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Climate change has had a widespread and profound impact on global temperature and precipitation patterns, especially in semi-arid areas. Plant δ13C and iWUE indicate the trade-off between carbon uptake and water loss, which is pivotal for understanding plant responses to climate change. Information about the long-term responses of the physiological and ecological processes of different tree species to climate change is also required. To investigate the impact of different forest stand structures and site conditions on long-term growth and physiological processes of coniferous and broad-leaved trees in the mountainous area of Beijing, we analyzed the tree-ring δ13C variation of four tree species (Platycladus orientalis, Pinus tabuliformis, Quercus variabilis, Robinia pseudoacacia) sampled from 64 plots with varying site and stand conditions. We found that the tree-ring δ13C of the four tree species varied from each other and was mainly affected by density and slope aspect, followed by slope and age. Both tree-ring δ13C and iWUE of the four tree species showed increasing trends over time, mechanistically linked to long-term changes in global CO2 concentration. This indicates the four native tree species have adapted well to climate change, and the risk of decline is relatively low. The increased iWUE translated into different growth patterns which varied with tree species, site, and stand condition. Different tree species have varying sensitivities to environmental factors. The iWUE of coniferous tree species is more sensitive to climate change than that of broad-leaved tree species, especially to temperature (T), the Standardized Precipitation Evapotranspiration Index (SPEI), and vapor pressure deficit (VPD). [ABSTRACT FROM AUTHOR]

Increasing evidence indicates that tree growth processes, including reproduction, can be either sink- or source-limited, or simultaneously co-limited by sink and source, depending on the interplay between internal and environmental factors. We tested the hypothesis that the relative strengths of photosynthate supply and demand by stem growth and reproduction create variable competition for substrate that is imprinted in the tree-ring isotopes (C and O) of stone pine (Pinus pinea L.), a masting gymnosperm with large costs of reproduction, under warming-induced drought. Across five representative stands of the Spanish Northern Plateau, we also identified reproductive phases where weather drivers of cone yield (CY) have varied over a 60-year period (1960–2016). We found that these drivers gradually shifted from winter–spring conditions 3 years before seed rain (cone setting) to a combination of 3- and 1-year lagged effects (kernel filling). Additionally, we observed positive regional associations between carbon isotope discrimination (Δ13C) of the year of kernel filling and CY arising at the turn of this century, which progressively offset similarly positive relationships between Δ13C of the year of cone setting and CY found during the first half of the study period. Altogether, these results pinpoint the increasing dependence of reproduction on fresh assimilates and suggest sink and source co-limitation superseding the sink-limited functioning of reproduction dominant before 2000. Under climate warming, it could be expected that drier conditions reinforce the role of source limitation on reproduction and, hence, on regeneration, forest structure and economic profit of the nutlike seeds of the species. [ABSTRACT FROM AUTHOR]

Human-caused anthropogenic greenhouse emissions impact the climate globally. In this pilot study, we aim to reveal the influence of hydrocarbon emissions on pine forests by applying a stable carbon isotope analysis in pine tree rings (δ13Cptrw). Our study was conducted in an industrial giant oil field reservoir (UVRT) and natural reserve (Raifa) sites, the Tatarstan Republic, Russia. Our results show a decreasing δ13Cptrw at the UVRT site in 1943, when oil extraction started, and in 1970, when it reached maximum production. We found that the δ13Cptrw from UVRT indicates developing unfavourable drier conditions and a suppressed tree growth caused by both human-induced oil and deposit infrastructures and natural processes compared to the undisturbed Raifa site. A 5-year running correlation analysis showed a significant difference between the sites in 1965 over the period of 1930 to 2021. The δ13Cptrw values from Raifa are more negative compared to UVRT, which can be explained by a higher forest sensitivity to human-induced impacts. From an eco-physiological point of view, the decreasing of intercellular (ci)-to-ambient (ca) CO2 concentration ratios at the leaf level and the increasing of intrinsic water-use efficiency (iWUE) along with a decreasing of tree-ring widths at the UVRT site (1970–2021) indicate the development of drought conditions. [ABSTRACT FROM AUTHOR]

Carbon isotope composition of tree‐ring (δ13CRing) is a commonly used proxy for environmental change and ecophysiology. δ13CRing reconstructions are based on a solid knowledge of isotope fractionations during formation of primary photosynthates (δ13CP), such as sucrose. However, δ13CRing is not merely a record of δ13CP. Isotope fractionation processes, which are not yet fully understood, modify δ13CP during sucrose transport. We traced, how the environmental intra‐seasonal δ13CP signal changes from leaves to phloem, tree‐ring and roots, for 7 year old Pinus sylvestris, using δ13C analysis of individual carbohydrates, δ13CRing laser ablation, leaf gas exchange and enzyme activity measurements. The intra‐seasonal δ13CP dynamics was clearly reflected by δ13CRing, suggesting negligible impact of reserve use on δ13CRing. However, δ13CP became increasingly 13C‐enriched during down‐stem transport, probably due to post‐photosynthetic fractionations such as sink organ catabolism. In contrast, δ13C of water‐soluble carbohydrates, analysed for the same extracts, did not reflect the same isotope dynamics and fractionations as δ13CP, but recorded intra‐seasonal δ13CP variability. The impact of environmental signals on δ13CRing, and the 0.5 and 1.7‰ depletion in photosynthates compared ring organic matter and tree‐ring cellulose, respectively, are useful pieces of information for studies exploiting δ13CRing. Summary statement: Intra‐annual tree‐ring δ13C signal of Scots pine recorded faithfully the environmental signal in leaf sucrose δ13C, with no clear signs of reserve use. The 13C‐enrichment of studied sink organs relative to leaves was dominated by post‐photosynthetic fractionations, such as sink organ respiration. [ABSTRACT FROM AUTHOR]

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 (CO2, 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. Summary statement: We analysed >300 studies making use of the dual carbon and oxygen isotope model (DI‐model). Based on the isotopic patterns, we can draw conclusions on how environmental variables impact photosynthesis and stomatal conductance. The concept was well received and applied to interpret plant environment interactions. Yet, some studies reported nonplausible model results. Evaluating all publications applying the DI‐model revealed most causes for nonplausibility. [ABSTRACT FROM AUTHOR]

The forests of south‐central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the Mediterranean‐Temperate gradient (35.9°−40.3°S) using carbon isotope discrimination (Δ13 C) and intrinsic water use efficiency (iWUE) in tree rings during 1967–2017. Moreover, tree ring δ18O was evaluated in the northernmost site to better understand the effects of the megadrought in this drier location. These forests have become more efficient in their use of water. However, trees from the densest stand are discriminating more against 13C, probably due to reduced photosynthetic rates associated with increasing light competition. The strongest associations between climate and Δ13C were found in the northernmost stand, suggesting that warmer and drier conditions could have reduced 13C discrimination. Tree growth in this site has not decreased, and δ18O was negatively related to annual rainfall. However, a shift in this relationship was found since 2007, when both precipitation and δ18O decreased, while correlations between δ18O and growth increased. This implies that tree growth and δ18O are coupled in recent years, but precipitation is not the cause, suggesting that trees probably changed their water source to deeper and more depleted pools. Our research demonstrates that forests are not reducing their growth in central Chile, mainly due to a shift toward the use of deeper water sources. Despite a common climate trend across the gradient, there is a non‐uniform response of N. obliqua forests to climate drying, being their response site‐specific. Plain Language Summary: The forests of south‐central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the Mediterranean‐Temperate gradient (35.9°−40.3°S) to climate drying using stable isotopes in tree rings during the period 1967–2017. These forests have become more efficient in their use of water. However, trees from the densest stand are showing a poor adaptability to climate change, probably due to a decrease in photosynthetic rates associated with the increase in competition. Our research demonstrates for the first time that forests are not reducing their growth in central Chile, mainly due to a shift toward the use of deeper water sources. However, it is not certain until which point this acclimation will persist as these pools get depleted in the future with continued drying. Despite a common climate trend across the latitudinal gradient, this research points to a non‐uniform response of N. obliqua second‐growth forests to a drying climate, being their response stand and site‐specific. Key Points: Despite a common climate trend, the response of N. obliqua to climate change is not uniform, but rather specific to the stand and siteTree growth and δ18O have been coupled during the megadrought in the northernmost site, but rainfall does not influence this patternN. obliqua northern forests are able to maintain radial growth rates and acclimate to persistent droughts by tapping deeper water sources [ABSTRACT FROM AUTHOR]

Recent climate extremes in Mongolia have ignited a renewed interest in understanding past climate variability over centennial and longer time scales across north-central Asia. Tree-ring width records have been extensively studied in Mongolia as proxies for climate reconstruction, however, the climate and environmental signals of tree-ring stable isotopes from this region need to be further explored. Here, we evaluated a 182-year record of tree-ring δ13C and δ18O from Siberian Pine (Pinus sibirica Du Tour) from a xeric site in central Mongolia (Khorgo Lava) to elucidate the environmental factors modulating these parameters. First, we analyzed the climate sensitivity of tree-ring δ13C and δ18O at Khorgo Lava for comparison with ring-width records, which have been instrumental in reconstructing hydroclimate in central Mongolia over two millennia. We also compared stable isotope records of trees with partial cambial dieback ('strip-bark morphology'), a feature of long-lived conifers growing on resource-limited sites, and trees with a full cambium ('whole-bark morphology'), to assess the inferred leaf-level physiological behavior of these trees. We found that interannual variability in tree-ring δ13C and δ18O reflected summer hydroclimatic variability, and captured recent, extreme drought conditions, thereby complementing ring-width records. The tree-ring δ18O records also had a spring temperature signal and thus expanded the window of climate information recorded by these trees. Over longer time scales, strip-bark trees had an increasing trend in ring-widths, δ13C (and intrinsic water-use efficiency, iWUE) and δ18O, relative to whole-bark trees. Our results suggest that increases in iWUE at this site might be related to a combination of leaf-level physiological responses to increasing atmospheric CO2, recent drought, and stem morphological changes. Our study underscores the potential of stable isotopes for broadening our understanding of past climate in north-central Asia. However, further studies are needed to understand how stem morphological changes might impact stable isotopic trends. [ABSTRACT FROM AUTHOR]

In the context of climate change, the stable isotope analysis of tree rings may play a crucial role in deciphering the eco-physiological mechanisms underlying forest decline and dieback phenomena. However, this technique is often considered expensive, time-consuming, and with several methodological constraints. Specifically, milling and transferring the material from jars to vials during the different steps of sample preparation involve risk of contamination among samples and loss of sample material. When dealing with declining trees (i.e., trees affected by loss of vitality with strong percentage of defoliation and reduction in growth) and trees subjected to extreme events or negative pointer years (characterized by extremely narrow ring) the sample preparation is particularly difficult because of scarce amount of wood material. In such a case, pooling rings from several years to achieve the minimum weight of wood is often necessary, thus losing information at the annual resolution. In order to overcome such limitations, we developed a novel protocol for quick and accurate whole-wood pre-processing, testing it on oak tree rings of different widths taken from living trees. The main novelty introduced by our protocol was freezing tree-ring samples at −80 °C and milling multiple samples at a time by using a 24-tube plate. The results showed that our novel simplified protocol significantly reduced the pre-processing time with respect to the standard protocol (12 vs. 284 sec/sample), while achieving the same wood particle size, limiting the loss of wood material and reducing the risk of contamination among samples. [ABSTRACT FROM AUTHOR]

Increasing intensity and frequency of droughts are leading to forest dieback, growth decline and tree mortality worldwide. Reducing tree-to-tree competition for water resources is a primary goal for adaptive climate silviculture strategies, particularly in reforested areas with high planting density. Yet, we need better insights into the role of stand type (i.e., natural forests versus plantations) on the resilience of pine forests to droughts across varying time scales. In this study, we combined dendrochronological data and stable C (δ13C) and H (δ2H) isotopes measured in tree-ring wood as well as in specific wood chromatographically isolated compounds to investigate contrasting responses to drought of natural versus planted stands of two representative pine species, i.e., Pinus pinaster and Pinus nigra in southeastern Europe. Natural stands exhibited about two-fold increase in tree-ring growth in average (basal area at 20 years-BAI20) as compared to planted stands. A response function analysis showed contrasting seasonal growth patterns for both species, which were related to monthly mean temperature and precipitation. Both stand type and species variables influenced growth resilience indices. Both pine species revealed contrasting resilience patterns among forest types; whereas planted stands seemed to be less sensitive to yearly droughts as determined by a higher recovery index (CRc) for P. pinaster, the contrary was found in the case of P. nigra. On the other hand, while resistance CRT and resilience CRS indices were higher for planted than natural forests in the case of P. pinaster, little differences were found for P. nigra. Beyond comparisons, carbon stable isotopes shed lights on the role of forest types in dry sites, being δ13C consistently lower in natural than in planted forests for both pine species (p < 0.05). We concluded that planted forest assimilated more carbon as per unit of water used than natural stands in response to droughts. Both δ13C and δ2H isotopic signals were positively correlated for both species for planted forests. However, a lack of correlation was evidenced for natural stands. Consistent with δ13C observations, δ2H concentrations in woody phenolic compounds (guaiacol and oleic acid) revealed contrasting patterns among forest types. This puts forward that δ2H concentrations in woody phenolic compounds (rather than in woody tree ring) accounts for other confounding factors in tree ring formation that can be associated with forest type. Our results highlight the value of stable isotope approaches versus conventional dendrochronological tools in drought studies and call for the consideration of forest type as an endogenous aspect defining the vulnerability of pine forests to climate. [ABSTRACT FROM AUTHOR]

Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (δ13C) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow δ13C signals within trees. To study how the choice of organic material may bias the interpretation of δ13C records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time δ13C of different leaf carbon pools with δ13C of assimilates estimated by a chamber-Picarro system (δ13CA_Picarro), and a photosynthetic discrimination model (δ13CA_model). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of δ13CA_Picarro and δ13CA_model. Consequently, in comparison with the other carbon pools, sucrose δ13C was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from δ13C of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose δ13C to understand plant physiological responses in depth. [ABSTRACT FROM AUTHOR]

Despite many years of establishment of exclosures in the semi‐arid environments of north Ethiopia for rehabilitation of degraded areas, its effectiveness is constrained by water deficit. There is lack of empirical data on the response of vegetation to additional water (spate irrigation) application. This study aimed to evaluate the effects of spate irrigation using storm runoff on the growth of trees in exclosures. Two exclosures (May Ba'ati and Adi Kolakol), in Dogu'a Temben district, Tigray, north Ethiopia, were used for this study. Each exclosure was divided in two blocks (irrigated and non‐irrigated) to allow comparison of change of vegetation using several diversity indices as well as contrasts in tree ring widths. The amount of spate irrigation water diverted onto the exclosures (762 mm) was in the same order of magnitude as the direct rainfall on the sites (645 mm). The results show that the irrigated exclosures have larger species richness, Shannon–Weaver diversity index, tree density, abundance, evenness and basal area than the non‐irrigated areas. Following low rainfall (353 mm) in 2015, these indices decreased in the non‐irrigated area, whereas most indices remained much higher in the irrigated area. The findings indicate that diverting runoff to exclosures results in an average 15% increase in ring width as compared to non‐irrigated trees. In addition, there is strong abstraction of storm runoff in gullies in the study area. Besides the promotion of forestry in washout semi‐arid areas of existing spate irrigation systems, we suggest to apply spate irrigation to exclosures in order to improve their productivity. [ABSTRACT FROM AUTHOR]

Keywords: dual isotope approach; leaf water enrichment; Péclet effect; stable oxygen isotope; stomatal conductance; tree-ring EN dual isotope approach leaf water enrichment Péclet effect stable oxygen isotope stomatal conductance tree-ring 803 808 6 10/13/22 20221101 NES 221101 Plant scientists have long been fascinated by the balance between photosynthetic carbon gain and transpirational water loss in terrestrial plants, because such a balance can have wide-ranging implications with respect to plant productivity and survival, ecosystem service and function, and even global-scale hydrology and climate forcing (Cowan & Farquhar, 1977; Bonan, 2008; Keenan I et al i ., 2013). Dual isotope approach, Péclet effect, stomatal conductance, leaf water enrichment, stable oxygen isotope, tree-ring Due to the well-established, theoretical link between iWUE and photosynthetic carbon isotope discrimination (Farquhar I et al i ., 1982), analysis of plant carbon isotope composition ( SP 13 sp C SB plant sb ) has emerged as a popular method of estimating time-integrated iWUE during plant growth. To address this limitation, researchers have suggested adding plant oxygen isotope composition ( SP 18 sp O) into the isotope toolbox to help tease apart the influences of I A i vs I g i SB s sb on SP 13 sp C SB plant sb -derived iWUE (Scheidegger I et al i ., 2000; Grams I et al i ., 2007). [Extracted from the article]

Keywords: AmeriFlux; dual isotope approach; Péclet effect; precipitation; stable oxygen isotopes; stomatal conductance; tree rings; vapour pressure deficit (VPD) EN AmeriFlux dual isotope approach Péclet effect precipitation stable oxygen isotopes stomatal conductance tree rings vapour pressure deficit (VPD) 809 812 4 10/13/22 20221101 NES 221101 A reply to Lin et al. (2022) 'Do changes in tree-ring 18 O indicate changes in stomatal con... There is growing scientific consensus that rising atmospheric CO SB 2 sb concentrations have caused an increase in forest water-use efficiency (WUE), but the magnitude of this effect and the relative roles of enhanced photosynthesis vs reduced stomatal conductance ( I g i SB s sb ) are still under debate (Walker I et al i ., 2020). Detecting long-term changes in stomatal conductance: challenges and opportunities of tree-ring 18O proxy Dual isotope approach, Péclet effect, AmeriFlux, precipitation, stable oxygen isotopes, stomatal conductance, tree rings, vapour pressure deficit (VPD) Simulating oxygen isotope ratios in tree ring cellulose using a dynamic global vegetation model. [Extracted from the article]

Tree-ring intra-annual stable isotopes (δ13C and δ18O) are powerful tools for revealing plant ecophysiological responses to climatic extremes. We analyzed interannual and fine-scale intra-annual variability of tree-ring δ13C and δ18O in Chinese red pine (Pinus massoniana) from southeastern China to explore environmental drivers and potential trade-offs between the main physiological controls. We show that wet season relative humidity (May–October RH) drove interannual variability of δ18O and intra-annual variability of tree-ring δ18O. It also drove intra-annual variability of tree-ring δ13C, whereas interannual variability was mainly controlled by February–May temperature and September–October RH. Furthermore, intra-annual tree-ring δ18O variability was larger during wet years compared with dry years, whereas δ13C variability was lower during wet years compared with dry years. As a result of these differences in intra-annual variability amplitude, process-based models (we used the Roden model for δ18O and the Farquhar model for δ13C) captured the intra-annual δ18O pattern better in wet years compared with dry years, whereas intra-annual δ13C pattern was better simulated in dry years compared with wet years. This result suggests a potential asymmetric bias in process-based models in capturing the interplay of the different mechanistic processes (i.e. isotopic source and leaf-level enrichment) operating in dry versus wet years. We therefore propose an intra-annual conceptual model considering a dynamic trade-off between the isotopic source and leaf-level enrichment in different tree-ring parts to understand how climate and ecophysiological processes drive intra-annual tree-ring stable isotopic variability under humid climate conditions. [ABSTRACT FROM AUTHOR]

Summary: The impact of climate extremes on forest ecosystems is poorly understood but important for predicting carbon and water cycle feedbacks to climate. Some knowledge gaps still remain regarding how drought‐related adjustments in intra‐annual tree‐ring characteristics directly impact tree carbon and water use.In this study we quantified the impact of an extreme summer drought on the water‐use efficiency and carbon sequestration of four mature Norway spruce trees. We used detailed observations of wood formation (xylogenesis) and intra‐annual tree‐ring properties (quantitative wood anatomy and stable carbon isotopes) combined with physiological water‐stress monitoring.During 41 d of tree water deficit, we observed an enrichment in 13C but a reduction in cell enlargement and wall‐thickening processes, which impacted the anatomical characteristics. These adjustments diminished carbon sequestration by 67% despite an 11% increase in water‐use efficiency during drought. However, with the resumption of a positive hydric state in the stem, we observed a fast recovery of cell formation rates based on the accumulated assimilates produced during drought.Our findings enhance our understanding of carbon and water fluxes between the atmosphere and forest ecosystems, providing observational evidence on the tree intra‐annual carbon sequestration and water‐use efficiency dynamics to improve future generations of vegetation models. See also the Commentary on this article by Anderson‐Teixeira & Kannenberg et al., 236: 5–8. [ABSTRACT FROM AUTHOR]

Understanding the water-use strategy of human-planted species used in response to climate change is essential to optimize afforestation programs in dry regions. Since 2000, trees on the central Loess Plateau have experienced a shift from strengthening drought to weakening drought. In this study, we combined tree-ring δ13C and δ18O records from Pinus tabuliformis (syn. tabulaeformis) Carr. (a tree) and Sophora viciifolia Hance (a shrub) on the central Loess Plateau to investigate species-specific responses to rising atmospheric CO2 (Ca) and drought. We found summer relative humidity controlled the fractionation of tree-ring δ18O, but the magnitude of the climate influence on δ13C differed between the species. The intrinsic water-use efficiency (iWUE) trends of both species suggested a strongly active response to maintain constant intercellular CO2 concentrations as Ca rose. The tree-ring δ13C and δ18O of both species using first-difference data were significantly and positively correlated, with stronger relationships for the shrub. This indicated the dominant regulation of iWUE by stomatal conductance in both species, but with greater stomatal control for the shrub. Moreover, the higher mean iWUE value of S. viciifolia indicated a more conservative water-use strategy than P. tabuliformis. Based on our commonality analysis, the main driver of the increased iWUE was the joint effect of Ca and vapor-pressure deficit (25.51%) for the tree, while it was the joint effect of Ca and the self-calibrated Palmer drought severity index (39.13%) for the shrub. These results suggest S. viciifolia will be more drought-tolerant than P. tabuliformis and as Ca continually rises, we should focus more on the effects of soil drought than atmospheric drought on the water-use strategy of S. viciifolia. [ABSTRACT FROM AUTHOR]

The response of plants to climate change has become a topical issue. However, there is no consensus on the synergistic processes of the canopy and trunk growth within different vegetation types, or on the consistency of the response of the canopy and trunk to climate change. This paper is based on Normalized Difference Vegetation Index (NDVI), tree-ring width index (TRW) and climate data from the Irtysh River basin, a sensitive area for climate change in Central Asia. Spatial statistical methods and correlation analysis were used to analyze the spatial and temporal trends of plants and climate, and to reveal the differences in the canopy and trunk response mechanisms to climate within different vegetation types. The results show a warming and humidifying trend between 1982 and 2015 in the study area, and NDVI and TRW increases in different vegetation type zones. On an interannual scale, temperature is the main driver of the canopy growth in alpine areas and precipitation is the main limiting factor for the canopy growth in lower altitude valley and desert areas. The degree of response of the trunk to climatic factors decreases with increasing altitude, and TRW is significantly correlated with mean annual temperature, precipitation and SPEI in desert areas. On a monthly scale, the earlier and longer growing season due to the accumulation of temperature and precipitation in the early spring and late autumn periods contributes to two highly significant trends of increase in the canopy from March to May and August to October. Climatic conditions during the growing season are the main limiting factor for the growth of the trunk, but there is considerable variation in the driving of the trunk in different vegetation type zones. The canopy growth is mainly influenced by climatic factors in the current month, while there is a 1–2-month lag effect in the response of the trunk to climatic factors. In addition, the synergy between the canopy and the trunk is gradually weakened with increasing altitude (correlation coefficient is 0.371 in alpine areas, 0.413 in valley areas and 0.583 in desert areas). These findings help to enrich the understanding of the response mechanisms to climate change in different vegetation type zones and provide a scientific basis for the development of climate change response measures in Central Asia. [ABSTRACT FROM AUTHOR]

Climate change exposes ecosystems to strong and rapid changes in their environmental boundary conditions mainly due to the altered temperature and precipitation patterns. It is still poorly understood how fast interlinked ecosystem processes respond to altered environmental conditions, if these responses occur gradually or suddenly when thresholds are exceeded, and if the patterns of the responses will reach a stable state. We conducted an irrigation experiment in the Pfynwald, Switzerland from 2003–2018. A naturally dry Scots pine (Pinus sylvestris L.) forest was irrigated with amounts that doubled natural precipitation, thus releasing the forest stand from water limitation. The aim of this study was to provide a quantitative understanding on how different traits and functions of individual trees and the whole ecosystem responded to increased water availability, and how the patterns and magnitudes of these responses developed over time. We found that the response magnitude, the temporal trajectory of responses, and the length of initial lag period prior to significant response largely varied across traits. We detected rapid and stronger responses from aboveground tree traits (e.g., tree‐ring width, needle length, and crown transparency) compared to belowground tree traits (e.g., fine‐root biomass). The altered aboveground traits during the initial years of irrigation increased the water demand and trees adjusted by increasing root biomass during the later years of irrigation, resulting in an increased survival rate of Scots pine trees in irrigated plots. The irrigation also stimulated ecosystem‐level foliar decomposition rate, fungal fruit body biomass, and regeneration abundances of broadleaved tree species. However, irrigation did not promote the regeneration of Scots pine trees, which are reported to be vulnerable to extreme droughts. Our results provide extensive evidence that tree‐ and ecosystem‐level responses were pervasive across a number of traits on long‐term temporal scales. However, after reaching a peak, the magnitude of these responses either decreased or reached a new stable state, providing important insights into how resource alterations could change the system functioning and its boundary conditions. [ABSTRACT FROM AUTHOR]

Wildfires are one of the most important environmental factors controlling forest ecosystem physiology and the carbon balance in the permafrost zone of North Siberia. We investigated tree-ring width (TRW) and stable isotope chronologies in tree-ring cellulose (δ13CCell, δ18OCell) of Larix Gmelinii (Rupr.) Rupr. from a wet (WS) and a dry (DS) site. These sites are characterized by different fire histories (fire in 1852 at the wet and 1896 at the dry sites, respectively). TRW and δ18OCell are identified to be the most sensitive parameters in the changing tree growth conditions after fire. The differences in the soil seasonal thermal regime of sites after fires are shown in the relationship between the studied parameters. The δ13CCell values in tree rings from the two sites are positively correlated independently of the fire impact. This fact indicates that δ13CCell chronologies might be more adequate for climatic reconstruction in the region due to the climate signal consistency. Relationships of δ18OCell values between the two sites are still significantly positive 60 years after the fire impact. Dendroclimatic analysis indicates significant changes in tree-ring growth and isotopic ratio responses to climate due to the increased demand of water for trees during the post-fire period (deeper seasonal subsidence of permafrost). [ABSTRACT FROM AUTHOR]

In 2010-2018, Northern Patagonia featured the longest severe drought of the last millennium. This extreme dry spell triggered widespread growth decline and forest dieback. Nonetheless, the roles played by the two major mechanisms driving dieback, hydraulic failure and carbon starvation, are still not clear and understudied in this seasonally dry region. Here, for the 1800-2017 period, we apply a retrospective analysis of radial growth, wood anatomical traits (lumen area, cell-wall thickness) and δ13C and δ18O stable isotopes to assess dieback causes of the iconic conifer Araucaria araucana. We selected three stands where declining (defoliated) and nondeclining (not defoliated) trees coexisted along a precipitation gradient from the warm-dry Coastal Range to the cool-wet Andes. At all sites declining trees showed lower radial growth and lower theoretical hydraulic conductivity, suggesting a long-lasting process of hydraulic deterioration in their water transport system compared to nondeclining, coexisting trees. Wood anatomical traits evidenced that this divergence between declining and nondeclining trees started at least seven decades before canopy dieback. In the drier stands, declining trees showed higher water-use efficiency (WUE) throughout the whole period, which we attributed to early stomatal closure, suggesting a greater carbon starvation risk consistent with thinner cell walls. In the wettest stand, we found the opposite pattern. Here, a reduction in WUE coupled with thicker cell walls suggested increased carbon assimilation rates and exposure to drought-induced hydraulic failure. The δ18O values indicated different strategies of gas exchange between sites, which are likely a consequence of microsite conditions and water sources. Multiproxy, retrospective quantifications of xylem anatomical traits and tree-ring isotopes provide a robust tool to identify and forecast, which stands or trees will show dieback or, on the contrary, which will likely withstand and be more resilient to future hotter droughts. [ABSTRACT FROM AUTHOR]

The Canary Islands, an archipelago east of Morocco's Atlantic coast, present steep altitudinal gradients covering various climatic zones from hot deserts to subalpine Mediterranean, passing through fog-influenced cloud forests. Unlike the majority of the Canarian flora, Pinus canariensis C. Sm. ex DC. in Buch grow along most of these gradients, allowing the study of plant functioning in contrasting ecosystems. Here we assess the water sources (precipitation, fog) of P. canariensis and its physiological behavior in its different natural environments. We analyzed carbon and oxygen isotope ratios of water and organics from atmosphere, soil and different plant organs and tissues (including 10-year annual time series of tree-ring cellulose) of six sites from 480 to 1990 m above sea level on the Canary Island La Palma. We found a decreasing δ18O trend in source water that was overridden by an increasing δ18O trend in needle water, leaf assimilates and tree-ring cellulose with increasing altitude, suggesting site-specific tree physiological responses to relative humidity. Fog-influenced and fog-free sites showed similar δ13C values, suggesting photosynthetic activity to be limited by stomatal closure and irradiance at certain periods. In addition, we observed an 18O-depletion (fog-free and timberline sites) and 13C-depletion (fog-influenced and fog-free sites) in latewood compared with earlywood caused by seasonal differences in: (i) water uptake (i.e. deeper ground water during summer drought, fog water frequency and interception) and (ii) meteorological conditions (stem radial growth and latewood δ18O correlated with winter precipitation). In addition, we found evidence for foliar water uptake and strong isotopic gradients along the pine needle axis in water and assimilates. These gradients are likely the reason for an unexpected underestimation of pine needle water δ18O when applying standard leaf water δ18O models. Our results indicate that soil water availability and air humidity conditions are the main drivers of the physiological behavior of pine along the Canary Island's altitudinal gradients. [ABSTRACT FROM AUTHOR]

Rising air temperature, changes in the hydrological cycle and atmospheric circulation patterns are leading to increasing extreme events affecting human and forests worldwide. Altai relict forests are under the risk of degradation under the recent warming. Investigations of climatic impacts on the southern Siberian forest, where temperature is not the main limiting growth factor are still rare and urgently needed. Therefore, we aim to reveal and evaluate the impact of recent extreme climatic events (1919–2019) on relict larch trees (Larix sibirica Ldb.) from two contrasting sites in the Russian Altai (Ersin—HO and Chadan—CH). Application of the state-of-the-art methods of stable isotope and tree-ring analyses is very promising in this region due to high sensitivity of trees to temperature and precipitation changes. We found site-specific differences (ca. 4‰) recorded in newly developed centennial stable carbon (δ13C) and oxygen (δ18O) isotope chronologies, showing their climate sensitivity and reflecting the contrasting conditions at the dry (HO) and wet (CH) sites. The most extreme drought anomalies (σ ≥ + 3σ) over the past 100 years (1919–2019) were revealed for the HO-site in 1935 and 2018. In contrast, a wet extreme anomaly (σ = − 2.7σ) was detected for the CH site only in 1994. Moreover, trees from the CH-site are receiving a greater amount of precipitation during the summer months, especially for the years with extremes σ ≥ − 2σ (1939, 1948, 1958, 1994, 2000, and 2019). Divergent δ13C and intrinsic water-use efficiency trends after the 1980s indicate increasing drought stress at the HO- compared to the CH-site, which is confirmed by changes in the running correlation between the isotopes and comparison with declining tree-ring width data. Drought or flooding extremes are recorded heterogeneously at the two study sites in southern Altai, indicating dependence from local habitats, type of soils and freezing active soil depths. Recent climate change affected both forest and humans as public and agricultural sectors are suffering from recent droughts or flooding events. To provide measures and apply mitigation plans for the unique relict forest and public sector, our reconstruction of past anomaly events in this region is important and helpful to predict future climate changes. [ABSTRACT FROM AUTHOR]

Stone pine (Pinus pinea L.) has been cultivated since centuries in Mediterranean areas for its products and economic benefits, including edible pine nuts, timber, mushrooms, firewood, and grazing. However, current management objectives of stone pine stands also include recreational use, biodiversity conservation, protection from soil erosion, and CO2 fixation. Stone pine stands are considered to be among the ecosystems most vulnerable to climate change, and the current increase in drought frequency in the Mediterranean Basin has been shown to negatively impact their long-term establishment. Understanding the effects of climate change on the distribution, tree-ring growth and water use of stone pine forests can help assessing the adaptive capacity of the species, and developing management programs aimed at its conservation. This paper reviews the impacts of climate change on stone pine in the Mediterranean region. The high sensitivity of stone pine to climate change has been widely demonstrated in that: (i) climatic models predict the loss of suitable habitats and the shift of its geographical distribution in the next future; (ii) tree-ring analysis showed that winter and spring rainfalls have positive effects on growth, whereas high spring temperature has a negative effect; (iii) the strategy of stone pine to cope with water deficit affects the processes regulating its growth, including wood formation, leading to peculiar tree-ring anatomical features such as intra-annual density fluctuations. The silvicultural interventions and the most effective management strategies for stone pine forests are reviewed and discussed in the context of current climate change in the Mediterranean Basin. [ABSTRACT FROM AUTHOR]

Background: Climate warming is amplifying and exacerbating drought stress worldwide. Long-term trends of increasing evaporative demand and decreasing soil moisture availability occur superimposed on severe spells of drought. These rare, extreme droughts have triggered episodes of forest dieback that have led to reduced productivity and rising mortality rates, usually at small scales (dieback hotspots), but affecting biomes worldwide. Aims: This review summarizes and discusses the drivers, patterns and mechanisms of forest dieback caused by drought. Methods: I review studies on forest dieback and tree death linked to dry spells with a focus on tools to forecast dieback. Results: Several mechanisms have been described as physiological drivers of dieback, including hydraulic failure and carbon starvation, however hydraulics-based models have shown little predictive power of dieback and mortality. Field proxies of tree vigour, including changes in canopy defoliation and water content, combined with surrogates of tree functioning (tree-ring growth, wood anatomy, tree-ring δ13C or δ18O composition) may improve predictions of forest dieback or at least render early-warning signals of impending tree death. Conclusions: Drought-induced dieback and mortality are concerning phenomena which lack forecasting tools with sufficient predictive power. Surrogates of tree vigour, growth and functioning should be used to build more accurate models of tree death in response to extreme climate events linked to drought. Here, I argue for combining and comparing those surrogates to better forecast forest dieback. [ABSTRACT FROM AUTHOR]

In many regions, drought is suspected to be a cause of Scots pine decline and mortality, but the underlying physiological mechanisms remain unclear. Because of their relationship to ecohydrological processes, δ18O values in tree rings are potentially useful for deciphering long-term physiological responses and tree adaptation to increasing drought. We therefore analyzed both needle- and stem-level isotope fractionations in mature trees exposed to varying water supply. In a first experiment, we investigated seasonal δ18O variations in soil and needle water of Scots pine in a dry inner Alpine valley in Switzerland, comparing drought-stressed trees with trees that were irrigated for more than 10 years. In a second experiment, we analyzed twentieth-century δ18O variations in tree rings of the same forest, including a group of trees that had recently died. We observed less 18O enrichment in needle water of drought-stressed compared with irrigated trees. We applied different isotope fractionation models to explain these results, including the Péclet and the two-pool correction, which considers the ratio of unenriched xylem water in the needles to total needle water. Based on anatomical measurements, we found this ratio to be unchanged in drought-stressed needles, although they were shorter. The observed lower 18O enrichment in needles of stressed trees was therefore likely caused by increased effective path length for water movement within the leaf lamina. In the tree-ring study, we observed lower δ18O values in tree rings of dead trees compared with survivors during several decades prior to their death. These lower values in declining trees are consistent with the lower needle water 18O enrichment observed for drought-stressed compared with irrigated trees, suggesting that this needle-level signal is reflected in the tree rings, although changes in rooting depth could also play a role. Our study demonstrates that long-term effects of drought are reflected in the tree-ring δ18O values, which helps to provide a better understanding of past tree physiological changes of Scots pine. [ABSTRACT FROM AUTHOR]

To identify source water for trees growing on permafrost in Siberia, we applied mechanistic models that quantify physical and biochemical fractionation processes, leading to oxygen isotope variation (δ18O) in plant organic matter. These models allowed us to investigate the influence of a variety of climatic factors on tree-ring cellulose from two dominant species: Larix cajanderi Mayr. from northeastern Yakutia (69° 22′ N, 148° 25′ E, ~ 250 m a.s.l.) and Pinus sylvestris L. from Central Yakutia (62°14′ N, 129°37′ E, ~ 220 m a.s.l.). The climate of the region is highly continental with short growing seasons, low amount of precipitation and these forest ecosystems are growing on permafrost, which in turn impact the water cycle and climate variation in the δ18O of source water. We compared outputs of the Land surface Processes and eXchanges (LPX-Bern v. 1.3), and Roden-Lin-Ehleringer (RLE) models for the common period from 1945 to 2004. Based on our findings, trees from northeastern and central Yakutia may have access to additional thawed permafrost water during dry summer periods. Owing to differences in the soil structure, active thaw soil depth and root systems of trees at two Siberian sites, Larix cajanderi Mayr. trees can access water not more than from 50 cm depth, in contrast to Pinus sylvestris L. in Central Yakutia which can acquire water from up to 80 cm soil depth. The results enhance our understanding of the growth and survival of the trees in this extreme environment. [ABSTRACT FROM AUTHOR]

Population density influences tree responses to environmental stresses, such as drought and high temperature. Prolonged drought negatively affects the health of Mongolian pines in forests planted by the Three-North Shelter Forest Program in North China. To understand the relationship between stand density and drought-induced forest decline, and to generate information regarding the development of future management strategies, we analyzed the vulnerability to drought of planted Mongolian pines at three stand densities. A tree-ring width index for trees from each density was established from tree-ring data covering the period 1988–2018 and was compared for differences in radial growth. Resistance (Rt), recovery (Rc), resilience (Rs), and relative resilience (RRs) in response to drought events were calculated from the smoothed basal area increment (BAI) curves. The high-density (HDT) group showed a consistently lower tree-ring width than the border trees (BT) and low-density (LDT) groups. The BAI curve of the HDT group started to decrease five years earlier than the LDT and BT groups. Pearson correlation analysis revealed that the radial growth of all of the groups was related to precipitation, relative humidity (RH), potential evapotranspiration (ET0), and standardized precipitation evapotranspiration index (SPEI) in the previous October and the most recent July, indicating that Mongolian pine trees of different densities had similar growth–climate relationships. Over the three decades, the trees experienced three severe drought events, each causing reduced tree-ring width and BAI. All of the groups showed similar Rc to each drought event, but the HDT group exhibited significantly lower Rt, Rs, and RRs than the BT group, suggesting that the HDT trees were more vulnerable to repeated drought stress. The RRs of the HDT group decreased progressively after each drought event and attained <0 after the third event. All of the groups showed similar trends regarding water consumption under varying weather conditions, but the HDT group showed significantly reduced whole-tree hydraulic capability compared with the other two groups. From these results, HDT trees exhibit ecophysiological memory effects from successive droughts, including sap flux dysfunction and higher competition index, which may prevent recovery of pre-drought growth rates. HDT trees may be at greater risk of mortality under future drought disturbance. [ABSTRACT FROM AUTHOR]

Recent analyses on the length of drought recovery in forests have shown multi-year legacies, particularly in semi-arid, coniferous ecosystems. Such legacies are usually attributed to ecophysiological memory, although drought frequency itself, and its effect on overlapping recovery times, could also contribute. Here, we describe a multi-decadal study of drought legacies using tree-ring carbon-isotope ratios (δ13C) and ring-width index (RWI) in Pinus ponderosa at 13 montane sites traversing a winter–summer precipitation gradient in the Southwestern U.S. Sites and trees were selected to avoid collection biases that exist in archived tree-ring databanks. The spatial hydroclimate gradient and winter–summer seasonal patterns were well predicted by seasonal and inter-annual correlations between δ13C and atmospheric vapor pressure deficit (VPD). Using VPD, we found that the probability of extreme drought has increased up to 70% in this region during the past two decades. When the recent increase in drought frequency was not considered, multi-year legacies in both δ13C and RWI were observed at most sites. When the increase in drought frequency was detrended from tree-ring chronologies, some sites exhibited short legacies (1–2 years) in both δ13C and RWI, and there was a sight trend for longer legacies in RWI. However, when considered broadly across the region and multiple decades, no significant legacies were observed, which contrasts with past studies. Our results reveal that a contribution to observed multi-year legacies is related to shifts in the climate system itself, an exogenous factor, that must be considered along with physiological memory. [ABSTRACT FROM AUTHOR]

Severe drought can cause lagged effects on tree physiology that negatively impact forest functioning for years. These "drought legacy effects" have been widely documented in tree‐ring records and could have important implications for our understanding of broader scale forest carbon cycling. However, legacy effects in tree‐ring increments may be decoupled from ecosystem fluxes due to (a) postdrought alterations in carbon allocation patterns; (b) temporal asynchrony between radial growth and carbon uptake; and (c) dendrochronological sampling biases. In order to link legacy effects from tree rings to whole forests, we leveraged a rich dataset from a Midwestern US forest that was severely impacted by a drought in 2012. At this site, we compiled tree‐ring records, leaf‐level gas exchange, eddy flux measurements, dendrometer band data, and satellite remote sensing estimates of greenness and leaf area before, during, and after the 2012 drought. After accounting for the relative abundance of tree species in the stand, we estimate that legacy effects led to ~10% reductions in tree‐ring width increments in the year following the severe drought. Despite this stand‐scale reduction in radial growth, we found that leaf‐level photosynthesis, gross primary productivity (GPP), and vegetation greenness were not suppressed in the year following the 2012 drought. Neither temporal asynchrony between radial growth and carbon uptake nor sampling biases could explain our observations of legacy effects in tree rings but not in GPP. Instead, elevated leaf‐level photosynthesis co‐occurred with reduced leaf area in early 2013, indicating that resources may have been allocated away from radial growth in conjunction with postdrought upregulation of photosynthesis and repair of canopy damage. Collectively, our results indicate that tree‐ring legacy effects were not observed in other canopy processes, and that postdrought canopy allocation could be an important mechanism that decouples tree‐ring signals from GPP. [ABSTRACT FROM AUTHOR]

Tree ring synthesis is a key process in wood production; however, little is known of the origin and fate of the carbon involved. We used natural 13C abundance to investigate the carbon-use process for the ring development in a temperate deciduous (Quercus petraea (Matt.) Liebl.) and a Mediterranean evergreen (Quercus ilex L.) oak. The sapwood carbon reserves, phloem sucrose contents, stem respired CO2 efflux and their respective carbon isotope compositions (δ 13C) were recorded over 1 year, in the native area of each species. The seasonal δ 13C variation of the current year ring was determined in the total ring throughout the seasons, as well as in slices from the fully mature ring after the growth season (intra-ring pattern). Although the budburst dates of the two oaks were similar, the growth of Quercus ilex began 50 days later. Both species exhibited growth cessation during the hot and dry summer but only Q. ilex resumed in the autumn. In the deciduous oak, xylem starch storage showed clear variations during the radial growth. The intra-ring δ 13C variations of the two species exhibited similar ranges, but contrasting patterns, with an early increase for Q. petraea. Comparison between δ 13C of starch and total ring suggested that Q. petraea (but not Q. ilex) builds its rings using reserves during the first month of growth. Shifts in ring and soluble sugars δ 13C suggested an interspecific difference in either the phloem unloading or the use of fresh assimilate inside the ring. A decrease in ring δ 13C for both oaks between the end of the radial growth and the winter is attributed to a lignification of ring cell walls after stem increment. This study highlighted the differences in carbon-use during ring growth for evergreen and deciduous oaks, as well as the benefits of exploring the process using natural 13C abundance. [ABSTRACT FROM AUTHOR]

Summary: While photosynthetic isotope discrimination is well understood, the postphotosynthetic and transport‐related fractionation mechanisms that influence phloem and subsequently tree ring δ13C are less investigated and may vary among species.We studied the seasonal and diel courses of leaf‐to‐phloem δ13C differences of water‐soluble organic matter (WSOM) in vertical crown gradients and followed the assimilate transport via the branches to the trunk phloem at breast height in European beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii). δ13C of individual sugars and cyclitols from a subsample was determined by compound‐specific isotope analysis.In beech, leaf‐to‐phloem δ13C differences in WSOM increased with height and were partly caused by biochemical isotope fractionation between leaf compounds. 13C‐Enrichment of phloem sugars relative to leaf sucrose implies an additional isotope fractionation mechanism related to leaf assimilate export. In Douglas fir, leaf‐to‐phloem δ13C differences were much smaller and isotopically invariant pinitol strongly influenced leaf and phloem WSOM. Trunk phloem WSOM at breast height reflected canopy‐integrated δ13C in beech but not in Douglas fir.Our results demonstrate that leaf‐to‐phloem isotope fractionation and δ13C mixing patterns along vertical gradients can differ between tree species. These effects have to be considered for functional interpretations of trunk phloem and tree ring δ13C. [ABSTRACT FROM AUTHOR]

Altitudinally separated bristlecone pine populations in the White Mountains (California, USA) exhibit differential climate–growth responses as temperature and tree-water relations change with altitude. These populations provide a natural experiment to explore the ecophysiological adaptations of this unique tree species to the twentieth century climate variability. We developed absolutely dated annual ring-width chronologies, and cellulose stable carbon and oxygen isotope chronologies from bristlecone pine growing at the treeline (~3500 m) and ~200 m below for the period AD 1710–2010. These chronologies were interpreted in terms of ecophysiological adaptations to climate variability with a dual-isotope model and a leaf gas exchange model. Ring widths show positive tree growth anomalies at treeline and consistent slower growth below treeline in relation to the twentieth century warming and associated atmospheric drying until the 1980s. Growth rates of both populations declined during and after the 1980s when growing-season temperature and atmospheric vapour pressure deficit continued to increase. Our model-based interpretations of the cellulose stable isotopes indicate that positive treeline growth anomalies prior to the 1980s were related to increased stomatal conductance and leaf-level transpiration and photosynthesis. Reduced growth since the 1980s occurred with a shift to more conservative leaf gas exchange in both the treeline and below-treeline populations, whereas leaf-level photosynthesis continued to increase in response to rising atmospheric CO2 concentrations. Our results suggest that warming-induced atmospheric drying confounds positive growth responses of apparent temperature-limited bristlecone pine populations at treeline. In addition, the observed ecophysiological responses of attitudinally separated bristlecone pine populations illustrate the sensitivity of conifers to climate change. [ABSTRACT FROM AUTHOR]

It is noteworthy that the largest part of global vegetation biomass depends on a thin layer of cells: the vascular cambium. Understanding the wood formation processes and relationships with environmental factors is a crucial and timely research question requiring interdisciplinary efforts, also to upscale the information gained and to evaluate implications for tree growth and forest productivity. We provide an overview of wood formation processes up to tree-ring development, bearing in mind that the combined action of intrinsic factors and environmental drivers determines the anatomical traits of a tree ring formed at a specific time and position within the tree's architecture. After briefly reviewing intrinsic factors, we focus attention on environmental drivers highlighting how a correct interpretation of environmental signals in tree rings must be grounded in a deep knowledge of xylogenesis and consequent wood anatomical traits. We provide guidelines on novel methods and approaches recently developed to study xylem formation. We refer to existing literature on established techniques for retrospective analyses in tree-ring series of anatomical and isotopic traits, to assess long-term ecophysiological responses to environmental variations, also giving advice on possible bias because of interand within-tree variability. Finally, we highlight that, once the temporal axis of intra-annual tree-ring variability of xylem traits is established by xylogenesis analysis, a multidisciplinary approach linking classical dendro-ecology, wood functional traits (dendro-anatomy) and eco-physiology (here focusing on dendro-isotopes) allows a better interpretation of past environmental events hidden in tree rings, and more reliable forecasts of wood growth in response to climate change. [ABSTRACT FROM AUTHOR]

As climate models predict increasing temperatures in Central Europe by 2.7–4°C, a decrease in summer precipitation by 21–28% toward the end of the 21st century and increased frequency and duration of extreme weather events like severe droughts, the question of how plants cope with imminent drought conditions is highly important. There have been numerous studies on tree mortality, but the underlying physiological mechanisms are still not fully understood. In this study we apply a conceptual model (Gessler et al. 2018) combining tree-ring width, and tree-ring carbon and oxygen isotopes. On the basis of the growth history and the stable isotope derived intrinsic water-use efficiency of trees the model implies the occurrence of four types of dying trees. These four combinations might be an indication of a tree's predisposition towards hydraulic failure or carbon starvation. Here we compare dead and surviving trees from a given species and location, investigating their annual radial growth and tree-ring isotopic signatures. We selected three species and sites for our study. Two species, Pinus sylvestris and Fagus sylvatica, were sampled in an inner-alpine dry valley, the Valais in Switzerland. Pine decline is a long-lasting issue there, and beech decline occurs more frequently since a couple of years with particularly dry growing seasons. The third site is located in the Thimphu Valley in the central mountainous region of Bhutan. Here our Bhutanese colleagues observe increasing decline of Pinus wallichiana. Using three different species growing in two different climatic zones under different ecological conditions (coniferous, broad-leafed, European and Himalayan), we hope to find not only intraspecific trait variability, but also interspecific differences in the respective mortality processes. Overall, this approach will provide us with a deepened understanding of the predisposing factors as well as the general physiological mechanisms of drought-induced mortality. Gessler A, Cailleret M, Joseph J, Schönbeck L, Schaub M, Lehmann M, Treydte K, Rigling A, Timofeeva G, Saurer M (2018) Drought induced tree mortality – a tree-ring isotope based conceptual model to assess mechanisms and predispositions. New Phytologist 219, 485-490. [ABSTRACT FROM AUTHOR]

Among the environmental factors that have an effect on the isotopic signature of tree rings, the specific impact of soil moisture on the Δ13C and, in particular, the δ18O ratios has scarcely been investigated. We studied the effects of soil type and soil moisture (from moderately moist [Cambisol] to wet [Gleysol]) on the growth and isotopic signature of tree rings of Norway spruce (Picea abies [L.] H. Karst.), a widely distributed forest tree species in Central Europe, at a small spatial scale in a typical mature forest plantation in the low mountain ranges of Western Germany. The δ18O ratios were lower in rings of trees growing at the wettest microsite (Gleysol) than in tree rings from the microsite with moderately moist soil (Cambisol). This indicates higher uptake rates of 18O-unenriched soil water at the Gleysol microsite and corresponds to less negative soil water potentials and higher transpiration rates on the Gleysol plots. Contrary to our expectations, the basal area increments, the Δ13C ratios and the intrinsic water-use efficiency (calculated on the basis of δ13C) did not differ significantly between the Cambisol and the Gleysol microsites. For average values of each microsite and year investigated, we found a significantly positive correlation between δ13C and δ18O, which indicates a consistent stomatal control over gas exchange along the soil moisture gradient at comparable relative air humidity in the stand. As δ18O ratios of tree rings integrate responses of wood formation to soil moisture over longer periods of time, they may help to identify microsites differing in soil water availability along small-scale gradients of soil moisture under homogeneous climatic conditions and to explain the occurrence of particular tree species along those gradients in forest stands. [ABSTRACT FROM AUTHOR]

Understanding how climate affects xylem formation is critical for predicting the impact of future conditions on tree growth and functioning in the Mediterranean region, which is expected to face warmer and drier conditions. However, mechanisms of growth response to climate at different temporal scales are still largely unknown, being complicated by separation between spring and autumn xylogenesis (bimodal temporal pattern) in most species such as Mediterranean pines. We investigated wood anatomical characteristics and carbon stable isotope composition in Mediterranean Pinus pinea L. along tree-ring series at intra-ring resolution to assess xylem formation processes and responses to intra-annual climate variability. Xylem anatomy was strongly related to environmental conditions occurring a few months before and during the growing season, but was not affected by summer drought. In particular, the lumen diameter of the first earlywood tracheids was related to winter precipitation, whereas the size of tracheids produced later was influenced by mid-spring precipitation. Diameter of latewood tracheids was associated with precipitation in mid-autumn. In contrast, tree-ring carbon isotope composition was mostly related to climate of the previous seasons. Earlywood was likely formed using both recently and formerly assimilated carbon, while latewood relied mostly on carbon accumulated many months prior to its formation. Our integrated approach provided new evidence on the short-term and carry-over effects of climate on the bimodal temporal xylem formation in P. pinea. Investigations on different variables and time scales are necessary to disentangle the complex climate influence on tree growth processes under Mediterranean conditions. [ABSTRACT FROM AUTHOR]

During recent decades, stable oxygen isotopes derived from tree-ring cellulose (δ18OTRC) have been frequently utilised as the baseline for palaeoclimatic reconstructions. In this context, numerous studies take advantage of the high sensitivity of trees close to their ecological distribution limit (high elevation or high latitudes). However, this increases the chance that indirect climatic forces such as cold ground induced by permafrost can distort the climate-proxy relationship. In this study, a tree stand of sub-alpine larch trees (Larix decidua Mill.) located in an inner alpine dry valley (Val Bever), Switzerland, was analysed for its δ18OTRC variations during the last 180 years. A total of eight L. decidua trees were analysed on an individual base, half of which are located on verified sporadic permafrost lenses approximately 500 m below the expected lower limit of discontinuous permafrost. The derived isotope time series are strongly dependent on variations in summer temperature, precipitation and large-scale circulation patterns (geopotential height fields). The results demonstrate that trees growing outside of the permafrost distribution provide a significantly stronger and more consistent climate-proxy relationship over time than permafrost-affected tree stands. The climate sensitivity of permafrost-affected trees is analogical to the permafrost-free tree stands (positive and negative correlations with temperature and precipitation, respectively) but attenuated partly leading to a complete loss of significance. In particular, decadal summer temperature variations are well reflected in δ18OTRC from permafrost-free sites (r = 0.62, p < 0.01), while permafrost-affected sites demonstrate a full lack of this dependency (r = 0.30, p > 0.05). Since both tree stands are located just a few meters away from one another and are subject to the same climatic influences, discrepancies in the isotope time series can only be attributed to variations in the trees' source water that constraints the climatic fingerprints on δ18OTRC. If the two individual time series are merged to one local mean chronology, the climatic sensitivity reflects an intermediate between the permafrost-free and –affected δ18OTRC time series. It can be deduced, that a significant loss of information on past climate variations arises by simply averaging both tree stands without prior knowledge of differing subsurface conditions. [ABSTRACT FROM AUTHOR]

The increased frequency and intensity of warming-induced droughts have triggered dieback episodes affecting many forest types and tree species worldwide. Tree plantations are not exempt as they can be more vulnerable to drought than natural forests because of their lower structural and genetic diversity. Therefore, disentangling the physiological mechanisms leading to growth decline and tree mortality can provide tools to adapt forest management to climate change. In this study, we investigated a Pinus nigra Arn. plantation situated in northern Spain, in which some trees showed canopy dieback and radial-growth decline. We analyzed how radial growth and its responses to drought events differed between non-declining (ND) and declining (D) trees showing low and high canopy defoliation, respectively, in combination with carbon (δ13C) and oxygen (δ18O) isotope ratios in tree rings. The radial growth of P. nigra was constrained by water availability during the growing season and the previous autumn. The radial growth of D trees showed higher sensitivity to drought than ND trees. This fact is in accordance with the lower drought resilience and negative growth trends observed in D trees. Both tree classes differed in their growth from 2012 onwards, with D trees showing a reduced growth compared to ND trees. The positive δ13C-δ18O relationship together with the uncoupling between growth and intrinsic water-use efficiency suggest that D trees have less tight stomatal regulation than ND trees, which could involve a high risk of xylem embolism in the former class. Our results suggest that different water use strategies between coexisting ND and D trees were behind the differences in growth patterns and point to hydraulic failure as a possible mechanism triggering dieback and growth decline. [ABSTRACT FROM AUTHOR]