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Linked to major volcanic eruptions around 536 and 540 CE, the onset of the Late Antique Little Ice Age has been described as the coldest period of the past two millennia. The exact timing and spatial extent of this exceptional cold phase are, however, still under debate because of the limited resolution and geographical distribution of the available proxy archives. Here, we use 106 wood anatomical thin sections from 23 forest sites and 20 tree species in both hemispheres to search for cell-level fingerprints of ephemeral summer cooling between 530 and 550 CE. After cross-dating and double-staining, we identified 89 Blue Rings (lack of cell wall lignification), nine Frost Rings (cell deformation and collapse), and 93 Light Rings (reduced cell wall thickening) in the Northern Hemisphere. Our network reveals evidence for the strongest temperature depression between mid-July and early-August 536 CE across North America and Eurasia, whereas more localised cold spells occurred in the summers of 532, 540-43, and 548 CE. The lack of anatomical signatures in the austral trees suggests limited incursion of stratospheric volcanic aerosol into the Southern Hemisphere extra-tropics, that any forcing was mitigated by atmosphere-ocean dynamical responses and/or concentrated outside the growing season, or a combination of factors. Our findings demonstrate the advantage of wood anatomical investigations over traditional dendrochronological measurements, provide a benchmark for Earth system models, support cross-disciplinary studies into the entanglements of climate and history, and question the relevance of global climate averages.

Recent climatic changes significantly affected forest ecosystems in northern Eurasia. Trees growing in Siberia are very sensitive to climate change due to strong temperature limitation of their growth. Our study covers high-latitude (northeastern Yakutia, eastern Taimyr, central Evenkia) and high-altitude (Russian Altai) zones in Eurasia, where tree-ring parameters (tree-ring width, cell-wall thickness, and maximum latewood density) mainly record summer air temperature variations. To reveal the impact of moisture changes (e.g., amount of precipitation, vapor pressure deficit, relative humidity and potential evapotranspiration) on tree growth in Siberian forest ecosystems, we evaluated δ13C in tree-ring cellulose over the past century. We found that at all the study sites mainly June-July precipitation and June-July evapotranspiration affect larch radial growth, while the strongest influence of vapor pressure deficit on the δ13C was observed in northeastern Yakutia. Further increase of vapor pressure deficit and rise of air temperature in the coming decades in Siberian regions will probably lead to drought and related forest mortality even under additional source of water due to permafrost thaw.

Significant air temperature and precipitation changes have occurred since the 2000s in vulnerable Siberian subarctic regions and urged updates of available chronologies towards the third millennium. It is important to better understand recent climatic changes compared to the past decades, centuries and even millennia. In this study, we present the first annually resolved triple tree-ring isotope dataset (δ13C, δ18O and δ2H) for the eastern part of the Taimyr Peninsula (TAY) and northeastern Yakutia (YAK) from 1900 to 2021. We found that the novel and largely unexplored δ2H of larch tree-ring cellulose was linked significantly with δ18O for the YAK site, which was affected by averaged April–June air temperatures and evaporation. Simulated by the Land Surface Processes and Exchanges (LPX-Bern 1.0) model, the water fraction per year for soil depths at 0–20 and 20–30 cm was significantly linked with the new eco-hydrological tree-ring δ2H data. Our results suggest increasing evapotranspiration and response of trees’ water relation to rising thaw water uptake from lower (20–30 cm) soil depth. A positive effect of July air temperature on tree-ring δ18O and a negative impact of July precipitation were found, indicating dry conditions. The δ13C in larch tree-ring cellulose for both sites showed negative correlations with July precipitation and relative humidity, confirming dry environmental conditions towards the third millennium.

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).

Rapid temperature and vapor pressure deficit (VPD) increase along with precipitation decrease over the past decades lead to massive wildfires and permafrost degradation in boreal forests. Conifer trees growing in subarctic regions are highly sensitive to these climatic changes due to their location at the high latitudes, where air temperature is the limiting factor, but also water relations have a strong impact on tree growth.In this study, we aimed (i) to assess the usability of local vs. gridded climate data; (ii) to reveal how conifer trees capture temperature and moisture signals based on the local weather station data vs. gridded data from the two Siberian sites in northeastern Yakutia and eastern Taimyr, and one site from northwestern Canada in Mackenzie Delta; (iii) to perform trend analysis of climatic data and δ18O in tree-ring cellulose; and (iv) to carry out spatial correlation analysis of oxygen isotope patterns and determine the distribution of climatic signal over broad geographical scales in the Siberian and Canadian subarctic.Comparative analysis of the local and gridded climatic data (air temperature, precipitation and VPD) for the three study sites showed that mainly temperatures are highly correlated between each other. Subarctic trees grow in a temperature-limited environment; therefore, the large spatial coherence of temperature signals is not surprising. Conversely, insignificant correlations between local and gridded for precipitation and rather low correlations for VPD is attributed to the more heterogeneous nature of moisture variables at larger spatial scales. Therefore, analyzing moisture changes in the subarctic using local weather station data is advantageous compared to gridded data.Trend analysis of the climate data showed that drastic changes in climate variability occurred from the 1980s in the investigated subarctic regions and were even more pronounced from 2000 to 2021. Recent warming and development of drought conditions were stronger in the Canadian subarctic than the Siberian subarctic sites. Drastic precipitation changes, temperature and VPD increase mainly occurred during winter, spring and autumn in the studied subarctic regions. New updated stable isotope chronologies from remote subarctic regions allowed us to accurately reconstruct moisture changes using precipitation and VPD data from the local weather stations while reconstructing air temperature using gridded data.This research was funded by the Russian Science Foundation (RSF) grant number 21-17-00006.

The boreal forests are widely expanded from subarctic forest to tundra, and from taigato forest-steppe zone (from 50 °N to 70 °N). We reviewed available stable isotope chronologies in tree-ring cellulose (δ13C, δ18O and δ2H) from 16 sites located in the Russian Federation; 4 research sites from Fennoscandia (Finland, Sweden and Norway); 5 sites from Canada, and 1 site from Alaska (USA) to evaluate impact of climatic changes from seasonal to annual scale across boreal forest ecosystems. Results of our review of carbon isotope data showed that droughtconditions (mainly high vapour pressure deficit) are prevalent for western and central regions of Eurasia, Alaska and Canada, while northeastern and eastern sites of Eurasian subarctic are showing water shortage developments resulting from decreasing precipitation. Oxygen isotopechronologies show increasing trends towards the end of the twentieth century mainly for all chronologies, except for the Siberian northern and southern sites. The application of the multiple stable isotope proxies (δ13C, δ18O, δ2H) is beneficial to study responses of boreal forests to climate change in temperature-limited environments. However, a deeper knowledge of hydrogen isotope fractionation processes at the tree-ring cellulose level is needed for a sound interpretation and application of δ2H for climate reconstructions, especially for the boreal forest zone where forest ecosystems are more sensitive to climatic and environmental changes.

X-ray microdensitometry on annually resolved tree-ring samples has gained an exceptional position in last-millennium paleoclimatology through the maximum latewood density (MXD) parameter, but also increasingly through other density parameters. For 50 years, X-ray based measurement techniques have been the de facto standard. However, studies report offsets in the mean levels for MXD measurements derived from different laboratories, indicating challenges of accuracy and precision. Moreover, reflected visible light-based techniques are becoming increasingly popular, and wood anatomical techniques are emerging as a potentially powerful pathway to extract density information at the highest resolution. Here we review the current understanding and merits of wood density for tree-ring research, associated microdensitometric techniques, and analytical measurement challenges. The review is further complemented with a careful comparison of new measurements derived at 17 laboratories, using several different techniques. The new experiment allowed us to corroborate and refresh "long-standing wisdom" but also provide new insights. Key outcomes include (i) a demonstration of the need for mass/volume-based recalibration to accurately estimate average ring density; (ii) a substantiation of systematic differences in MXD measurements that cautions for great care when combining density data sets for climate reconstructions; and (iii) insights into the relevance of analytical measurement resolution in signals derived from tree-ring density data. Finally, we provide recommendations expected to facilitate futureinter-comparability and interpretations for global change research.

Although it has been recognized that rising temperatures and shifts in the hydrological cycle affect the depth of the seasonally thawing upper permafrost stratum, it remains unclear to what extent the frequency and intensity of wildfires, and subsequent changes in vegetation cover, influence the soil active layer on different spatiotemporal scales. Here, we use ring width measurements of the subterranean stem part of 15 larch trees from a Sphagnum bog site in Central Siberia to reconstruct long-term changes in the thickness of the active layer since the last wildfire occurred in 1899. Our approach reveals a three-step feedback loop between above- and belowground ecosystem components. After all vegetation is burned, direct atmospheric heat penetration over the first ~20 years caused thawing of the upper permafrost stratum. The slow recovery of the insulating ground vegetation reverses the process and initiates a gradual decrease of the active layer thickness. Due to the continuous spreading and thickening of the peat layer during the last decades, the upper permafrost horizon has increased by 0.52 cm/year. This study demonstrates the strength of annually resolved and absolutely dated tree-ring series to reconstruct the effects of historical wildfires on the functioning and productivity of boreal forest ecosystems at multi-decadal to centennial time-scale. In so doing, we show how complex interactions of above- and belowground components translate into successive changes in the active permafrost stratum. Our results are particularly relevant for improving long-term estimates of the global carbon cycle that strongly depends on the source and sink behavior of the boreal forest zone.

Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794–2016 CE at 0.79 (p, Tree rings are a crucial archive for Common Era climate reconstructions, but the degree to which methodological decisions influence outcomes is not well known. Here, the authors show how different approaches taken by 15 different groups influence the ensemble temperature reconstruction from the same data.

Climate warming is expected to positively alter upward and poleward treelines which are controlled by low temperature and a short growing season. Despite the importance of treelines as a bioassay of climate change, a global field assessment and posterior forecasting of tree growth at annual scales is lacking. Using annually resolved tree-ring data located across Eurasia and the Americas, we quantified and modeled the relationship between temperature and radial growth at treeline during the 20th century. We then tested whether this temperature–growth association will remain stable during the 21st century using a forward model under two climate scenarios (RCP 4.5 and 8.5). During the 20th century, growth enhancements were common in most sites, and temperature and growth showed positive trends. Interestingly, the relationship between temperature and growth trends was contingent on tree age suggesting biogeographic patterns in treeline growth are contingent on local factors besides climate warming. Simulations forecast temperature–growth decoupling during the 21st century. The growing season at treeline is projected to lengthen and growth rates would increase and become less dependent on temperature rise. These forecasts illustrate how growth may decouple from climate warming in cold regions and near the margins of tree existence. Such projected temperature–growth decoupling could impact ecosystem processes in mountain and polar biomes, with feedbacks on climate warming. climate warming, forest limit, growth model, mountain ecosystems, tree rings acceptedVersion

The world’s largest terrestrial biome, Boreal forest, is prone to the greatest rates of recent and predicted warming. Much of this circumpolar vegetation belt is underlain by permafrost, which further challenges our understanding of the direct and indirect consequences of increasing temperature on the functioning and productivity of these northern latitudinal forests.Here, we present the results of an on-going study of tree-ring growth of conifers in Russia’s continuous permafrost zone in northern Siberia, from 61-72°N and 90-148°E. Tree-ring data from a variety of habitats between 20 and 600 m asl with different climate and thermo-hydrological regimes of soils are analyzed. While in some cases up to 60-70% of the year-to-year tree-ring width and maximum latewood density variability can be explained by summer temperature variations alone, we find that the seasonal dynamics of permafrost also plays an important role in defining the overall rate of radial tree growth. Wider rings are generally formed on sites with a deeper active soil layer, which itself depends on the geographical location of a site, as well as its ground vegetation, stand parameters and fire history. Waterlogged permafrost may further act as a source of water for trees under exceptionally dry summer seasons.Our study indicates that the growth response of conifers to temperature and precipitation across the continuous permafrost zone of Siberia is both, site- and species-specific. This implies a range of possible scenarios of further development of northern forests under projected climate change. Seasonal dynamics of the active soil layer and possible permafrost degradation must be taken into account when modelling tree growth variability and forest productivity.

Warming alters the variability and trajectories of tree growth around the world by intensifying or alleviating energy and water limitation. This insight from regional to global-scale research emphasizes the susceptibility of forest ecosystems and resources to climate change. However, globally-derived trends are not necessarily meaningful for local nature conservation or management considerations, if they lack specific information on present or prospective tree species. This is particularly the case towards the edge of their distribution, where shifts in growth trajectories may be imminent or already occurring.Importantly, the geographic and bioclimatic space (or “niche”) occupied by a tree species is not only constrained by climate, but often reflects biotic pressure such as competition for resources with other species. This aspect is underrepresented in many species distribution models that define the niche as a climatic envelope, which is then allowed to shift in response to changes in ambient conditions. Hence, distinguishing climatic from competitive niche boundaries becomes a central challenge to identifying areas where tree species are most susceptible to climate change.Here we employ a novel concept to characterize each position within a species’ bioclimatic niche based on two criteria: a climate sensitivity index (CSI) and a habitat suitability index (HSI). The CSI is derived from step-wise multiple linear regression models that explain variability in annual radial tree growth as a function of monthly climate anomalies. The HSI is based on an ensemble of five species distribution models calculated from a combination of observed species occurrences and twenty-five bioclimatic variables. We calculated these two indices for 11 major tree species across the Northern Hemisphere.The combination of climate sensitivity and habitat suitability indicated hotspots of change, where tree growth is mainly limited by competition (low HSI and low CSI), as well as areas that are particularly sensitive to climate variability (low HSI and high CSI). In the former, we expect that forest management geared towards adjusting the competitive balance between several candidate species will be most effective under changing environmental conditions. In the latter areas, selecting particularly drought-tolerant accessions of a given species may reduce forest susceptibility to the predicted warming and drying.

Societal upheaval occurred across Eurasia in the sixth and seventh centuries. Tree-ring reconstructions suggest a period of pronounced cooling during this time associated with several volcanic eruptions. Climatic changes during the first half of the Common Era have been suggested to play a role in societal reorganizations in Europe1,2 and Asia3,4. In particular, the sixth century coincides with rising and falling civilizations1,2,3,4,5,6, pandemics7,8, human migration and political turmoil8,9,10,11,12,13. Our understanding of the magnitude and spatial extent as well as the possible causes and concurrences of climate change during this period is, however, still limited. Here we use tree-ring chronologies from the Russian Altai and European Alps to reconstruct summer temperatures over the past two millennia. We find an unprecedented, long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 AD (ref. 14), which was probably sustained by ocean and sea-ice feedbacks15,16, as well as a solar minimum17. We thus identify the interval from 536 to about 660 AD as the Late Antique Little Ice Age. Spanning most of the Northern Hemisphere, we suggest that this cold phase be considered as an additional environmental factor contributing to the establishment of the Justinian plague7,8, transformation of the eastern Roman Empire and collapse of the Sasanian Empire1,2,5, movements out of the Asian steppe and Arabian Peninsula8,11,12, spread of Slavic-speaking peoples9,10 and political upheavals in China13.

Although the effect of pollution on forest health and decline received much attention in the 1980s, it has not been considered to explain the ‘Divergence Problem’ in dendroclimatology; a decoupling of tree growth from rising air temperatures since the 1970s. Here we use physical and biogeochemical measurements of hundreds of living and dead conifers to reconstruct the impact of heavy industrialisation around Norilsk in northern Siberia. Moreover, we develop a forward model with surface irradiance forcing to quantify long‐distance effects of anthropogenic emissions on the functioning and productivity of Siberia’s taiga. Downwind from the world’s most polluted Arctic region, tree mortality rates of up to 100% have destroyed 24,000 km2 boreal forest since the 1960s, coincident with dramatic increases in atmospheric sulphur, copper, and nickel concentrations. In addition to regional ecosystem devastation, we demonstrate how ‘Arctic Dimming’ can explain the circumpolar ‘Divergence Problem’, and discuss implications on the terrestrial carbon cycle., Ecology Letters, 23 (12), ISSN:1461-023X, ISSN:1461-0248

Stratospheric volcanic eruptions have had significant impacts on the radiation budget, atmospheric and surface temperatures, precipitation and regional weather patterns, resulting in global climatic changes. The changes associated with such eruptions most commonly result in cooling during several years after events. This study aimed to reveal eco-physiological response of larch trees from northeastern Yakutia (YAK), eastern Taimyr (TAY) and Altai (ALT) regions to climatic anomalies after major volcanic eruptions CE 535, 540, 1257, 1641, 1815 and 1991 using new multiple tree-ring parameters: tree-ring width (TRW), maximum latewood density (MXD), cell wall thicknesses (CWT), δ13C and δ18O in tree-ring cellulose. This investigation showed that TRW, CWT, MXD and δ18O chronologies recorded temperature signal, while information about precipitation and vapor pressure deficit was captured by δ13C chronologies. Sunshine duration was well recorded in δ18O from YAK and ALT. Tree-ring parameters recorded cold, wet and cloudy summer anomalies during the 6th and 13th centuries. However, significant summer anomalies after Tambora (1815) and Pinatubo (1991) eruptions were not captured by any tree-ring parameters

Though the extraction of increment cores is common practice in tree-ring research, there is no standard for the number of samples per tree, or trees per site needed to accurately describe the common growth pattern of a discrete population of trees over space and time. Tree-ring chronologies composed of living, subfossil and archaeological material often combine an uneven distribution of increment cores and disc samples. The effects of taking one or two cores per tree, or even the inclusion of multiple radii measurements from entire discs, on chronology development and quality remain unreported. Here, we present four new larch (Larix cajanderi Mayr) ring width chronologies from the same 20 trees in northeastern Siberia that have been independently developed using different combinations of core and disc samples. Our experiment reveals: i) sawing is much faster than coring, with the latter not always hitting the pith; ii) the disc-based chronology contains fewer locally absent rings, extends further back in time and exhibits more growth coherency; iii) although the sampling design has little impact on the overall chronology behaviour, lower frequency information is more robustly obtained from the disc measurements that also tend to reflect a slightly stronger temperature signal. In quantifying the influence of sampling strategy on the quality of tree-ring width chronologies, and their suitability for climate reconstructions, this study provides useful insights for optimizing fieldwork campaigns, as well as for developing composite chronologies from different wood sources.

Warming-induced biological and ecological responses have been reported from high-northern latitude sites, where changes in dwarf shrub communities translate into complex vegetation-climate feedbacks. Most of the available Arctic tree-ring evidence is, however, restricted to a limited number of species and locations. A combination of wood anatomical and ‘dendro’-ecological techniques provides insights into past growth rates, recruitment dynamics and even community assemblages of Arctic vegetation. Here, we use thin sectioning and ring counting of 1432 dwarf shrub samples from eight species and two tundra regions in coastal east Greenland to assess community recruitment history and its relation to climate. Site and species-specific annual stem increments, as well as estimated plant ages, range from 0.013-0.720 mm and from 4 to 204 years, respectively. The mean ring width is 0.086 mm, with a mean age of 50 years. Decadal-scale recruitment dynamics of the studied vegetation cover respond to Greenlandic summer temperature variations back to the late 19th century (r = 0.7; 1881–2000).

Tracheids fulfil most wood functions in conifers (mechanical support and water transport) and earlywood tracheids account for most hydraulic conductivity within the annual tree ring. Dry conditions during the early growing season, when earlywood is formed, could lead to the formation of narrow tracheid lumens and a dense earlywood. Here, we assessed if there is a negative association between minimum wood density and early growing-season (spring) precipitation. Using dendrochronology, we studied growth and density data at nine forest stands of three Pinaceae species (Larix sibirica, Pinus nigra, and Pinus sylvestris) widely distributed in three cool–dry Eurasian regions from the forest-steppe (Russia, Mongolia) and Mediterranean (Spain) biomes. We measured for each annual tree ring and the common 1950–2002 period the following variables: earlywood and latewood width, and minimum and maximum wood density. As expected, dry early growing season (spring) conditions were associated with low earlywood production but, most importantly, to high minimum density in the three conifer species. The associations between minimum density and spring precipitation were stronger (r = -0.65) than those observed with earlywood width (r = 0.57). We interpret the relationship between spring water availability and high minimum density as a drought-induced reduction in lumen diameter, hydraulic conductivity, and growth. Consequently, forecasted growing-season drier conditions would translate into increased minimum wood density and reflect a reduction in hydraulic conductivity, radial growth, and wood formation. Given the case-study-like nature of this work, more research on other cold–dry sites with additional conifer species is needed to test if minimum wood density is a robust proxy of early season water availability. We acknowledge the support of Spanish Ministry of Economy Projects (Fundiver, CGL2015-69186-C2-1-R). Tree-ring density data were obtained and analysed under support of Russian Science Foundation (Project 14-14-00295).

It is generally accepted that animal heartbeat and lifespan are often inversely correlated, however, the relationship between productivity and longevity has not yet been described for trees growing under industrial and pre-industrial climates. Using 1768 annually resolved and absolutely dated ring width measurement series from living and dead conifers that grew in undisturbed, high-elevation sites in the Spanish Pyrenees and the Russian Altai over the past 2000 years, we test the hypothesis of grow fast—die young. We find maximum tree ages are significantly correlated with slow juvenile growth rates. We conclude, the interdependence between higher stem productivity, faster tree turnover, and shorter carbon residence time, reduces the capacity of forest ecosystems to store carbon under a climate warming-induced stimulation of tree growth at policy-relevant timescales. Fertilization under greenhouse warming conditions is expected to accelerate tree growth and potentially increase the biological storage of CO2. Here the authors analyse ring width measurements from 1768 conifers from the Spanish and Russian mountains and demonstrate that longevity requires slow growth rates at least in mountainous regions.

Considering the importance and complexity of natural (e.g., volcanic eruptions and wildfires) and anthropogenic (e.g., mining, oil and shipping industries) aerosol emissions to Arctic warming is particularly timely given the recent temperature extremes recorded at high-northern latitudes (Cohen et al., 2020; Overland and Wang, 2021). Despite our knowledge about the observed and modelled climatic effects of rising Arctic aerosol concentrations (Schmale et al., 2021), which may exhibit regional and seasonal differences and call for diverse research priorities from local to circumpolar scales, we feel that the ecological consequences of an aerosol-induced reduction in surface irradiance (i.e., Arctic Dimming) justify more thorough tree-ring investigations in the future. We argue that this is particularly true if the goal is to enhance our understanding of all aerosol generating processes that impact terrestrial vegetation and its ability to assimilate carbon dioxide from the atmosphere.

Climate change affects forest dynamics with potential consequences for essential ecosystem services. The retrospective analysis of secondary growth unveils the effect of climate on forests. However, most tree-ring studies focus on dominant trees, and less is known about the climatic response of their neighbor suppressed trees. We evaluated the influence of tree social status (dominant/suppressed) on climate response in Pinus sylvestris L. trees from two sites with contrasting water availability conditions in the forest-steppe ecotone in southern Siberia. Tree-ring width and intra-annual density fluctuations (IADFs) were used as proxies. Late spring to early summer conditions were the main climate drivers in both tree social status, but the climate response of suppressed trees was stronger and had a longer time window (May-June). IADFs’ occurrence was controlled by temperature and its frequency was modulated by local conditions, being more common at the dry site, with tree status just marginally significant. Our results suggest that under the projected warmer and drier climate, suppressed trees in southern Siberia will be prone to increased water shortage, leading to possible higher mortality of more sensitive suppressed trees, with potential consequences for carbon sequestration in the forest-steppe ecosystems in southern Siberia.

Arctic driftwood represents a unique proxy archive at the interface of marine and terrestrial environments. Combined wood anatomical and dendrochronological analyses have been used to detect the origin of driftwood and may allow past timber floating activities, as well as past sea ice and ocean current dynamics to be reconstructed. However, the success of driftwood provenancing studies depends on the length, number, and quality of circumpolar boreal reference chronologies. Here, we introduce a Eurasian-wide high-latitude network of 286 ring width chronologies from the International Tree Ring Data Bank (ITRDB) and 160 additional sites comprising the three main boreal conifers Pinus, Larix, and Picea. We assess the correlation structure within the network to identify growth patterns in the catchment areas of large Eurasian rivers, the main driftwood deliverers. The occurrence of common growth patterns between and differing patterns within catchments indicates the importance of biogeographic zones for ring width formation and emphasizes the degree of spatial precision when provenancing. Reference chronologies covering millennial timescales are so far restricted to a few larch sites in Central and Eastern Siberia (eastern Taimyr, Yamal Peninsula and north-eastern Yakutia), as well as several pine sites in Scandinavia, where large rivers are missing though. The general good spatial coverage of tree-ring sites across northern Eurasia indicates the need for updating and extending existing chronologies rather than developing new sites.

It is generally assumed in dendroecological studies that annual tree-ring growth is adequately determined by a linear function of local or regional precipitation and temperature with a set of coefficients that are temporally invariant. However, various researchers have maintained that tree-ring records are the result of multivariate, often nonlinear biological and physical processes. To describe critical processes linking climate variables with tree-ring formation, the process-based tree-ring Vaganov–Shashkin model (VS-model) was successfully used. However, the VS-model is a complex tool requiring a considerable number of model parameters that should be re-estimated for each forest stand. Here we present a new visual approach of process-based tree-ring model parameterization (the so-called VS-oscilloscope) which allows the simulation of tree-ring growth and can be easily used by researchers and students. The VS-oscilloscope was tested on tree-ring data for two species (Larix gmeliniiand Picea obovata) growing in the permafrost zone of Central Siberia. The parameterization of the VS-model provided highly significant positive correlations (p < 0.0001) between simulated growth curves and original tree-ring chronologies for the period 1950–2009. The model outputs have shown differences in seasonal tree-ring growth between species that were well supported by the field observations. To better understand seasonal tree-ring growth and to verify the VS-model findings, a multi-year natural field study is needed, including seasonal observation of the thermo-hydrological regime of the soil, duration and rate of tracheid development, as well as measurements of their anatomical features.

German Science Foundation [161/9-1]; National Natural Science Foundation of China [41325008]; [RNF 15-14-30011]

This research is an attempt to study seasonal translocation patterns of photoassimilated carbon within trees of one of the high latitudes widespread deciduous conifer species Larix gmelinii (Rupr. Rupr). For this purpose, we applied whole-tree labeling by 13CO2, which is a powerful and effective tool for tracing newly developed assimilates translocation to tissues and organs of a tree. Experimental plot has been established in a mature 105-year-old larch stand located within the continuous permafrost area near Tura settlement (Central Siberia, 64°17′13″N, 100°11′55″E, 148 m a.s.l.). Measurements of seasonal photosynthetic activity and foliage parameters (i.e., leaf length, area, biomass, etc.), and sampling were arranged from early growing season (June 8, 2013; May 14, 2014) until yellowing and senescence of needles (September 17, 2013; September 14, 2014). Labeling by 13C of the tree branch (June 2013, for 3 branch replicates in 3 different trees) and the whole tree was conducted at early (June 2014), middle (July 2014), and late (August 2013) phase of growing season (for different trees in 3 replicates each time) by three pulses [(CO2)max = 3000–4000 ppmv, 13CO2 (30 % v/v)]. We found at least two different patterns of carbon translocation associated with larch CO2 assimilation depending on needle phenology. In early period of growing season (June), 13C appearing in newly developed needles is a result of remobilized storage material use for growth purposes. Then approximately at the end of June, growth processes is switching to storage processes lasting to the end of growing season.

Plants exhibit morphological and anatomical adaptations to cope the environmental constraints of their habitat. How can mechanisms for adapting to contrasting environmental conditions change the patterns of tree rings formation? In this study, we explored differences in climatic conditions of permafrost and non-permafrost zones and assessed their influence on radial growth and wood traits of Larix gmelinii Rupr (Rupr) and Larix sibirica L., respectively. We quantified the contribution of xylem cell anatomy to the tree-ring width variability. Comparison of the anatomical tree-ring parameters over the period 1963–2011 was tested based on non-parametric Mann-Whitney U test. The generalized linear modeling shows the common dependence between TRW and the cell structure characteristics in contrasting environments, which can be defined as non-specific to external conditions. Thus, the relationship between the tree-ring width and the cell production in early- and latewood are assessed as linear, whereas the dependence between the radial cell size in early- and latewood and the tree-ring width becomes significantly non-linear for both habitats. Moreover, contribution of earlywood (EW) and latewood (LW) cells to the variation of TRW (in average 56.8% and 24.4% respectively) was significantly higher than the effect of cell diameters (3.3% (EW) and 17.4% (LW)) for the environments. The results show that different larch species from sites with diverging climatic conditions converge towards similar xylem cell structures and relationships between xylem production and cell traits. The work makes a link between climate and tree-ring structure, and promotes a better understanding the anatomical adaptation of larch species to local environment conditions.

The forest-steppe ecotone in southern Siberia is characterized by a strong dependence of tree growth on summer drought, which is expected to increase under ongoing climate change, with potential consequences for regional and global water and carbon cycles. Since climate conditions control tree secondary growth throughout the growing season, it is assumed that climate change will differently impact the formation of particular tree-ring sectors. In this study, we evaluated spatiotemporal trends in Pinus sylvestris L. tree-ring traits: tree-ring (RW), earlywood (EW) and latewood (LW) widths, as well as their climate response in order to understand potential reactions of P. sylvestris radial growth to climate change along a 4900 km longitudinal transect of increasing continentality in southern Siberia. Results indicated an increasing tree radial growth from the West to the East along the transect. Tree-ring parameters were sensitive to drought, showing a temporal delay in the climatic signals of LW (summer) relative to EW (spring). Climate control of tree growth was stronger at the western site, while it was alleviated over time in eastern sites. This study highlighted the wide plasticity of P. sylvestris to thrive within a wide range of climatic conditions, also suggesting that future drought, as predicted by climate change simulations, will potentially impact P. sylvestris growth differently along the studied gradient, being more susceptible at the western sites due to the current growth limitation.

Wildfires are an important factor in controlling forest ecosystem dynamics across the circumpolar boreal zone. An improved understanding of their direct and indirect, short- to long-term impacts on vegetation cover and permafrost–vegetation coupling is particularly important to predict changes in carbon, nutrient and water cycles under projected climate warming. Here, we apply dendrochronological techniques on a multi-parameter dataset to reconstruct the effect of wildfires on tree growth and seasonal permafrost thaw depth in Central Siberia. Based on annually-resolved and absolutely dated information from 19 Gmelin larch (Larix gmelinii (Rupr.) Rupr.) trees and active soil layer thickness measurements, we find substantial stand-level die-off, as well as the removal of ground vegetation and the organic layer following a major wildfire in 1896. Reduced stem growth coincides with increased δ 13C in the cellulose of the surviving trees during the first decade after the wildfire, when stomatal conductance was reduced. The next six to seven decades are characterized by increased permafrost active soil layer thickness. During this period of post-wildfire ecosystem recovery, enhanced tree growth together with positive δ 13C and negative δ 18O trends are indicative of higher rates of photosynthesis and improved water supply. Afterwards, a thinner active soil layer leads to reduced growth because tree physiological processes become limited by summer temperature and water availability. Revealing long-term effects of forest fires on active soil layer thickness, ground vegetation composition and tree growth, this study demonstrates the importance of complex vegetation–permafrost interactions that modify the trajectory of post-fire forest recovery across much of the circumpolar boreal zone. To further quantify the influence of boreal wildfires on large-scale carbon cycle dynamics, future work should consider a wide range of tree species from different habitats in the high-northern latitudes.

The original version of this Article contained an error in the Data Availability section, which incorrectly read ‘All data will be freely available via https://www.ams.ethz.ch/research.html.’ The correct version states ‘http://www.ams.ethz.ch/research/published-data.html’ in place of ‘https://www.ams.ethz.ch/research.html’. This has been corrected in both the PDF and HTML versions of the Article.

Tree-ring width of Larix gmelinii (Rupr.) Rupr., ratios of stable isotopes of C (delta(13)C) and O (delta(18)O) of whole wood and cellulose chronologies were obtained for the northern part of central Siberia (Tura, Russia) for the period 1864-2006. A strong decrease in the isotope ratios of O and C (after atmospheric delta(13)C corrections) and tree-ring width was observed for the period 1967-2005, while weather station data show a decrease in July precipitation, along with increasing July air temperature and vapor pressure deficit (VPD). Temperature at the end of May and the whole month of June mainly determines tree radial growth and marks the beginning of the vegetation period in this region. A positive correlation between tree-ring width and July precipitation was found for the calibration period 1929-2005. Positive significant correlations between C isotope chronologies and temperatures of June and July were found for whole wood and cellulose and negative relationships with July precipitation. These relationships are strengthened when the likely physiological response of trees to increased CO(2) is taken into account (by applying a recently developed delta(13)C correction). For the O isotope ratios, positive relationships with annual temperature, VPD of July and a negative correlation with annual precipitation were observed. The delta(18)O in tree rings may reflect annual rather than summer temperatures, due to the late melting of the winter snow and its contribution to the tree water supply in summer. We observed a clear change in the isotope and climate trends after the 1960s, resulting in a drastic change in the relationship between C and O isotope ratios from a negative to a positive correlation. According to isotope fractionation models, this indicates reduced stomatal conductance at a relatively constant photosynthetic rate, as a response of trees to water deficit for the last half century in this permafrost region.

Permafrost exerts strong controls on forest development through nutrient availability. The key question of this study was to assess the effect of site conditions on macroelement concentration and stable isotope (&delta, 13C and &delta, 15N) dynamics during the growing season, and nutrient stoichiometry and resorption efficiency in the foliage of two common larch species in Siberia. Foliar nutrient (N, P and K) concentrations of larches grown on permafrost soils were exceptionally high in juvenile needles compared to those from a permafrost-free region (+50% and 130% for P and K), but were two-fold lower at needle maturation. Within permafrost terrain trees, sites with a warmer and deeper soil active layer had 15&ndash, 60% greater nutrient concentrations and higher &delta, 15N in their needles compared to shallower, colder soils. Larch of permafrost-free sites demonstrated an enrichment of foliage in 15N (+1.4% to +2.4&permil, ) in comparison to permafrost terrain (&minus, 2.0% to &minus, 6.9&permil, ). At all sites, foliar &delta, 13C decreased from June to August, which very likely results from an increasing contribution of current photoassimilates to build foliar biomass. With senescence, nutrient concentrations in larch needles decreased significantly by 60&ndash, 90%. This strong ability of larch to retain nutrients through resorption is the essential mechanism that maintains tree growth early in the growing season when soil remains frozen. The high resorptive efficiency found for K and P for larches established on permafrost suggests nutrient limitation of tree growth within the Central Siberian Plateau not only by N, as previously reported, but also by P and K. The increasing nutrient concentrations and a 15N enrichment of foliage towards warmer sites was paralleled by an up to 50-fold increase in biomass production, strongly suggesting that accelerated nutrient cycling with permafrost degradation contributes to an increased productivity of Siberian larch forests.

Stable oxygen isotope ratios (δ(18) O) in trees from high latitude ecosystems are valuable sources of information for recent and past environmental changes, but the interpretation is hampered by the complex hydrology of forests growing under permafrost conditions, where only a shallow layer of soil thaws in summer. We investigated larch trees (Larix gmelinii) at two sites with contrasting soil conditions in Siberia and determined δ(18) O of water from different soil depths, roots, twigs, and needles as well as δ(18) O of soluble carbohydrates regularly over two growing seasons. A comparison of results from the 2 yrs revealed an unexpected 'inverse' climate-isotope relationship, as dry and warm summer conditions resulted in lower soil and root δ(18) O values. This was due to a stronger uptake of isotopically depleted water pools originating from melted permafrost or previous winter snow. We developed a conceptual framework that considers the dependence of soil water profiles on climatic conditions for explaining δ(18) O in needle water, needle soluble carbohydrates and stem cellulose. The negative feedback of drought conditions on the source isotope value could explain decreasing tree-ring δ(18) O trends in a warming climate and is likely relevant in many ecosystems, where a soil isotope gradient with depth is observed.

Estimations of tree annual biomass increments are used by a variety of studies related to forest productivity or carbon fluxes. Biomass increment estimations can be easily obtained from diameter surveys or historical diameter reconstructions based on tree rings' records. However, the biomass models rely on the assumption that wood density is constant. Converting volume increment into biomass also requires assumptions about the wood density. Wood density has been largely reported to vary both in time and between trees. In Norway spruce, wood density is known to increase with decreasing ring width. This could lead to underestimating the biomass or carbon deposition in bad years. The variations between trees of wood density have never been discussed but could also contribute to deviations. A modelling approach could attenuate these effects but will also generate errors. Here a model of wood density variations in Norway spruce, and an allometric model of volume growth were developed. We accounted for variations in wood density both between years and between trees, based on specific measurements. We compared the effects of neglecting each variation source on the estimations of annual biomass increment. We also assessed the errors of the biomass increment predictions at tree level, and of the annual productivity at plot level. Our results showed a partial compensation of the decrease in ring width in bad years by the increase in wood density. The underestimation of the biomass increment in those years reached 15 %. The errors related to the use of an allometric model of volume growth were modest, around ±15 %. The errors related to variations in wood density were much larger, the biggest component being the inter-tree variability. The errors in plot-level annual biomass productivity reached up to 40 %, with a full account of all the error sources.

Key message This study provides new data and an alternative framework to the debate of tree carbon economy in a context of increasing stress. Abstract For long-living trees, the resilience in times of stress is directly linked to the amount of accessible re- serves. Despite the simplicity of this principle, the under- standing of how carbon reserves limit growth and/or induce mortality under global change is still debated. In this study, we quantify how anatomical properties of rays—one of the main container for carbon reserves in tree stems—vary among sites, individuals, and annual rings of Larix gmelinii growing in contrasting sites in Siberia to verify if (1) the ray proportion and anatomical structure is linked to the environment, and/or (2) to changes in other wood tissues. Our observations have highlighted that ray proportion mainly varies among individuals, but little among sites and consecutive annual rings. We also observed that ray size and density scale to the wood structure with a relatively constant ratio of *2.5 rays per tracheid, independent of site conditions. These results suggest that the functional connection between the anatomy of rays and tracheid is unaffected by environment and highlight the importance of considering allometric relations in ecological comparisons. Comparative studies of long-term trajectory of ray pro- portion of living and dead trees might unravel observed variability among individuals validating the link between long-term depleted reserves and mortality.

Summary The effects of climate change on Arctic ecosystems can range between various spatiotemporal scales and may include shifts in population distribution, community composition, plant phenology, primary productivity and species biodiversity. The growth rates and age structure of tundra vegetation as well as its response to temperature variation, however, remain poorly understood because high-resolution data are limited in space and time. Anatomical and morphological stem characteristics were recorded to assess the growth behaviour and age structure of 871 dwarf shrubs from 10 species at 30 sites in coastal East Greenland at ˜70°N. Recruitment pulses were linked with changes in mean annual and summer temperature back to the 19th century, and a literature review was conducted to place our findings in a pan-Arctic context. Low cambial activity translates into estimated average/maximum plant ages of 59/204 years, suggesting relatively small turnover rates and stable community composition. Decade-long changes in the recruitment intensity were found to lag temperature variability by 2 and 6 years during warmer and colder periods, respectively (r = 0.851961–2000 and 1881–1920). Synthesis. Our results reveal a strong temperature dependency of Arctic dwarf shrub reproduction, a high vulnerability of circumpolar tundra ecosystems to climatic changes, and the ability of evaluating historical vegetation dynamics well beyond the northern treeline. The combined wood anatomical and plant ecological approach, considering insights from micro-sections to community assemblages, indicates that model predictions of rapid tundra expansion (i.e. shrub growth) following intense warming might underestimate plant longevity and persistence but overestimate the sensitivity and reaction time of Arctic vegetation.

Global warming is most pronounced in high-latitude regions by altering habitat conditions and affecting permafrost degradation, which may significantly influence tree productivity and vegetation changes. In this study, by applying a “space-for-time” approach, we selected three plots of Larix gmelinii forest from a continuous permafrost zone in Siberia with different thermo-hydrological soil regimes and ground cover vegetation with the objective of assessing how tree growth and productivity will change under different stages of permafrost degradation. A tree-ring multi-proxy characterization of mature trees was used to identify shift in ecophysiological responses related to the modified plant-soil system. Variability of tree-ring width (1975–2009), stable isotope ratios (oxygen and carbon, 2000–2009) and xylem structural characteristics (2000–2009) under climatic conditions of particular years indicated that an increased depth of the soil active layer will initially lead to increase of tree productivity. However, due to an expected water use increase through transpiration, the system might progressively shift from a temperature to a moisture-limited environment.

Understanding climate change impacts on drought-prone forests is a critical issue. We investigated ring-width and stable isotopes (Δ13C and δ18O) in two Pinus sylvestris stands of the cold-dry Siberian forest-steppe growing under contrasting climatic trends over the last 75 years. Despite regional warming, there was increasing precipitation during the growing period at the southern site (MIN) but increasing water deficit (WD) at the northern site (BER). Intrinsic water use efficiency (WUEi) increased similarly (ca. 22%) in response to warming and rising atmospheric CO2. However, the steady increase in WUEi was accompanied by divergent growth patterns since 1980: increasing basal area increment (BAI) in MIN (slope = 0.102 cm2 year−2) and decreasing BAI in BER (slope = −0.129 cm2 year−2). This suggests that increased precipitation, mediated by CO2 effects, promoted growth in MIN, whereas intensified drought stress led to decreased carbon gain and productivity in BER. When compared to warm-dry stands of eastern Spain, the WUEi dependence on WD was three-fold greater in Siberia. Conversely, BAI was more affected by the relative impact of water stress within each region. These results indicate contrasting future trajectories of P. sylvestris forests, which challenge forecasting growth and carbon sequestration in cold-dry areas. This study was funded by the Spanish Government (grant number AGL2015-68274-C3-3-R) and the Russian Science Foundation (project numbers 14-14-00295, sampling and tree-ring data obtainment and 14-14-00219-P, mathematical approach). We acknowledge P. Sopeña and M.J. Pau for technical assistance.

The radial growth of Siberian larch under the impact of pollutants emitted by enterprises of Norilsk has been analyzed to reconstruct the die-off dynamics of larch stands located along the Rybnaya River, along the main direction of pollutant air transport. Dendrochronological cross dating is used to detect the year of die off of 268 trees growing in 4 sites located 22, 45, 68, and 85 km from Norilsk. The death of individual trees in the sites closest to Norilsk was recorded immediately after the first enterprises started to operate in the early 1940s. The mass mortality of the trees started in the 1960s due to the operation of new smelters and the consequent increase in pollutant emissions. The complete destruction of the stands (100% die off of larch trees) occurred in the 1970s. At the most distant site (85 km), the highest rate of larch death was observed between 1975 and 1980, and in 2004 only 23% of larch trees were alive. A comparative analysis of the tree-ring width of the studied trees testified to the decrease in tree radial growth at the period before the complete degradation of stands. Unfavorable climatic conditions became an additional factor that enhanced the rate of tree die off due to the impact of pollutants. Although the increase in tree radial growth had been found in late 1990s to early 2000s at the site 85 km from Norilsk, the status of studied trees indicated that the area of completely degraded forest ecosystems might become larger under the present conditions.