambient aerosol samples from Switzerland and Lithuania. The method achieves sufficient compound separation for isotope analysis in atmospheric particulate matter, enabling δ 18 O analysis of levoglucosan by gas chromatography/pyrolysis-isotope ratio mass spectrometry (GC/Pyr-IRMS), with a precision better than 1.0 ‰ and an accuracy of 0.3 ‰., Results: The δ 18 O value of the levoglucosan released during controlled wood-burning experiments was not significantly different from the cellulose δ 18 O values, which implies very little or no isotope fractionation during wood burning under the given conditions. While the δ 18 O values of levoglucosan in Swiss samples were as expected for the source region, those in Lithuania were 1-4 ‰ lower than expected. This may be due to differences in vegetation (grass vs wood) or burning conditions (high vs low temperatures)., Conclusions: Low oxygen isotope fractionation between cellulose and levoglucosan and clear differences in levoglucosan δ 18 O values between the Swiss and Lithuanian ambient samples demonstrate that our new method is useful for source appointment studies on wood-burning-derived aerosols., (Copyright © 2017 John Wiley & Sons, Ltd.)

Rationale: In this study, we tested stable hydrogen isotope ratios of wood lignin methoxyl groups (δ 2 H methoxyl values) as a palaeoclimate proxy in dendrochronology. This is a quite new method in the field of dendrochronology and the sample preparation is much simpler than the methods used before to measure δ 2 H values from wood., Methods: We measured δ 2 H methoxyl values in high elevation larch trees (Larix decidua Mill.) from Simplon Valley (southern Switzerland). Thirty-seven larch trees were sampled and five individuals analysed for their δ 2 H methoxyl values at annual (1971-2009) and pentadal resolution (1746-2009). The δ 2 H methoxyl values were measured as CH 3 I released upon treatment of the dried wood samples with hydroiodic acid. 10-90 μL from the head-space were injected into the gas chromatography/high-temperature conversion/isotope ratio mass spectrometry (GC/HTC-IRMS) system., Results: Testing the climate response of the δ 2 H methoxyl values, the annually resolved series show a positive correlation of r = 0.60 with June/July precipitation. The pentadally resolved δ 2 H methoxyl series do not show any significant correlation to climate parameters., Conclusions: Increased precipitation during June and July, which are on average warm and relatively dry months, results in higher δ 2 H values of the xylem water and, therefore, higher δ 2 H values in the lignin methoxyl groups. Therefore, we suggest that δ 2 H methoxyl values of high elevation larch trees might serve as a summer precipitation proxy., (Copyright © 2017 John Wiley & Sons, Ltd.)

Adjustment mechanisms of trees to changes in soil-water availability over long periods are poorly understood, but crucial to improve estimates of forest development in a changing climate. We compared mature trees of Scots pine (Pinus sylvestris) and European larch (Larix decidua) growing along water-permeable channels (irrigated) and under natural conditions (control) at three sites in inner-Alpine dry valleys. At two sites, the irrigation had been stopped in the 1980s. We combined measurements of basal area increment (BAI), tree height and gas-exchange physiology (Δ 13 C) for the period 1970-2009. At one site, the Δ 13 C of irrigated pine trees was higher than that of the control in all years, while at the other sites, it differed in pine and larch only in years with dry climatic conditions. During the first decade after the sudden change in water availability, the BAI and Δ 13 C of originally irrigated pine and larch trees decreased instantly, but subsequently reached higher levels than those of the control by 2009 (15 years afterwards). We found a high plasticity in the gas-exchange physiology of pine and larch and site-specific responses to changes in water availability. Our study highlights the ability of trees to adjust to new conditions, thus showing high resilience., (© 2017 John Wiley & Sons Ltd.)

A growing number of studies have described the direct absorption of water into leaves, a phenomenon known as foliar water uptake. The resultant increase in the amount of water in the leaf can be important for plant function. Exposing leaves to isotopically enriched or depleted water sources has become a common method for establishing whether or not a plant is capable of carrying out foliar water uptake. However, a careful inspection of our understanding of the fluxes of water isotopes between leaves and the atmosphere under high humidity conditions shows that there can clearly be isotopic exchange between the two pools even in the absence of a change in the mass of water in the leaf. We provide experimental evidence that while leaf water isotope ratios may change following exposure to a fog event using water with a depleted oxygen isotope ratio, leaf mass only changes when leaves are experiencing a water deficit that creates a driving gradient for the uptake of water by the leaf. Studies that rely on stable isotopes of water as a means of studying plant water use, particularly with respect to foliar water uptake, must consider the effects of these isotopic exchange processes.

Almost no δ 18 O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ 18 O relationship between leaf water and cellulose. We measured δ 18 O values of bulk leaf water (δ 18 O LW ) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ 18 O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally 18 O-enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε bio ) of more than 27‰ relative to δ 18 O LW , which might be explained by isotopic leaf water and sucrose synthesis gradients. δ 18 O LW and δ 18 O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε bio  = 25.1‰). Interestingly, damping factor p ex p x , which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ 18 O LW but not if modelled directly from δ 18 O of individual carbohydrates. We conclude that δ 18 O LW is not always a good substitute for δ 18 O of synthesis water due to isotopic leaf water gradients. Thus, compound-specific δ 18 O analyses of individual carbohydrates are helpful to better constrain (post-)photosynthetic isotope fractionation processes in plants., (© 2017 John Wiley & Sons Ltd.)

We analyzed the chronologies of cellulose stable isotopes (δ 13 C and δ 18 O) and tree-ring widths from European larch (Larix decidua) in a high-altitude site (2190ma.s.l.) at the bottom of a glacial valley in the Italian Alps, and investigated their dependence on monthly meteorological variables and δ 18 O precipitation values. The δ 18 O of tree-ring cellulose appears to be strongly driven by the δ 18 O of winter snowfall (November to March), which suggests that larch trees mostly use the snow-melt water of the previous winter during the growing season. This water, which also comes from the slope streams and from the underground flow of nearby steep slopes, infiltrates the soil in the valley bottom. The tree-ring cellulose δ 18 O values were also found to be influenced by the August precipitation δ 18 O and mean temperature. The associated regression model shows that the δ 18 O chronology from the tree rings explains up to 34% of the variance in the winter precipitation δ 18 O record, demonstrating the potential for reconstructing the δ 18 O isotopic composition of past winter precipitation in the study region. Unlike most other tree-ring studies that focus on growing season signals, in our study the summer signal was small and the winter signal dominant due to the special conditions of the glacial valley. Site topography, geomorphology and soil characteristics in particular influence the stable isotope signal in tree-ring cellulose., (Copyright © 2016 Elsevier B.V. All rights reserved.)

How will carbon source-sink relations of 35-yr-old larch trees (Larix decidua) at the alpine treeline respond to changes in atmospheric CO(2) and climate? We evaluated the effects of previously elevated CO(2) concentrations (9 yr, 580 ppm, ended the previous season) and ongoing soil warming (4 yr, + 4°C). Larch branches were pulse labeled (50 at% (13)CO(2)) in July 2010 to trace fresh assimilates through tissues (buds, needles, bark and wood) and non-structural carbon compounds (NCC; starch, lipids, individual sugars) using compound-specific isotope analysis. Nine years of elevated CO(2) did not lead to increased NCC concentrations, nor did soil warming increase NCC transfer velocities. By contrast, we found slower transfer velocities and higher NCC concentrations than reported in the literature for lowland larch. As a result of low dilution with older carbon, sucrose and glucose showed the highest maximum (13)C labels, whereas labels were lower for starch, lipids and pinitol. Label residence times in needles were shorter for sucrose and starch (c. 2 d) than for glucose (c. 6 d). Although our treatments showed no persistent effect on larch carbon relations, low temperature at high altitudes clearly induced a limitation of sink activities (growth, respiration, root exudation), expressed in slower carbon transfer and higher NCC concentrations., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

•  The effects of nitrogen dioxide (NO 2 ) exposure are reported on the physiology, morphology and carbon partitioning of hybrid poplar clone cuttings (Populus ×euramericana) grown under high and low soil nitrogen supply. •  Plants were exposed to filtered air or NO 2 -enriched air (80-135 nl l -1 ) over 12 wk growth in phytotrons. Stable isotope analysis, combined with CO 2 and H 2 O gas exchange measurements, biomass analysis and morphological development, was used to assess the integrated long-term effects of NO 2 . •  NO 2 had no toxic effects. A reduced 15 N-isotope ratio indicated incorporation of NO 2 while nitrate reductase activity in leaves was stimulated. The two nitrogen sources had differential effects on water use efficiency (WUE): NO 2 exposure increased long-term WUE; soil N supply decreased WUE; a result not detectable using growth and short-term gas exchange experiments. Plants benefited from airborne NO 2 , increasing CO 2 assimilation rate and biomass; both N sources increased shoot production at the expense of root growth. NO 2 exposure induced leaf formation with reduced stomatal density and increased leaf area. •  NO 2 exposure might be beneficial although the reduced root: shoot biomass could have a detrimental effect on nutrient balance and drought resistance.