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Nearly 13,000 years ago, the warming trend into the Holocene was sharply interrupted by a reversal to near glacial conditions. Climatic causes and ecological consequences of the Younger Dryas (YD) have been extensively studied, however proxy archives from the Mediterranean basin capturing this period are scarce and do not provide annual resolution. Here, we report a hydroclimatic reconstruction from stable isotopes (δ 18 O, δ 13 C) in subfossil pines from southern France. Growing before and during the transition period into the YD (12 900-12 600 cal BP), the trees provide an annually resolved, continuous sequence of atmospheric change. Isotopic signature of tree sourcewater (δ 18 O sw ) and estimates of relative air humidity were reconstructed as a proxy for variations in air mass origin and precipitation regime. We find a distinct increase in inter-annual variability of sourcewater isotopes (δ 18 O sw ), with three major downturn phases of increasing magnitude beginning at 12 740 cal BP. The observed variation most likely results from an amplified intensity of North Atlantic (low δ 18 O sw ) versus Mediterranean (high δ 18 O sw ) precipitation. This marked pattern of climate variability is not seen in records from higher latitudes and is likely a consequence of atmospheric circulation oscillations at the margin of the southward moving polar front.

As the worldwide standard for radiocarbon (14C) dating over the past ca. 50,000 years, the International Calibration Curve (IntCal) is continuously improving towards higher resolution and replication. Tree-ring-based 14C measurements provide absolute dating throughout most of the Holocene, although high-precision data are limited for the Younger Dryas interval and farther back in time. Here, we describe the dendrochronological characteristics of 1448 new 14C dates, between ~11,950 and 13,160 cal BP, from 13 pines that were growing in Switzerland. Significantly enhancing the ongoing IntCal update (IntCal20), this Late Glacial (LG) compilation contains more annually precise 14C dates than any other contribution during any other period of time. Thus, our results now provide unique geochronological dating into the Younger Dryas, a pivotal period of climate and environmental change at the transition from LG into Early Holocene conditions. [ABSTRACT FROM AUTHOR]

Aim: The aim was to decipher Europe‐wide spatio‐temporal patterns of forest growth dynamics and their associations with carbon isotope fractionation processes inferred from tree rings as modulated by climate warming. Location: Europe and North Africa (30‒70° N, 10° W‒35° E). Time period: 1901‒2003. Major taxa studied: Temperate and Euro‐Siberian trees. Methods: We characterize changes in the relationship between tree growth and carbon isotope fractionation over the 20th century using a European network consisting of 20 site chronologies. Using indexed tree‐ring widths (TRWi), we assess shifts in the temporal coherence of radial growth across sites (synchrony) for five forest ecosystems (Atlantic, boreal, cold continental, Mediterranean and temperate). We also examine whether TRWi shows variable coupling with leaf‐level gas exchange, inferred from indexed carbon isotope discrimination of tree‐ring cellulose (Δ13Ci). Results: We find spatial autocorrelation for TRWi and Δ13Ci extending over a maximum of 1,000 km among forest stands. However, growth synchrony is not uniform across Europe, but increases along a latitudinal gradient concurrent with decreasing temperature and evapotranspiration. Latitudinal relationships between TRWi and Δ13Ci (changing from negative to positive southwards) point to drought impairing carbon uptake via stomatal regulation for water saving occurring at forests below 60° N in continental Europe. An increase in forest growth synchrony over the 20th century together with increasingly positive relationships between TRWi and Δ13Ci indicate intensifying impacts of drought on tree performance. These effects are noticeable in drought‐prone biomes (Mediterranean, temperate and cold continental). Main conclusions: At the turn of this century, convergence in growth synchrony across European forest ecosystems is coupled with coordinated warming‐induced effects of drought on leaf physiology and tree growth spreading northwards. Such a tendency towards exacerbated moisture‐sensitive growth and physiology could override positive effects of enhanced leaf intercellular CO2 concentrations, possibly resulting in Europe‐wide declines of forest carbon gain in the coming decades. [ABSTRACT FROM AUTHOR]

Tree rings are natural archives that annually record distinct types of past climate variability depending on the parameters measured. Here, we use ring-width and stable isotopes in cellulose of trees from the northwestern Iberian Peninsula (IP) to understand regional summer hydroclimate over the last 400 years and the associated atmospheric patterns. Correlations between tree rings and climate data demonstrate that isotope signatures in the targeted Iberian pine forests are very sensitive to water availability during the summer period, and are mainly controlled by stomatal conductance. Non-linear methods based on extreme events analysis allow for capturing distinct seasonal climatic variability recorded by tree-ring parameters and asymmetric signals of the associated atmospheric features. Moreover, years with extreme high (low) values in the tree-ring records were characterised by coherent large-scale atmospheric circulation patterns with reduced (enhanced) moisture transport onto the northwestern IP. These analyses of extremes revealed that high/low proxy values do not necessarily correspond to mirror images in the atmospheric anomaly patterns, suggesting different drivers of these patterns and the corresponding signature recorded in the proxies. Regional hydroclimate features across the broader IP and western Europe during extreme wet/dry summers detected by the northwestern IP trees compare favourably to independent multicentury sea level pressure and drought reconstructions for Europe. Historical records also validate our findings that attribute non-linear moisture signals recorded by extreme tree-ring values to distinct large-scale atmospheric patterns and allow for 400-year reconstructions of the frequency of occurrence of extreme conditions in late spring and summer hydroclimate. [ABSTRACT FROM AUTHOR]

According to the Intergovernmental Panel on Climate Change (IPCC), all of Africa is very likely to warm up more than the global average during this century. Especially (semi-)arid regions are expected to experience particularly high warming and possibly catastrophic droughts. However, assessments of the impacts of climate change on these regions are currently impeded by a lack of transregional high temporal resolution proxy data for the African continent. Baobab trees are widely distributed in (semi-)arid Africa and can reach ages of up to 2000 years. This pilot study was aimed at investigating African baobabs, Adansonia spp., from a site in Botswana using multiple dendroclimatological methods. Increment cores from 16 individual baobabs growing on Kubu Island (20°53' S, 25°49' E), a granite pluton located in the Kalahari, were collected in June 2011 to test for annual growth and the species' utility for palaeoclimatic studies. Due to the particular wood fabric and relatively high water content, baobab increment cores were packed in air-tight opaque tubes and stored in a freezer to prevent drying and mould formation. The complicated wood anatomical structure was found to be analysed best using a microscope with incident UV light, allowing tree-ring boundaries to be distinguished. Nonetheless, potential differences in individual site conditions, as well as diverse tree ages, caused conventional dendrochronological crossdating to fail. Missing and false tree rings could be identified due to the strong relationship between tree-ring width and annual precipitation amount allowing the development of a preliminary 50 year-long baobab chronology (1960–2009). Subsequently, stable carbon and oxygen isotope analyses revealed significant correlations of Δ13C and δ18O of tree rings with climate data. Year-to-year isotope variability and trends were found to be in good agreement with established models of fractionation. Intrinsic water-use efficiency has mainly increased over the study period (2–30 %). Despite the demonstrated high potential of African baobabs as a valuable high-resolution climatearchive, we conclude that more dendrochronological calibration studiesare required at various sites in southern Africa. Furthermore, ecophysiological monitoring of climate and stable isotope signal transfer from the atmosphere, through soil and leaves into the tree rings is necessary to fully understand tree-ring formation and climate response of the African baobab. [ABSTRACT FROM AUTHOR]

The Last Glacial–Interglacial Transition (LGIT; 15,000–11,000 cal BP) was characterized by complex spatiotemporal patterns of climate change, with numerous studies requiring accurate chronological control to decipher leads from lags in global paleoclimatic, paleoenvironmental, and archaeological records. However, close scrutiny of the few available tree-ring chronologies and radiocarbon-dated sequences composing the IntCal13 14C calibration curve indicates significant weakness in 14C calibration across key periods of the LGIT. Here, we present a decadally resolved atmospheric 14C record derived from New Zealand kauri spanning the Lateglacial from ~13,100–11,365 cal BP. Two floating kauri 14C time series, curve-matched to IntCal13, serve as a 14C backbone through the Younger Dryas. The floating Northern Hemisphere (NH) 14C data sets derived from the YD-B and Central European Lateglacial Master tree-ring series are matched against the new kauri data, forming a robust NH 14C time series to ~14,200 cal BP. Our results show that IntCal13 is questionable from ~12,200–11,900 cal BP and the ~10,400 BP 14C plateau is approximately 5 decades too short. The new kauri record and repositioned NH pine 14C series offer a refinement of the international 14C calibration curves IntCal13 and SHCal13, providing increased confidence in the correlation of global paleorecords. [ABSTRACT FROM AUTHOR]

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO concentration ( c) and climate in the Iberian Peninsula using annually resolved width and δC tree-ring chronologies since 1600. C discrimination (Δ≈ c/ c), leaf intercellular CO concentration ( c) and intrinsic water-use efficiency (iWUE) were inferred from δC values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant c/ c ratio, leading to significant c and iWUE increases (active response to c); whereas a significant increase in c/ c resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to c). A significant decrease in c/ c led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the δC series after removing the low-frequency trends due to the physiological responses to increasing c. We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to c changes modulated δC and masked the climate signal. Since 1970 higher δC values revealed iWUE improvements at all the sites exceeding values expected by an active response to the c increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO'fertilization' effect on growth. The iWUE improvements found for all the forests, reflecting both a c rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth. [ABSTRACT FROM AUTHOR]

This dendroclimatological research is based on two close pine forests ( Pinus sylvestris and Pinus uncinata) located at the Northern Iberian System (Spain), and three tree-ring variables (ring widths, δ13C and δ18O). The climate-tree growth system was assessed at local and regional scales using three climate datasets. Calibration of tree-ring records with climate showed a diversity of information recorded in the different variables, such as a general response to temperature and precipitation of current growing period, and an important contribution of previous year conditions understood as the use of food reserves. The analysis of the stability of climate-tree growth relationships throughout the twentieth century showed a shift of those climatic variables to which trees responded and results suggested an enhancement of reserve use on current tree growth. The results obtained in this research made clear a physiological adaptation of trees to changing climate. The results provided hints that the recent warming coupled to slight precipitation decay are forcing growth of studied trees to a higher stress status and to a higher climate-growth synchronisation. These instabilities also have implications on future dendroclimatic reconstructions performed with trees growing under changing environments. [ABSTRACT FROM AUTHOR]

This paper examines tree-ring width and δ13C chronologies from a network of five Iberian pine forests to determine their sensitivity to climate variability under different site conditions. Interseries comparisons revealed better and more homogenous agreement among δ13C records than among tree-ring width series of the different sites. This suggests that δ13C ratios may preferentially record large-scale climatic signals, whereas ring-width variations may reflect more local factors. A negative relationship was found between ring-width and δ13C. As inferred from response function analyses, ring-width and δ13C showed significant relationships with climate. The analyses of different sites and species revealed unshared tree-ring width responses to summer temperature and precipitation, whereas all δ13C series were highly sensitive to current year summer precipitation and, to a lesser extent, to current summer temperature. A strong summer precipitation signal seems to dominate the δ13C of trees growing under Mediterranean climate, even when the mean climatic site conditions do not indicate distinct summer drought. Therefore, δ13C values reflect precipitation variability during the summer season better than tree-ring widths. This demonstrates that δ13C from tree-rings can be a very useful tool for climatic reconstruction in the Mediterranean region, especially when climate-growth relationships are weak. [ABSTRACT FROM AUTHOR]

Twentieth-century warming could lead to increases in the moisture-holding capacity of the atmosphere, altering the hydrological cycle and the characteristics of precipitation. Such changes in the global rate and distribution of precipitation may have a greater direct effect on human well-being and ecosystem dynamics than changes in temperature itself. Despite the co-variability of both of these climate variables, attention in long-term climate reconstruction has mainly concentrated on temperature changes. Here we present an annually resolved oxygen isotope record from tree-rings, providing a millennial-scale reconstruction of precipitation variability in the high mountains of northern Pakistan. The climatic signal originates mainly from winter precipitation, and is robust over ecologically different sites. Centennial-scale variations reveal dry conditions at the beginning of the past millennium and through the eighteenth and early nineteenth centuries, with precipitation increasing during the late nineteenth and the twentieth centuries to yield the wettest conditions of the past 1,000 years. Comparison with other long-term precipitation reconstructions indicates a large-scale intensification of the hydrological cycle coincident with the onset of industrialization and global warming, and the unprecedented amplitude argues for a human role. [ABSTRACT FROM AUTHOR]

Previous studies have suggested that the Late Glacial period (LG; ∼14 600–11 700 cal BP) was characterised by abrupt and extreme climate variability over the European sector of the North Atlantic. The limited number of precisely dated, high-resolution proxy records, however, restricts our understanding of climate dynamics through the LG. Here, we present the first annually-resolved tree-cellulose stable oxygen and carbon isotope chronology (δ18O tree , δ13C tree) covering the LG between ∼14 050 and 12 795 cal BP, generated from a Swiss pine trees (P. sylvestris ; 27 trees, 1255 years). Comparisons of δ18O tree with regional lake and ice core δ18O records reveal that LG climatic changes over the North Atlantic (as recorded by Greenland Stadials and Inter-Stadials) were not all experienced to the same degree in the Swiss trees. Possible explanations include: (1) LG climate oscillations may be less extreme during the summer in Switzerland, (2) tree-ring δ18O may capture local precipitation and humidity changes and/or (3) decayed cellulose and various micro-site conditions may overprint large-scale temperature trends found in other δ18O records. Despite these challenges, our study emphasises the potential to investigate hydroclimate conditions using subfossil pine stable isotopes. • Millennial stable isotope chronologies of δ13C and δ18O from 27 Swiss subfossil pine trees, covering the Late Glacial (∼14 050–12 795 cal BP). • Two extreme δ18O tree depletions parallel North Atlantic 'cool periods' (GI-1c, GS-1), another LG event (GI-1b) is not expressed in δ18O tree. • Trees with less extensive decay have significantly higher inter-isotope (δ18O tree : δ13C tree) correlations. [ABSTRACT FROM AUTHOR]

In recent decades, the Tibetan plateau (TP) experienced a distinctive temperature increase, with fundamental consequences for the hydrological system. As meteorological time-series extending back more than 60 years are scarce, there is a strong need for proxy data providing insight into the regional hydroclimatic history as well as the long-term variability. Within this study, a recently developed annually resolved 1500 year-long tree-ring stable oxygen isotope (δ 18 O) time from the southeastern TP is presented. Climate–proxy relationships reveal a strong impact of relative humidity (rH) during the summer months (May to September) on tree-ring δ 18 O, explaining around 45% of its variance. The derived reconstruction of relative humidity reveals a recent trend towards drier conditions on the TP. However, the present low rH level is not unprecedented in regard to the last 1500 years. In comparison to recent climate conditions, the Medieval Warm Period (MWP) was characterized by more stable conditions with higher humidity values. Hydroclimatic conditions during the Little Ice Age (LIA) suggest a contrasting two-phased period, with a clear shift from drier conditions prevailing between 1400 and 1650 AD to more humid conditions since the second half of the 17th century. Comparisons with other local proxies from lake levels and Pollen data from the southern part of the Tibetan plateau indicate a common regional climate forcing during the MWP and the LIA which can be related to changes in summer monsoon activity. However, the strength of the distinct dryness trend during the 20th century seems unique. [ABSTRACT FROM AUTHOR]

Abstract: We explore the 20th century climate sensitivity of annually resolved carbon and oxygen isotope ratios in five Pinus uncinata individuals from the upper treeline in ~2400m asl of the Spanish Pyrenees. Time series of δ13C and δ18O are calibrated against temperature, precipitation, and drought indices over the period 1901–2009. Negative correlations of δ13C with summer precipitation and drought indices, as well as positive correlations with summer temperatures, confirm previous evidence from similar habitats in the Pyrenees. In contrast to this summer climate signal in the carbon isotopes, the δ18O record reveals mainly negative correlations with spring precipitation and drought. We explore the coherence between δ13C and δ18O time series derived from individual trees and assess the influence of widely applied δ13C correction procedures on the climate signal strength. Spatial correlation patterns and decomposition of the time series into high- and low-frequency components are used to develop a calibration setup for carbon and oxygen isotope ratios, which will improve long-term climate reconstructions in a region, where classical tree-ring width and density data are limited. [Copyright &y& Elsevier]

Abstract: In tropical Australia, palaeoclimatic proxies derived from tree rings are sought after sources for reconstructing climate variations. However, dendroclimatology has not been widely applied in tropical forests and even less so in the Australian tropics due to the extreme rarity of species producing anatomically distinct annual growth rings. Furthermore, most Australian tree species exhibit rather strong opportunistic growth with non-annual growth zones that are less suitable for dendrochronology. Recent studies on the Australian Red Cedar (Toona ciliata) in the Upper Kangaroo Valley near Sydney revealed that tree-ring based climate reconstructions are feasible with this species. This study moved 2500 km further north and concentrated on the tropical stands of T. ciliata because it is one of the few deciduous tree species in tropical Australia likely characterised by a dormant period of the cambium and thus annual tree rings. Although dendroclimatological studies indicate that some Australian tree species are suitable for reconstructing climate patterns, a well replicated tree-ring record from Far North Queensland has not been developed until now. Tree cores of T. ciliata were developed into a 140-year tree-ring widths index chronology. The analyses showed that the ring-widths indices correlate with March–June precipitation as recorded at Kairi research station. March–June precipitation was reconstructed using the tree-ring data with 35% of the variance explained. The reconstructed series contains both high- and low-frequency climate signals. This suggests that growth of T. ciliata is influenced by climate phenomena of different wave lengths which can be associated with El Niño Southern Oscillation (ENSO) and Interdecadal Pacific Oscillation (IPO). [Copyright &y& Elsevier]

Abstract: We present one millennium-long (1171-year), and three 100 year long annually resolved δ13C tree-ring chronologies from ecologically varying Juniperus stands in the Karakorum Mountains (northern Pakistan), and evaluate their response to climatic and atmospheric CO2 changes. All δ13C records show a gradual decrease since the beginning of the 19th century, which is commonly associated with a depletion of atmospheric δ13C due to fossil fuel burning. Climate calibration of high-frequency δ13C variations indicates a pronounced summer temperature signal for all sites. The low-frequency component of the same records, however, deviates from long-term temperature trends, even after correction for changes in anthropogenic CO2. We hypothesize that these high-elevation trees show a response to both climate and elevated atmospheric CO2 concentration and the latter might explain the offset with target temperature data. We applied several corrections to tree-ring δ13C records, considering a range of potential CO2 discrimination changes over the past 150 years and calculated the goodness of fit with the target via calibration/verification tests (R 2, residual trend, and Durbin–Watson statistics). These tests revealed that at our sites, carbon isotope fixation on longer timescales is affected by increasing atmospheric CO2 concentrations at a discrimination rate of about 0.012‰/ppmv. Although this statistically derived value may be site related, our findings have implications for the interpretation of any long-term trends in climate reconstructions using tree-ring δ13C, as we demonstrate with our millennium-long δ13C Karakorum record. While we find indications for warmth during the Medieval Warm Period (higher than today’s mean summer temperature), we also show that the low-frequency temperature pattern critically depends on the correction applied. Patterns of long-term climate variation, including the Medieval Warm Period, the Little Ice Age, and 20th century warmth are most similar to existing evidence when a strong influence of increased atmospheric CO2 on plant physiology is assumed. [Copyright &y& Elsevier]

• Evaluating trees' responses to climate change can be challenging. • We propose the use of correlation networks to aid the integration of wood traits. • Trees coordinate carbon and water related traits along a tropical climate gradient. • Correlation networks revealed trade-offs that reflects carbon metabolic pathways. • Cell-wall xylose and mannose are central in trees responses to climate variability. Tropical forests uptake more atmospheric CO 2 and transpire more water than any other forest in the world and are critical components of the global carbon and hydrological cycles. Both cycles depend to a great extent on the carbon and water balance of individual trees. Such adjustments are usually evaluated through well-established and newly-emerging traits but integrating them for a systemic understanding of trees' responses to climate change can be challenging. We propose using complex correlation networks to integrate and understand how trees coordinate water- and carbon-related traits under changing climate conditions. We built a correlation network based on 20 traits measured in the wood of Hymenaea courbaril (Leguminosae) trees, a species known for its extreme anisohydric water-use strategy, sampled along a climate gradient in Southeastern Brazil. Intercellular to ambient CO 2 concentrations ratio (c i /c a , estimated from tree-ring δ13C) is a central network trait for being coordinated with several hydraulic and carbon allocation traits. Trees of H. courbaril coordinate these traits along the climate gradient, favoring high c i /c a under warm and dry conditions. A high c i /c a is only possible through a consistent water supply provided by wider vessels together with the investment on soluble sugars, at the detriment of starch, likely for hydraulic maintenance. Trees also favor heat resistance by investing in cell-wall xylose, another central network trait, from xyloglucans and xylans, at the expense of mannose from glucomannans. Such trade-offs within, and between, structural and non-structural carbon allocation reflect well-known metabolic pathways in plants. In summary, this systemic approach confirms previously reported patterns on leaf physiology, stem hydraulics and carbon adjustments while bringing to light the previously unreported role of cell-wall composition and its fine adjustments to cope with climate change. [ABSTRACT FROM AUTHOR]