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The European Dendroecological Fieldweek (EDF) provides an intensive learning experience in tree-ring research for anyone approaching or working in dendrochronology. Here we present an overview of scientific activities of the 31st EDF, held in Val Müstair, Switzerland, in summer 2021. Despite the COVID-19 pandemic, the EDF gathered 20 participants and 10 instructors (7 dendrochronologists and 3 local experts) from 10 European countries and provided valuable outcomes for the local stakeholders such as Biosfera, the forest service and the private-public of Val Müstair.During the eight days of the EDF, six groups developed different tree-ring projects, carefully designed with respect to the loal environmental setting. The dendroarchaeology group dated two buildings, an abandonned stable and a house in Val Müstair, providing private owners with accurate construction dates. The dendroclimatology group explored the potential of a relict Scots pine forest growing at ~2000 m asl for climate reconstruction, and created a chronology from 1648 to 2020 CE. The wood anatomy group found that the larger vessel sizes and and higher radial growth rates of two alpine shrub species at moist compared to dry sites, while tree ages were similar at both sites. The Blue Intensity group identified a robust climate signal in the BI chronology of high-elevation Norway spruce trees, which was significantly stronger than in the tree-ring width chronology. One dendroecology group found that growth of local larch trees recorded outbreaks of the grey larch budmoth between 1880 and 1980, a stop of outbreaks after and its return in 2018. A second dendroecology group investigated larch trees along an abandoned irrigation channel and could not detect a significant effect of the irrigation stop on growth.The EDFs continuously provide an essential service to the dendrochronological community, and this even during challenging times. The 31st EDF was again an educational, scientific and multi-cultural experience in a unique environmental setting. It resulted in highly interesting and valuable scientific outreach and opened up new avenues for future tree-ring research in Val Mustair.

[Motivation] Trait variation within species can reveal plastic and/or genetic responses to environmental gradients, and may indicate where local adaptation has occurred. Here, we present a dataset of rangewide variation in leaf traits from seven of the most ecologically and economically important tree species in Europe. Sample collection and trait assessment are embedded in the GenTree project (EU-Horizon 2020), which aims at characterizing the genetic and phenotypic variability of forest tree species to optimize the management and sustainable use of forest genetic resources. Our dataset captures substantial intra- and interspecific leaf phenotypic variability, and provides valuable information for studying the relationship between ecosystem functioning and trait variability of individuals, and the response and resilience of species to environmental changes., [Main types of variable contained] We chose morphological and chemical characters linked to trade-offs between acquisition and conservation of resources and water use, namely specific leaf area, leaf size, carbon and nitrogen content and their ratio, and the isotopic signature of stable isotope 13C and 15N in leaves., [Spatial location and grain] We surveyed between 18 and 22 populations per species, 141 in total, across Europe., [Time period] Leaf sampling took place between 2016 and 2017., [Major taxa and level of measurement] We sampled at least 25 individuals in each population, 3,569 trees in total, and measured traits in 35,755 leaves from seven European tree species, i.e. the conifers Picea abies, Pinus pinaster and Pinus sylvestris, and the broadleaves Betula pendula, Fagus sylvatica, Populus nigra and Quercus petraea., [Software format] The data files are in ASCII text, tab delimited, not compressed., All authors acknowledge support from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 676876 (Project GenTree). Laboratory work was also supported by other projects: COMEDIAS (CGL2017- 83170-R, Spanish Ministry of Science, Innovation and Universities), REMEDINAL IV (TE-CM S2018/EMT-4338, Project of the Autonomous Community of Madrid, Spain), SPONFOREST (BIODIVERSA-3, PCIN-2016-055), and International Laboratory of Global Change (LINCGlobal 4540-143AP). The Swiss contribution was supported by the Swiss Secretariat for Education, Research and Innovation (SERI) under contract no. 6.0032. BC was funded by a Scholarship from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES (Brazil).

The impacts of climate extremes on forest ecosystems are still poorly understood but important for predicting carbon and water cycle feedbacks to climate. Despite evidences of the different climatic thresholds of tree carbon source and sink activities, implementations of mechanistic growth components into dynamic global vegetation models (DGVM) remain challenged by lacking observational data into the most important terrestrial biological sink, i.e.; into the cambial zone of tree stems.In this study, we aim to provide a framework for mechanistic understanding of drought impacts on carbon and water dynamics based on accurate analyses of the physiological processes that indirectly regulate these budgets. We quantified the drought impact and resilience of intra-annual carbon sequestration and water use in four mature Norway spruces from a Swiss subalpine site by comparing high-resolution growth (i.e., xylogenesis and wood anatomy) and physiological (i.e., stable carbon isotope ratios) data from an exceptional dry summer (year 2015) with those from a regular growing season (year 2014).Our observations described the cascade of impacts from leaf physiology to cambial activity during and after a 41-day period of physiological water deficit. During water deficit, all wood formation processes were strongly reduced diminishing carbon sequestration by 67% despite a 11% increased water-use efficiency. However, with the recovery of the positive hydric state in the stem, we observed a fast recovery of the rates of the different cell formation phases at the expenses of the accumulated assimilates produced during the drought event.Our results clarify how the interaction between source and sink is modulated via external environmental factors and provide evidence that, under specific circumstances, tree growth can be extremely resilient. These novel findings should provide a framework to improve sink model components in DGVMs and consequently help to bridge understanding of carbon and water fluxes between atmosphere and forest ecosystems.

Oxygen isotopes (δ18O) in tree rings carry a strong potential to retrospectively evaluate tree water uptake and physiological response to climate. Their interpretation can, however, be challenging due to the complexity of the isotopic fractionations along the soil-tree-atmosphere continuum. Indeed, several processes play a role in defining the final tree-ring isotopic signal: source water variations, evaporative processes at the soil surface and leaf level, and mixing of xylem water that might exchange with new assimilates associated with phloem transport and synthesis of wood constituents. Disentangling these influences along the growing season and how climate conditions modify them are remaining challenges to exploit the full potential of δ18O tree-ring records as a climate proxy.In this study, we aim at identifying the contribution of leaf water enrichment and source water on the tree-ring δ18O signature by assessing intra-annual variations of δ18O along the soil-leaf-tree ring pathway of larch (Larix decidua Mill.). We focus on two sites with contrasting water availability in the Lötschental valley (Swiss Alps) and three consecutive growing seasons (2011-2013). Our approach takes into consideration specific timing of the involved processes with a high spatio-temporal resolution: environmental conditions, diurnal sapflow-derived transpiration rates, δ18O analysis of xylem and leaf water, and intra-annual tree-ring δ18O measurements coupled with wood formation kinetics. Structural equation models (SEM) were applied to statistically assess the relations among δ18O values of the different pathway components. Furthermore, we calibrated mechanistic models of leaf-water and tree-ring cellulose δ18O to explore site-specific contributions of the fractionation processes (e.g., Péclet effect and the proportion of xylem-cellulose synthesis exchange [Pex]) and investigated their climatic drivers.Our results showed that intra-annual xylem water δ18O and transpiration rates differed between sites and years whereas needle water δ18O did not differ significantly between sites (but between years). However, tree-ring cellulose δ18O values were higher at the dry site resembling those differences observed in xylem water δ18O. SEMs reinforced these results since xylem water δ18O contributed more to cellulose δ18O in comparison to needle water δ18O, and this effect was more prominent at the dry site. Vapor pressure deficit (VPD) had strong control on the overall leaf water-related 18O-fractionations. However, mechanistic leaf-water δ18O models did not indicate a relevant role of the Péclet effect in our study. Most importantly, mechanistic models of cellulose δ18O revealed that Pex was variable along the growing season and its variability was significantly associated with variations in VPD.Our study suggests that the imprint of the source water signal on the δ18O signature in tree rings is highly dominant, particularly during episodes of high VPD, potentially overwriting signals coming from leaf fractionation processes.

Stable carbon (δ13Cc) and oxygen (δ18Oc)isotopes measured in tree-ring cellulose, together with tree-ring width (TRW), have been used extensively to investigate the effects of past climatic conditions on tree growth. By contrast, the information recorded by the third major chemical component of tree-ring cellulose, the non-exchangeable carbon-bound hydrogen, has been explored far less due to methodological drawbacks and lack of understanding of 2H-specific fractionations. In this first Europe-wide assessment we investigate the signals stored in the hydrogen isotope ratios in tree-ring cellulose (δ2Hc), from a unique collection of 100-years records, from two major genera (Pinus and Quercus) across 17 sites (36°N to 68°N).The climate correlation analyses showed weak climate signals in the δ2Hc high-frequency chronologies, compared to those recorded by δ13Cc and δ18Oc, but similar to the TRW ones. The δ2Hc climate signal strength varied across the continent and was stronger and more consistent for Pinus than for Quercus. The δ2Hc inter-annual variability was strongly site-specific. Focusing on the effect of extreme climatic conditions during years with extremely dry summers, we observed a significant 2H-enrichment in tree-ring cellulose for both genera. Our findings clearly indicate that δ2Hc registers information about hydrology and climate, but it also records non-climatic signals such as physiological mechanisms associated with carbohydrates storage remobilization 2H-specific fractionations and growth.To disentangle the climatic and non-climatic signals in δ2Hc, we investigated its relationships with δ18Oc and TRW. We found significant relationships negative between δ2Hc and TRW at 7 sites and positive between δ2Hc and δ18Oc at 10 sites, while the rest of the sites did not show any significant relationships. The agreement with the TRW chronologies confirms the relationship between growth and δ2Hc, while the divergencefrom δ18O suggests a loss of the hydrological signal in δ2Hc. These highlights, once again,a stronger physiological component in the δ2H signature independent from climate. Advancements in the understanding of 2H-fractionations and their relationships with climate, physiology, and species-specific traits are therefore needed to improve the mechanistic modeling and interpretation accuracy of δ2Hc in plant physiology and paleoclimatology. Such advancements could lead to new insights into trees’ carbon allocation mechanisms, and responses to abiotic and biotic stressors.

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., New Phytologist, 236 (1), ISSN:0028-646X, ISSN:1469-8137

In this chapter we introduce the climate signal in stable isotope tree-ring records, with the emphasis on temperate forests. The development of the subdiscipline is recapped followed by an exploration of isotope dendroclimatic records by geography and, broadly, by isotopic species. Whilst there are still questions to be answered around signal strength and age-related effects in different environments and in different species, the proxy is now contributing to palaeoclimatology in a far greater way than in the days of the first hints of ‘isotope tree thermometers’. We include two summary tables. Table 19.1 exemplifies the range of climate information available from stable carbon isotope time series and Table 19.2 explores oxygen isotope proxy signals. Due to the greater complexity seen in stable carbon isotope interpretations we explore response groupings with example references given for each category of proxy response. Finally, we summarize the state of the art in isotope dendroclimatology and discuss possible future directions.

Temperature and precipitation variability throughout the year control the intra-annual dynamics of tree-ring formation. Physiological adaptation of trees to climate change is among the key issues to better understand and predict future forest performance and composition. In this study, we investigated the species’ coexistence and performance of Scots pine and pubescent oak growing in a mixed sub-Mediterranean forest in the northeast of the Iberian Peninsula. We assessed intra-annual cumulative growth patterns derived from band dendrometers during four consecutive growing seasons and long-term changes in basal area increment for the period 1950–2014. Our results revealed that Scots pine followed an intra-annual bimodal growth pattern. Scots pine growth was mainly limited by water availability at intra-annual, interannual and decadal time scales, which resulted in a negative long-term growth trend. Conversely, oak displayed a unimodal growth pattern, which was less climatically constrained. A significant increase in basal area of oak denotes an overall better potential acclimation to prevailing climatic conditions at the expenses of a higher risk of physiological failure during extreme climate events.

Warmer climate and more frequent extreme droughts will pose major threats to forest ecosystems. Past demography processes due to post-glacial recolonization and adaptation to local environmental conditions are among the main contributors to genetic differentiation processes among provenances. Assessing the intra-specific variability of tree growth responses to such changes is crucial to explore a species' potential to cope with climate warming. We combined growth-related traits derived from tree-ring width series with neutral genetic information of 18 European provenances of silver fir (Abies alba Mill.) growing in two common garden experiments in Switzerland. Analyses based on neutral single nucleotide polymorphisms (SNPs) revealed that the studied provenances grouped into three longitudinal clusters. These three genetic clusters showed differences in growth traits (height and DBH), with the provenances from the eastern cluster exhibiting the highest growth. The Pyrenees cluster showed significantly lower recovery and resilience to the extreme drought of 2003 as well as lower values of growth autocorrelation. QST-FST and correlation analyses with climate of provenance origin suggest that the differences among provenances found in some traits result from natural selection. Our study suggests that the last post-glacial re-colonization and natural selection are the major drivers explaining the intra-specific variability in growth of silver fir across Europe. These findings highlight the importance of combining dendroecology and genetic analyses on fitness-related traits to assess the potential of a species to cope with global environmental change and provide insights to support assisted gene flow to ensure the persistence of the species in European forests., Science of The Total Environment, 779, ISSN:0048-9697, ISSN:1879-1026

1. Plant species that grow across environmental gradients show a range of trait ex-pression, but traits do not vary independently. In fact, phenotypes are integrated expressions of multiple traits that covary due to trade-offs among functions and processes. Understanding trait covariation structures will ultimately help with predicting species' responses to change and developing management actions., 2. We measured variation and covariation (a proxy of phenotypic integration) among functional traits of Pinus sylvestris from paired populations across its European distribution. Populations within a pair were close enough to be in gene flow con-tact but located in contrasting environmental conditions across a local gradient. Measured traits represented three axes of variation (groups of traits) related to a tree's competitive ability and the trade-off between resource acquisition and conservation, namely plant size measures and stem and foliar traits., 3. Results revealed important intra- and inter-population trait variability. In particular, at the population level, trait means shifted across the climatic gradient mainly de-scribed by mean annual temperature. Moreover, we found a higher degree of trait covariation in populations under harsher environments (i.e. lower environmental suitability for the species). This pattern was consistent within population pairs, suggesting that higher trait covariation may be adaptive, being more coordinated in sites with harsher conditions. At larger spatial scales, we found a less conclusive pattern with a trend of increasing covariation at the northern edge of the species distribution. This result suggests that at larger scales different processes may be in-volved in the trade-off between the adaptive value of phenotypic covariation ver-sus its constraints on trait combinations that may limit plant's response capability., 4. Synthesis. Trait covariation varies at different spatial scales, increasing under harsher conditions, and the robustness and repeatability of this pattern suggests its adaptive role for the species' responses to different environments., Funding was provided by the European Union Horizon 2020 Project GenTree (Grant Agreement No. 676876). Extra support came from the Spanish projects REMEDINAL TE-CM (Autonomous Community of Madrid, S2018/EMT-4338), Phenotypes (PGC2018-099115-B-100), COMEDIAS (CGL2017-83170-R, Spanish Ministry of Science, Innovation and Universities), and Swiss Secretariat for Education, Research and Innovation (SERI) under contract No. 6.0032. R.B. is now funded by the Contrato-Programa CEEC Individual, Fundação para a Ciência e Tecnologia, Portugal. B.C. was funded by CAPES fellowship (DOC-PLENO -Programa de Doutorado Pleno no Exterior. Grant Agreement No. 99999.001266/2015-02, Brazil). C.C.B. was supported by a post-doctoral fellowship of the Ramón Areces Foundation.

Recent studies have identified strong relationships between delayed recovery of tree growth after drought and tree mortality caused by subsequent droughts. These observations raise concerns about forest ecosystem services and post-drought growth recovery given the projected increase in drought frequency and extremes. For quantifying the impact of extreme droughts on tree radial growth, we used a network of tree-ring width data of 1689 trees from 100 sites representing most of the distribution of two drought tolerant, deciduous oak species (Quercus petraea and Quercus robur). We first examined which climatic factors and seasons control growth of the two species and if there is any latitudinal, longitudinal or elevational trend. We then quantified the relative departure from pre-drought growth during droughts, and how fast trees were able to recover the pre-drought growth level. Our results showed that growth was more related to precipitation and climatic water balance (precipitation minus potential evapotranspiration) than to temperature. However, we did not detect any clear latitudinal, longitudinal or elevational trends except a decreasing influence of summer water balance on growth of Q. petraea with latitude. Neither species was able to maintain the pre-drought growth level during droughts. However, both species showed rapid recovery or even growth compensation after summer droughts but displayed slow recovery in response to spring droughts where none of the two species was able to fully recover the pre-drought growth-level over the three post-drought years. Collectively, our results indicate that oaks which are considered resilient to extreme droughts have also shown vulnerability when droughts occurred in spring especially at sites where long-term growth is not significantly correlated with climatic factors. This improved understanding of the role of drought seasonality and climate sensitivity of sites is key to better predict trajectories of post-drought growth recovery in response to the drier climate projected for Europe., Science of The Total Environment, 784, ISSN:0048-9697, ISSN:1879-1026

Climate change may reduce forest growth and increase forest mortality, which is connected to high carbon costs through reductions in gross primary production and net ecosystem exchange. Yet, the spatiotemporal patterns of vulnerability to both short-term extreme events and gradual environmental changes are quite uncertain across the species' limits of tolerance to dryness. Such information is fundamental for defining ecologically relevant upper limits of species tolerance to drought and, hence, to predict the risk of increased forest mortality and shifts in species composition. We investigate here to what extent the impact of short- and long-term environmental changes determines vulnerability to climate change of three evergreen conifers (Scots pine, silver fir, Norway spruce) and two deciduous hardwoods (European beech, sessile oak) tree species at their southernmost limits of distribution in the Mediterranean Basin. Finally, we simulated future forest growth under RCP 2.6 and 8.5 emission scenarios using a multispecies generalized linear mixed model. Our analysis provides four key insights into the patterns of species' vulnerability to climate change. First, site climatic marginality was significantly linked to the growth trends: increasing growth was related to less climatically limited sites. Second, estimated species-specific vulnerability did not match their a priori rank in drought tolerance: Scots pine and beech seem to be the most vulnerable species among those studied despite their contrasting physiologies. Third, adaptation to site conditions prevails over species-specific determinism in forest response to climate change. And fourth, regional differences in forests vulnerability to climate change across the Mediterranean Basin are linked to the influence of summer atmospheric circulation patterns, which are not correctly represented in global climate models. Thus, projections of forest performance should reconsider the traditional classification of tree species in functional types and critically evaluate the fine-scale limitations of the climate data generated by global climate models.

The dataset presented here was collected by the GenTree project (EU-Horizon 2020), which aims to improve the use of forest genetic resources across Europe by better understanding how trees adapt to their local environment. This dataset of individual tree-core characteristics including ring-width series and whole-core wood density was collected for seven ecologically and economically important European tree species: silver birch (Betula pendula), European beech (Fagus sylvatica), Norway spruce (Picea abies), European black poplar (Populus nigra), maritime pine (Pinus pinaster), Scots pine (Pinus sylvestris), and sessile oak (Quercus petraea). Tree-ring width measurements were obtained from 3600 trees in 142 populations and whole-core wood density was measured for 3098 trees in 125 populations. This dataset covers most of the geographical and climatic range occupied by the selected species. The potential use of it will be highly valuable for assessing ecological and evolutionary responses to environmental conditions as well as for model development and parameterization, to predict adaptability under climate change scenarios., Measurement(s)growth ring • wood densityTechnology Type(s)measuring table • calculationFactor Type(s)tree speciesSample Characteristic - OrganismBetula pendula • Fagus sylvatica • Picea abies • Populus nigra • Pinus pinaster • Pinus sylvestris • Quercus petraeaSample Characteristic - LocationEurope Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.11294993

The increase of greenhouse gas (GHG) emissions into the atmosphere is promoting and accelerating climate warming. Among GHG sources, soils are an important natural source of GHG to the atmosphere through aerobic soil respiration that release carbon dioxide (CO2). However, in riparian areas, soils can also release relevant amounts of methane (CH4) and nitrous oxide (N2O) through anaerobic processes promoted by high groundwater levels or flooded conditions. Recent studies have highlighted the role of trees in CH4 emissions, but little is still known about the origin of these emissions, the processes involved, and their contribution to the global carbon and nitrogen cycles. To shed light on this issue, we measured GHG emissions (i.e. CO2, CH4, and N2O) from the stems of two riparian tree species (Fraxinus agustifolia and Quercus robur) located along a gradient of soil moisture conditions (i.e. from wet to completely flooded soils) in a Mediterranean floodplain forest. Moreover, we also analyzed the isotopic carbon signature of the GHG emitted and the microbial communities inhabiting within tree stems by 16S rRNA gene analysis. Our results showed that CH4 emitted by riparian tree stems was 100-fold higher at the flooded than at wet soil locations, while CO2 and N2O emissions did not vary across moisture conditions. When considering together emissions form soil surface and tree stems under flooded conditions, riparian trees contributed up to 20%, 40% and 60% of the total CH4, CO2, and N2O emissions, respectively. Keeling plots suggested that CO2 emitted through tree stems was produced within the soil compartment and thus transported to the atmosphere through the tree stems, whereas CH4 emissions may have a different origin. However, methanogens were almost absent on the wood microbiome. The substantially higher presence of methanotrophs on the wood than on the soil compartment suggested that, despite CH4 emitted by stems could come from soil microbial activity, the microbial consumption of that CH4 within the tree stem could have changed its isotopic signature. Overall, our findings suggest that the riparian trees growing in this Mediterranean floodplain forest may mainly act as passive transporters of GHG produced in soils instead of being active GHG producers.

The dataset presented here was collected by the GenTree project (EU-Horizon 2020), which aims to improve the use of forest genetic resources across Europe by better understanding how trees adapt to their local environment. This dataset of individual tree-core characteristics including ring-width series and whole-core wood density was collected for seven ecologically and economically important European tree species: silver birch (Betula pendula), European beech (Fagus sylvatica), Norway spruce (Picea abies), European black poplar (Populus nigra), maritime pine (Pinus pinaster), Scots pine (Pinus sylvestris), and sessile oak (Quercus petraea). Tree-ring width measurements were obtained from 3600 trees in 142 populations and whole-core wood density was measured for 3098 trees in 125 populations. This dataset covers most of the geographical and climatic range occupied by the selected species. The potential use of it will be highly valuable for assessing ecological and evolutionary responses to environmental conditions as well as for model development and parameterization, to predict adaptability under climate change scenarios. Measurement(s) growth ring • wood density Technology Type(s) measuring table • calculation Factor Type(s) tree species Sample Characteristic - Organism Betula pendula • Fagus sylvatica • Picea abies • Populus nigra • Pinus pinaster • Pinus sylvestris • Quercus petraea Sample Characteristic - Location Europe Published

The study of tree rings can reveal long-term records of a tree's response to the environment. This dendroecological approach, when supplemented with finer-scale observations of the xylem anatomy, can provide novel information about a tree's year-to-year anatomical and hydraulic adjustments. Here we use this method in aspen (Populus tremuloides Michx.) to identify xylem response to drought and insect defoliation. Surprisingly, we found that precipitation influenced vessel diameter mostly in the trees' youth, while this correlation was less pronounced at maturity. This is likely due to a reduction in stress the stand experiences as it ages, and reflects an ability to mediate drought stress as trees mature. Defoliation events caused consistent and profound changes in fiber anatomy likely leading to reduced structural support to vessels. We therefore expect that in years of defoliation trees may be vulnerable to drought-induced cavitation when leaf area recovers. This study highlights how the inclusion of cellular level measurements in tree ring studies provides additional information on how stress events may alter tree functioning through alterations in structure.

The ratio of leaf internal (ci ) to ambient (ca ) partial pressure of CO2 , defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation model uncertainties for predicting terrestrial carbon uptake and water use. We test a theory based on the least-cost optimality hypothesis for modelling historical changes in χ over the 1951-2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely dated tree-ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure. The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the intersite variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model. Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant-available soil water and other site-specific characteristics might improve the predictions.

During the last 25 years, the dendrochronology community has benefited from the application of stable carbon isotopes in tree rings to answer a number of research questions, which has resulted in an exponential burst in the number of publications related to this discipline. Here, we perform a literature review of the most influential topics (scope, methodology, targeted species, and climatic signals) that have shaped the research agenda and its impact on the scholarly output over the recent decades. Based on a total of 550 publications, we observed that: (1) conifers are the most investigated tree functional type, being included in over two-thirds of studies; (2) although being a subject of debate, the material of choice for carbon isotope analyses is still cellulose, for which the extraction procedure has been refined with methodological improvements over the years; (3) a shift in the application of carbon isotopes has occurred from proxies of climatic information for paleoclimatic reconstructions to tools for the understanding of short- and long-term tree functioning; and (4) such shift in research scope is apparently linked to an increasing number of publications showing drought-related signals governing isotope time series in the context of current climate change. Besides, we also assessed the number and outcome of publications analyzing 13C-derived intrinsic water-use efficiency (WUEi) trends within the framework formulated by Saurer et al. (2004). We found that leaf intercellular CO2 concentration responding proportionally to the increase in atmospheric CO2 is the most common scenario reported across continents, but with changing importance relative to other scenarios depending on the biome examined. We also detected that climate plays a dominant role over any potential CO2 fertilization linked to increasing WUEi. Looking for new and thrilling research avenues, future studies should further examine δ13C fluctuations at intra-annual resolution, explore its links to wood characteristics, and disentangle δ13C signals of the distinct wood constituents for a better understanding of tree functioning at varying spatiotemporal scales, also benchmarking vegetation models of different complexities.

In forests, the increase of atmospheric CO2 concentrations (Ca) has been related to enhanced tree growth and intrinsic water-use efficiency (iWUE). However, in drought-prone areas such as the Mediterranean Basin it is not yet clear to what extent this ‘fertilizing’ effect may compensate for drought-induced growth reduction. We investigated tree growth and physiological responses at five Scots pine (Pinus sylvestris L.) and five sessile oak (Quercus petraea (Matt.) Liebl.) sites located at their southernmost distribution limits in Europe for the period 1960-2012 using annually resolved tree-ring width and δ13C data to track ecophysiological processes. Results indicated that all ten natural stands significantly increased their leaf intercellular CO2 concentration (Ci), and consequently iWUE. Different trends in the theoretical gas exchange scenarios as a response to increasing Ca were found: generally, Ci tended to increase proportionally to Ca, except for trees at the driest sites in which Ci remained constant. Ci from the oak sites displaying higher water availability tended to increase at a comparable rate to Ca. Multiple linear models fitted at site level to predict basal area increment (BAI) using iWUE and climatic variables better explained tree growth in pines (31.9 - 71.4%) than in oak stands (15.8 - 46.8%). iWUE was negatively linked to pine growth whereas its effect on growth of oak differed across sites. Tree growth in the western and central oak stands was negatively related to iWUE, whereas BAI from the easternmost stand was positively associated with iWUE. Thus, some Q. petraea stands might have partially benefited from the ‘fertilizing’ effect of rising Ca, whereas P. sylvestris stands due to their strict closure of stomata did not profit from increased iWUE and consequently showed in general growth reductions across sites. Additionally, the inter-annual variability of BAI and iWUE displayed a geographical polarity in the Mediterranean. This article is protected by copyright. All rights reserved.

Climate change-induced rise of air temperatures and the increase of extreme climatic events, such as droughts, will largely affect plant growth and hydraulics, leading to mortality events all over the globe. In this study, we investigated the growth and hydraulic responses of seedlings of contrasting functional types. Pinus sylvestris, Quercus spp. and Castanea sativa seedlings were grown in a common garden experiment under four treatments: control, air warming, drought and their combination during two consecutive growing periods. Height and diameter increments, stomatal conductance and stem water potentials were measured during both growing seasons. Additionally, hydraulic parameters such as xylem-specific native and maximum hydraulic conductivities, and native percentage of loss of conductivity were measured at the end of the entire experiment. Our results clearly pointed to different adaptive strategies of the studied species. Scots pine displayed a relatively isohydric behavior with a strict stomata control prohibiting native embolism whereas sweet chestnut and oak as relatively anisohydric species displayed an increased loss of native conductivity as a results of low water potentials. Seasonal timing of shoot and diameter growth also differed among functional types influencing drought impacts. Additionally, the possibility of embolism reversal seemed to be limited under the study conditions.

Warmer and drier climatic conditions are projected for the 21st century; however, the role played by extreme climatic events on forest vulnerability is still little understood. For example, more severe droughts and heat waves could threaten quaternary relict tree refugia such as Circum-Mediterranean fir forests (CMFF). Using tree-ring data and a process-based model, we characterized the major climate constraints of recent (1950–2010) CMFF growth to project their vulnerability to 21st-century climate. Simulations predict a 30% growth reduction in some fir species with the 2050s business-as-usual emission scenario, whereas growth would increase in moist refugia due to a longer and warmer growing season. Fir populations currently subjected to warm and dry conditions will be the most vulnerable in the late 21st century when climatic conditions will be analogous to the most severe dry/heat spells causing dieback in the late 20th century. Quantification of growth trends based on climate scenarios could allow defining vulnerability thresholds in tree populations. The presented predictions call for conservation strategies to safeguard relict tree populations and anticipate how many refugia could be threatened by 21st-century dry spells.

Drought is a key limiting factor for tree growth in the Mediterranean Basin. However, the variability in acclimation via xylem traits is largely unknown. We studied tree growth and vessel features of Quercus petraea (Matt.) Lieb. in five marginal stands across southern Europe. Tree-ring width (TRW), mean earlywood vessel area (MVA) and number of earlywood vessels (NV) as well as theoretical hydraulic conductivity (Kh) chronologies were developed for the period 1963-2012. Summer drought signals were consistent among TRW chronologies; however, climatic responses of vessel features differed considerably among sites. At the three xeric sites, previous year's summer drought had a negative effect on MVA and a positive effect on NV. In contrast, at the two mesic sites, current year's spring drought negatively affected NV, while exerting a positive influence on MVA. In both cases, Kh was not altered by this xylem adjustment. All variables revealed identical east-west geographical patterns in growth and anatomical features. Sessile oak copes with drought in different ways at xeric sites and after unfavourable previous summer conditions more but smaller vessels are built, lowering vulnerability to cavitation, whereas at mesic sites, dry springs partly lead to tree-rings with wider but fewer vessels. The variability of vessel-related features displays a similar geographical dipole in the Mediterranean Basin previously described for tree growth by other studies. © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email journals.permissions@oup.com.

Overlaid to a general reduction of European beech and sessile oak tree growth over the recent decades in the Mediterranean Basin, tree-ring records from western Mediterranean populations display a stronger growth decrease than eastern populations. We investigate here to what extent the impact of sustained atmospheric circulation patterns in summertime can explain the observed spatial patterns of tree growth. We use Canonical Correlation Analysis, a statistical method that identifies the coupled patterns that are optimally correlated between two multivariate data sets. A general change in growth trends, shifting from a general increase during the period 1950–1981 to a decrease during the last three decades (1982–2012), can be attributed to increasing summer temperatures, which exert a dominant and negative influence on growth in both tree species across sites. However, summer precipitation has gained importance for growth, coinciding with the intensification of the geographical polarity in climate conditions across the Mediterranean Basin. This intensification during the last three decades can be traced back to a strengthening of the Summer North Atlantic Oscillation (SNAO), which imparts an east-west dipole to summer climate in this region. Under predicted persistent stronger SNAO in the future, western populations would face harsher summer conditions than central and eastern rear-edge populations, due to decreasing precipitation and increasing temperatures in the western Mediterranean Basin. These results evidence the determinant role that changes in the atmospheric circulation patterns may play in the persistence of rear-edge temperate deciduous forests in the near future. © 2017 Elsevier B.V.

Many European temperate tree species reach their southern distribution limits in the Mediterranean region, and ongoing climate change will further restrict their climatic niche in this area. In this study, we investigated the effects of forest management and climate change on tree growth and the spatial extension of a silver fir forest (Abies alba Mill.) located at the species’ southern distribution limit on the Iberian Peninsula (Montseny Mountains Natural Park, Spain). Different growth variables such as tree-ring width (RW), basal area increment (BAI), earlywood width (EwW) and latewood width (LwW) were assessed, and climate-growth relationships were established for the period 1914–2010. Our results revealed that the main growth reductions and releases in the raw tree-ring width series were related to both volcanic activity and intensive logging. Since the establishment of the Natural Park in 1977, RW series have levelled off, and this has translated into an increase in BAI. This positive performance may have also facilitated the spatial expansion of the stand. Low precipitation during spring and summer was found to be the most limiting factor for tree growth during the period 1914–2010. Temperature had only a minor influence on tree growth. LwW was the growth variable most sensitive to climatic conditions. Such sensitivity explained the decreasing LwW trend since 1975. In contrast, EwW mostly depended on the previous year’s climatic conditions, and was not climatically limited during the growing season, resulting in an increasing trend over the study period. However, the temporal instability of most of these climate-growth relations indicated that climate change might have been beneficial for tree performance. Past logging events have fostered tree growth in the stand due to the increase in the availability of water, light, and nutrients, potentially alleviating the negative impacts of climate change. Furthermore, it is possible that the increase in the EwW improved water transport in the silver firs, which may also have helped them to endure ongoing climate change. Therefore, it is crucial to assess the role of forest management, as well as the potential acclimation of the tree species when considering the effects of climate change.