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Classical tree-ring parameters like tree-ring width, maximum latewood density and cell wall thickness record summer air temperature at high latitudes well and allow for the reconstruction of summer temperature over centuries and millennia. However, information about other climatic factors (e.g. moisture and sunshine duration) is limited in these proxies, especially for temperature-limited environments in subarctic forests growing on permafrost soils. The application of stable hydrogen and oxygen isotopes can provide complementary information about moisture changes and solar irradiation.Solar irradiation and its seasonal distribution play an important role for trees in terms of photosynthesis and carbohydrate production, which can influence deuterium variations in organic matter in addition to hydrological processes. We found that hydrogen and oxygen isotopes in larch tree-ring cellulose from Siberia show a strong link with summer sunshine duration during the growth season. Negative correlations with sunshine duration during autumn of the previous year and winter of the current year can be related to the lack of light during polar nights at subarctic sites (> 60º N) and lack of needles in larch trees to produce photosynthates in a cold environment. Indirect effects might also arise as a high amount of winter sunshine means persistent high-pressure systems with low precipitation, therefore negatively affecting snow accumulation, which is an important water source after snow melt.Uptake of winter precipitation is possible in the warming spring and summer months after snowmelt and thawing of the active soil layer, resulting in significant correlations between tree-ring oxygen values and winter-spring air temperatures. Oxygen isotopes in organic matter are influenced by variations in the isotopic composition of source water, which is closely related to that of precipitation and soil water (though modified by evaporation at the soil surface).Furthermore, we observed that summer vapor pressure deficit is positively and significantly recorded in stable oxygen isotopes from northeastern Siberia, while continuously represented through spring-summer months in western Canada. Hydrogen tree-ring isotopes from the Canadian subarctic recorded a negative significant correlation with summer relative humidity, opposite to northeastern Siberia.The application of dual hydrogen and oxygen stable isotopes in tree rings can expand our knowledge beyond traditional summer air temperature reconstructions and will help to improve climate reconstructions over the past millennia.Acknowledgements: This work was supported by the project RSF 21-17-00006.

Background To assess the extent of belowground carbon transfer to ectomycorrhizal fungi in natural forests, we used δ13C and loge C/N measurements to calculate spatial dynamics of carbon movement into ectomycorrhizal sporocarps. Methods Fourteen broad-leaved trees were labeled with 13C-depleted CO2 from 2001–2005 in Switzerland and 13C traced into ectomycorrhizal sporocarps collected at different distances. We then used stepwise regressions on patterns of δ13C and loge C/N in ectomycorrhizal sporocarps as a function of distance (zone), solar radiation, fungal genus, and association type. Results CO2-labeled trees contributed 76 ± 5%, 36 ± 6%, and 19 ± 7% of sporocarp carbon at 0–6 m, 6–12 m, and 12–18 m from labeled trees, respectively. Literature estimates of hyphal development in different taxa did not correlate with carbon acquisition patterns. After drought in 2003, sporocarp loge C/N was low in 2004 and 13C-depleted carbon from elevated CO2 trees contributed less than in other years to sporocarps. In contrast, sporocarp loge C/N peaked in 2005 and contributions from elevated CO2 trees to the 6–12 m zone increased. Therefore, carbohydrate transport belowground decreased in 2004, reflecting plant allocation priorities, and increased in 2005. Sporocarp loge C/N varied less among years under elevated CO2 than elsewhere. Conclusions These patterns indicated that 1) belowground transport was influenced by climate and plant allocation, 2) root transport rather than ectomycorrhizal transport drove carbon spatial dynamics of ectomycorrhizal fungi, and 3) elevated CO2 decreased the sensitivity of belowground allocation to climatic fluctuations, suggesting improved drought resistance in a high-CO2 world.

The combined study of C and 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 (CO2, water availability, air humid-ity, temperature, nutrients). We review the mechanistic basis for a conceptual model, in light of recently published research, and discuss where isotopic observations don’t match our current understanding of plant physiological response to environment. We demonstrate that 1) the mod-el was applied successfully in many, but not all studies, 2), while 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 plau-sible conclusions, this mismatch between gas-exchange and isotope response provides valuable insights on 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.

Background. The spatial extent and timing of carbon fluxes from mature trees to ectomycorrhizal fungi associated with different hosts is challenging to assess in natural forests but could provide insights into carbon dynamics of fungi differing in exploration capabilities. Methods. We analyzed carbon movement into ectomycorrhizal sporocarps at the Swiss Forest Free-Air CO2 Enrichment (FACE) site in 2010 and 2011 during continuous labeling of five mature Picea abies (L.) H. Karst. with 13C-depleted CO2. Sporocarps were collected 0-6 m, 6-12 m, 12-18 m, and > 18 m from 13C-labeled trees. We then used stepwise regressions on patterns of 13C:12C ratios (δ13C) in ectomycorrhizal sporocarps as a function of distance (zone), solar radiation, fungal genus, and association type. Results. 18%, 10%, 40%, and 32% of ectomycorrhizal sporocarps were associated with conifers, broad-leaved trees, both, or of unknown association, respectively. Conifer-associated sporocarps derived 58 ± 7%, 41 ± 10%, and 20 ± 7% of their carbon from labeled trees in the 0-6 m, 6-12 m and 12-18 m zones, respectively, whereas other sporocarps in the 0-6 m zone only derived 34 ± 4% of their carbon from labeled trees. Sporocarp δ13C correlated positively with solar radiation for the 16-19 days prior to sporocarp harvest. Conclusions. These patterns indicated that (1) carbon spread further through conifer-associated mycorrhizal networks than through other fungal networks, and (2) carbon accumulation, storage, and transfer dynamics for sporocarp formation can take several weeks. Mature conifers supplied carbon to mycorrhizal fungi up to ~15 m from host trees.

∙ To assess how belowground mycorrhizal networks may share resources, we used δ13C, δ15N, and C/N measurements to calculate spatial and temporal dynamics of carbohydrate and amino acid movement through ectomycorrhizal networks of mature trees.∙ Canopies of 14 deciduous trees were continuously labeled with 13C-depleted CO2 from 2001-2005 (Swiss Forest FACE) and the 13C label traced into ectomycorrhizal sporocarps.∙ Sporocarps derived 69±5%, 30±6%, and 16±7% of their carbon from labeled trees in the elevated (beneath labeled trees), 0-6 m, and 6-12 m distances, respectively. Sporocarp δ13C correlated positively with C/N under elevated CO2 and negatively elsewhere, reflecting that high-δ13C carbohydrates from surrounding trees contributed to sporocarps under elevated CO2 and low-δ13C carbohydrates from elevated CO2 trees contributed to sporocarps elsewhere. Sporocarp δ15N increased in Cortinarius with decreasing δ13C, suggesting that greater hyphal growth with elevated CO2 sequestered 15N-depleted N from sporocarp formation. Sporocarp loge C/N decreased during the 2004 growing season and the contribution of 13C-depleted carbon from elevated CO2 plants decreased at the 0-6 m and 6-12 m distances, suggesting decreased carbohydrate availability and network transport that year. In contrast, sporocarp loge C/N increased during the 2005 growing season and the contribution of 13C-depleted carbon from elevated CO2 plants increased at the 0-6 m distance, suggesting increased carbohydrate availability and network transport that year. Relative to other taxa, elevated CO2 reduced C/N by 15% and ambient CO2 increased C/N by 5% in taxa exclusively associated with deciduous trees, suggesting increased carbohydrate sharing by the deciduous-associated taxa.∙ These patterns indicated that 1) carbohydrates (high C/N), not amino acids (low C/N), were preferentially transferred between regions differing in source δ13C, 2) sporocarp C/N reflected yearly plant productivity, 3) network transport was influenced by climate, and 4) taxonomy influenced transport dynamics belowground.

The combined study of C and O isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional response 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 draw inferences about changes in photosynthetic assimi-lation and stomatal conductance driven by changes in environmental parameters (CO2, water availability, air humidity, temperature, nutrients). We review the mechanistic basis for model and research to date, and discuss where isotopic observations don’t match our current under-standing of plant physiological response to environment. We demonstrate that 1) the model has been applied successfully in many, but not all studies, and 2), while originally conceived for leaf isotopes, the model has been applied extensively to tree ring isotopes in the context of tree phys-iology and dendrochronology. Where isotopic observations deviate from physiologically plau-sible conclusions, this mismatch between gas-exchange and isotope response provides valuable insights on underlying physiological processes. Overall, we found that isotope responses can be grouped into situations of increasing resource limitation versus higher resource availability. Thus, the dual isotope model helps to interpret plant responses to a multitude of environmental factors.

Carbon reserves are important for maintaining tree function during and after stress. Increasing tree mortality driven by drought globally has renewed the interest in how plants regulate allocation of recently fixed C to reserve formation. Three-year-old seedlings of two species (Tilia platyphyllos and Pinus sylvestris) were exposed to two intensities of experimental drought during ~10 weeks, and

Measurements of the stable carbon isotope ratio (δ13C) on annual tree rings offer new opportunities to evaluate mechanisms of variations in photosynthesis and stomatal conductance under changing CO2 and climate, especially in conjunction with process-based biogeochemical model simulations. The isotopic discrimination is indicative of the ratio between the CO2 partial pressure in the intercellular cavities and the atmosphere (ci / ca) and of the ratio of assimilation to stomatal conductance, termed intrinsic water-use efficiency (iWUE). We performed isotope-enabled simulations over the industrial period with the land biosphere module (CLM4.5) of the Community Earth System Model and the LPX-Bern dynamic global vegetation model. Results for C3 tree species show good agreement with a global compilation of δ13C measurements on leaves, though modeled 13C discrimination by C3 trees is smaller in arid regions than measured. A compilation of seventy-six tree-ring records, mainly from Europe, boreal Asia, and western North America, suggest on average small 20th-century changes in isotopic discrimination and an increase in iWUE of about 27 % since 1900. LPX-Bern results match these century-scale reconstructions, supporting the idea that the physiology of stomata has evolved to optimize trade-offs between carbon gain by assimilation and water loss. In contrast, CLM4.5 simulates an increase in discrimination and in turn a change in iWUE that is almost twice as large as revealed by the tree-ring data. Factorial simulations show that these changes are mainly in response to rising atmospheric CO2. The results suggest that the down-regulation of ci / ca and of photosynthesis by nitrogen limitation is possibly too strong in the standard setup of CLM4.5 or there may be more fundamental problems associated with the prescribed relationship between conductance and assimilation.

The presence of vegetation alters hydrological cycles of ecosystems. Complex plant – soil interactions govern the fate of precipitation input and water transitions through ecosystem compartments. Disentangling these interactions is a major challenge in the field of ecohydrology and pivotal foundation for understanding the carbon cycle of semi – arid ecosystems. Stable water isotopes can be used in this context as tracer to quantify water movement through soil – vegetation – atmosphere interfaces. The aim of this study is to disentangle vegetation effects on soil water infiltration and distribution as well as dynamics of soil evaporation and grassland water – use in a Mediterranean cork – oak woodland during dry conditions. An irrigation experiment using δ18O labeled water was carried out in order to quantify distinct effects of tree and herbaceous vegetation on infiltration and distribution of event water in the soil profile. Dynamic responses of soil and herbaceous vegetation fluxes to precipitation regarding event water – use, water uptake depth plasticity and contribution to ecosystem evapotranspiration were quantified. Total water loss to the atmosphere from bare soil was as high as from vegetated soil, utilizing large amounts of unproductive water loss for biomass production, carbon sequestration and nitrogen fixation. During the experiment no adjustments of main root water uptake depth to changes of water availability could be observed, rendering light to medium precipitation events under dry conditions useless. This forces understory plants to compete with adjacent trees for soil water in deeper soil layers. Thus understory plants are faster subject to chronic drought, leading to premature senescence at the onset of drought. Despite this water competition, the presence of Cork oak trees fosters infiltration to large degrees. That reduces drought stress, caused by evapotranspiration, due to favourable micro climatic conditions under tree crown shading. This study highlights complex soil – plant – atmosphere and inter – species interactions in both space and time controlling the fate of rain pulse transitions through a typical Mediterranean savannah ecosystem, disentangled by the use of stable water isotopes.

The Péclet correction is often used to predict leaf evaporative enrichment and requires an estimate of effective path length (L). Studies to estimate L in conifer needles have produced unexpected patterns based on Péclet theory and leaf anatomy. We exposed seedlings of six conifer species to different vapour pressure deficits (VPD) in controlled climate chambers to produce steady-state leaf water enrichment (in (18) O). We measured leaf gas exchange, stable oxygen isotopic composition (δ(18) O) of input and plant waters as well as leaf anatomical characteristics. Variation in bulk needle water δ(18) O was strongly related to VPD. Conifer needles had large amounts of water within the vascular strand that was potentially unenriched (up to 40%). Both standard Craig-Gordon and Péclet models failed to accurately predict conifer leaf water δ(18) O without taking into consideration the unenriched water in the vascular strand and variable L. Although L was linearly related to mesophyll thickness, large within-species variation prevented the development of generalizations that could allow a broader use of the Péclet effect in predictive models. Our results point to the importance of within needle water pools and isolating mechanisms that need further investigation in order to integrate Péclet corrections with 'two compartment' leaf water concepts.

Above- and belowground processes in plants are tightly coupled via carbon and water fluxes through the soil-plant-atmosphere system. The oxygen isotopic composition of atmospheric CO₂ and water vapour (H₂Ov) provides a valuable tool for investigating the transport and cycling of carbon and water within this system. However, detailed studies on the coupling between ecosystem components and environmental drivers are sparse. Therefore, we conducted a H2 (18)O-labelling experiment to investigate the effect of drought on the speed of the link between below- and aboveground processes and its subsequent effect on C(18)OO released by leaves and soils. A custom-made chamber system, separating shoot from soil compartments, allowed separate measurements of shoot- and soil-related processes under controlled conditions. Gas exchange of oxygen stable isotopes in CO₂ and H₂Ov served as the main tool of investigation and was monitored in real time on Fagus sylvatica saplings using laser spectroscopy. H₂(18)O-labelling showed that drought caused a slower transport of water molecules from soil to shoot, which was indicated by its direct derivation from independently measured concentrations and (18)O/(16)O ratios of CO₂ and H₂Ov, respectively. Furthermore, drought reduced the (18)O equilibrium between H₂O and CO₂ at the shoot level, resulting in less-enriched C(18)OO fluxes from leaf to atmosphere compared with control plants. Compared with the shoot, (18)O equilibrium was not instantaneous in the soil and no drought effect was apparent.

Carbon (C) allocation strongly influences plant and soil processes. Short-term C allocation dynamics in ecosystems and their responses to environmental changes are still poorly understood. Using in situ (13) CO(2) pulse labeling, we studied the effects of 1 wk of shading on the transfer of recent photoassimilates between sugars and starch of above- and belowground plant organs and to soil microbial communities of a mountain meadow. C allocation to roots and microbial communities was rapid. Shading strongly reduced sucrose and starch concentrations in shoots, but not roots, and affected tracer dynamics in sucrose and starch of shoots, but not roots: recent C was slowly incorporated into root starch irrespective of the shading treatment. Shading reduced leaf respiration more strongly than root respiration. It caused no reduction in the amount of (13) C incorporated into fungi and Gram-negative bacteria, but increased its residence time. These findings suggest that, under interrupted C supply, belowground C allocation (as reflected by the amount of tracer allocated to root starch, soil microbial communities and belowground respiration) was maintained at the expense of aboveground C status, and that C source strength may affect the turnover of recent plant-derived C in soil microbial communities.

Drought-induced forest decline, like the Scots pine mortality in inner-Alpine valleys, will gain in importance as the frequency and severity of drought events are expected to increase. To understand how chronic drought affects tree growth and tree-ring delta(13)C values, we studied mature Scots pine in an irrigation experiment in an inner-Alpine valley. Tree growth and isotope analyses were carried out at the annual and seasonal scale. At the seasonal scale, maximum delta(13)C values were measured after the hottest and driest period of the year, and were associated with decreasing growth rates. Inter-annual delta(13)C values in early- and latewood showed a strong correlation with annual climatic conditions and an immediate decrease as a response to irrigation. This indicates a tight coupling between wood formation and the freshly produced assimilates for trees exposed to chronic drought. This rapid appearance of the isotopic signal is a strong indication for an immediate and direct transfer of newly synthesized assimilates for biomass production. The fast appearance and the distinct isotopic signal suggest a low availability of old stored carbohydrates. If this was a sign for C-storage depletion, an increasing mortality could be expected when stressors increase the need for carbohydrate for defence, repair or regeneration.

Acute winter frost drought in Rhododendron ferrugineum at the alpine timberline was provoked by removing the snow that had covered the shrubs and by irradiating and rewarming the twigs in situ in a climatized chamber simulating snow melt conditions. Water potential decreased to -2.3 MPa within three days. After activation of photosynthesis on the 1st day, the COr-uptake decreased during the 2nd day and approached zero at the 3rd day. With advanced desiccation, the internal COr concentration of the leaves increased indicating an impairment of chloroplast functions. The procedure of the disturbances appears to be similar to that of other acute drought events.