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The response of evapotranspiration to anthropogenic warming is of critical importance for the water and carbon cycle. Contradictory conclusions about evapotranspiration changes are caused primarily by their brevity in time and sparsity in space, as well as the strong influence of internal variability. Here, we present the first gridded reconstruction of the summer (June, July, and August) vapor pressure deficit (VPD) for the past 4 centuries at the European level. This gridded reconstruction is based on 26 European tree ring oxygen isotope records and is obtained using a random forest approach. According to validation scores obtained with the Nash–Sutcliffe model efficiency, our reconstruction is robust over large parts of Europe since 1600, in particular for the westernmost and northernmost regions, where most tree ring records are located. Based on our reconstruction, we show that from the mid-1700s a trend towards higher summer VPD occurred in central Europe and the Mediterranean region that is related to a simultaneous increase in temperature and decrease in precipitation. This increasing summer VPD trend continues throughout the observational period and in recent times. Moreover, our summer VPD reconstruction helps to visualize the local and regional impacts of the current climate change, as well as to minimize statistical uncertainties of historical VPD variability. This paper provides also new insights into the relationship between summer VPD and large-scale atmospheric circulation, and we show that summer VPD has two preferred modes of variability, namely a NW–SE dipole-like mode and a N–S dipole-like mode. Furthermore, the interdisciplinary use of the data should be emphasized, as summer VPD is a crucial parameter for many climatological feedback processes in the Earth's surface system. The reconstructed summer VPD gridded data over the last 400 years are available at the following link: 10.5281/zenodo.5958836 (Balting et al., 2022). [ABSTRACT FROM AUTHOR]

The response of evapotranspiration to anthropogenic warming is of critical importance for the water and carbon cycle. Contradictory conclusions about evapotranspiration changes are caused primarily by their brevity in time and sparsity in space, as well as the strong influence of internal variability. Here, we present the first gridded reconstruction of the summer vapour pressure deficit (VPD) for the past four centuries at the European level. This gridded reconstruction is based on 26 European tree-ring oxygen isotope records and is obtained using a Random Forest approach. Based on our reconstruction, we show that from the mid- 1700s, a trend towards higher VPD occurred in Central Europe and the Mediterranean region which is related to a simultaneous increase in temperature and decrease in precipitation. This increasing VPD trend continues throughout the observational period and in recent times. Moreover, our VPD reconstruction helps to visualize the local and regional impacts of the current climate change as well as to minimize statistical uncertainties of historical VPD variability. Furthermore, the interdisciplinary use of the data should be emphasized, as VPD is a crucial parameter for many climatological feedback processes in the earth surface system. The reconstructed VPD gridded data, over the last 400 years, is available at the following link: https://doi.org/10.5281/zenodo.5958836 (Balting, D. F. et al., 2022). [ABSTRACT FROM AUTHOR]

Changes to the Amazon hydrological cycle have important consequences for world's largest tropical forest, and the biodiversity it contains. However, a scarcity of long‐term climate data in the region makes it hard to contextualize recent observed changes in Amazon hydrology. Here, we explore to what extent tree‐ring oxygen isotope (δ18OTR) chronologies can inform us about hydrological changes in the Amazon over the past two centuries. Two δ18OTR records from northern Bolivia and the Ecuadorian Andes are presented. The Ecuador record spans 1799–2012 (n = 16 trees) and the Bolivia record spans 1860–2014 (n = 32 trees), making them the longest δ18OTR records from the Amazon, and among the most highly‐replicated δ18OTR records from the tropics to date. The two chronologies correlate well at interannual and decadal timescales, despite coming from sites more than 1,500 km apart. Both δ18OTR records are strongly related to interannual variation in Amazon River discharge measured at Óbidos, and accumulated upwind precipitation, suggesting a common climatic driver. In both records a strong increase in δ18OTR was observed up until approximately 1950, consistent with positive trends in the few other existing δ18O proxy records from across the Amazon. Considering all possible drivers of this long‐term increase, a reduction in rainout fraction over the basin driven by rising sea surface temperatures in the North Atlantic is suggested as the most likely cause. The upward trend in δ18OTR reverses over the past 1–2 decades, consistent with the observed strengthening of the Amazon hydrological cycle since approximately 1990. Plain Language Summary: Long‐term climate measurements from the Amazon are few and far between, so information about the climate in the past needs to come from alternative sources. This is essential to provide context for current climate variability such as the strengthening of the Amazon water cycle since the 1990s. One way we can reconstruct historical climate is using stable oxygen isotopes recorded in tree rings. Previous work has shown that tree‐ring oxygen isotopes from the western Amazon can tell you about the amount of rainfall over the entire basin. Here, we present tree‐ring isotope records from Ecuador and Bolivia that span the past two centuries, making them the longest tree‐ring oxygen isotope records from the region to date. We find that the two records are very similar to one another, despite coming from sites that are more than 1,500 km apart, suggesting they are both influenced by the same climatic factors. We explore the climate signals that these records contain, including interpreting a long‐term positive trend in the isotope record until around 1950. We conclude that the most likely cause of this trend is a long‐term reduction in the fraction of incoming water vapor that falls as rainfall over the Amazon. Key Points: We present two well‐replicated tree‐ring oxygen isotope (δ18OTR) records from tropical South America spanning the past two centuriesThese records capture important large‐scale climate signals, including interannual variation in Amazon basin‐wide precipitationA multi‐decadal long‐term increase in δ18OTR is interpreted as being primarily driven by a long‐term reduction in rainout fraction [ABSTRACT FROM AUTHOR]

Poor drought tolerance of European beech trees raised concerns in Europe. We hypothesized that beech could show an opposite physiological response to the same level of climatic drought with change in edaphic drought. We performed a combined analysis of δ13C and δ18O in tree rings to reveal retrospective temporal physiological responses of trees to drought. The edaphic drought was assessed by quantifying the capacity of soil to store water in plots (classified as "dry" and "less-dry") near the drought limit of the species in three near-natural oak-beech ecotones in Germany and Switzerland. Neighbourhood competition was quantified. A climatic drought index was calculated from meteorological records and related to the δ13C and δ18O values of the trees. Trees from dry plots showed a higher response to drought and climatic dependency than less-dry plots. Neighbourhood competetion increased δ18O values significantly. Dual isotope analysis shows a tendency of greater stomatal resistance in dry plots and higher stomatal conductance in less-dry plots. We conclude that beech trees belonging to the same population under changing soil water availability can show different physiological responses under climatic drought stress. Our finding indicates the high plasticity of the beech trees to survive drought stress with changing site conditions. [ABSTRACT FROM AUTHOR]

Light and water availability are likely to vary over the lifespan of closed-canopy forest trees, with understory trees experiencing greater limitations to growth by light and canopy trees greater limitation due to drought. As drought and shade have opposing effects on isotope discrimination (Δ13C), paired measurement of ring width and Δ13C can potentially be used to differentiate between water and light limitations on tree growth. We tested this approach for Cedrela trees from three tropical forests in Bolivia and Mexico that differ in rainfall and canopy structure. Using lifetime ring width and Δ13C data for trees of up to and over 200 years old, we assessed how controls on tree growth changed from understory to the canopy. Growth and Δ13C are mostly anti-correlated in the understory, but this anti-correlation disappeared or weakened when trees reached the canopy, especially at the wettest site. This indicates that understory growth variation is controlled by photosynthetic carbon assimilation due to variation in light levels. Once trees reached the canopy, inter-annual variation in growth and Δ13C at one of the dry sites showed positive correlations, indicating that inter-annual variation in growth is driven by variation in water stress affecting stomatal conductance. Paired analysis of ring widths and carbon isotopes provides significant insight in what environmental factors control growth over a tree's life; strong light limitations for understory trees in closed-canopy moist forests switched to drought stress for (sub)canopy trees in dry forests. We show that combined isotope and ring width measurements can significantly improve our insights in tree functioning and be used to disentangle limitations due to shade from those due to drought. [ABSTRACT FROM AUTHOR]

Author(s): Kerstin S. Treydte (corresponding author) [1]; Gerhard H. Schleser [2]; Gerhard Helle [2]; David C. Frank [1]; Matthias Winiger [3]; Gerald H. Haug [4]; Jan Esper [1] Twentieth-century warming [...]

The response of evapotranspiration to anthropogenic warming is of critical importance to the water and carbon cycle. Conflicting observations about changes of evapotranspiration stem mostly from the brevity of observations in time and space as well as a high degree of internal variability. Here we present the first gridded reconstruction of the European summer vapour pressure deficit (VPD) for the past four centuries. The gridded reconstruction is based on 26 European tree-ring oxygen isotope records and is performed using a Random Forest approach. Based on our reconstruction, we show that from the mid- 1700s a trend towards higher VPD occurred in Central Europe and the Mediterranean region which is based on the simultaneous increase in temperature and decrease in precipitation. This increasing VPD trend continues throughout the observational period and recent times. Climate model projections show this increase in VPD for the Mediterranean region continuing until the end of the 21st century, whereby the extent depends on the amount of greenhouse gas emissions. In contrast, projected VPD in North and Central Europe shows a tendency towards higher VPD only in the highest emission scenario. The reconstructed, observed and modelled VPD for past present and future is available here: https://doi.org/10.5281/zenodo.5958837 (Balting, D. F. et al., 2022). [ABSTRACT FROM AUTHOR]

Robust reconstruction of past climate and environmental change based on proxy data obtained from natural archives requires an in-depth understanding of the processes and mechanisms that form and determine these proxies. Here we present comprehensive long-term monitoring projects for seasonally laminated (varved) lake sediments and tree rings in the northern German lowlands. The two monitoring sites are located in the nature park Nossentiner/Schwinzer Heide (Tiefer See) and in the Müritz National Park (tree rings) and are an integral part of the Helmholtz TERrestrial ENvironmental Observatories (TERENO) infrastructure initiative. Both sites are located in the close vicinity of moraine deposits of the main ice advance of the Pomeranian phase of the Weichselian glaciation. This field guide provides an introduction to the local morphologies and landscapes as well as details of the monitoring concepts and some selected results. [ABSTRACT FROM AUTHOR]

Large earthquakes can increase the amount of water feeding stream flows, raise groundwater levels, and thus grant plant roots more access to water in water‐limited environments. We examine growth and photosynthetic responses of Pine plantations to the Maule Mw 8.8 earthquake in headwater catchments of Chile's Coastal Range. We combine high‐resolution wood anatomic (lumen area) and biogeochemical (δ13C of wood cellulose) proxies of daily to weekly tree growth sampled from trees on floodplains and close to ridge lines. We find that, immediately after the earthquake, at least two out of six tree trees on valley floors had increased lumen area and decreased δ13C, while trees on hillslopes had a reverse trend. Our results indicate a control of soil water on this response, largely consistent with models that predict how enhanced postseismic vertical soil permeability causes groundwater levels to rise on valley floors, but fall along the ridges. Statistical analysis with boosted regression trees indicates that streamflow discharge gained predictive importance for photosynthetic activity on the ridges, but lost importance on the valley floor after the earthquake. We infer that earthquakes may stimulate ecohydrological conditions favoring tree growth over days to weeks by triggering stomatal opening. The weak and short‐lived signals that we identified, however, show that such responses are only valid under water‐limited, rather than energy‐limited tree, growth. Hence, dendrochronological studies targeted at annual resolution may overlook some earthquake effects on tree vitality. Plain Language Summary: Earthquakes deform and shake Earth's surface and the ground below. These changes may affect groundwater flow. Groundwater level may rise in the valley bottom and drop along higher elevated ridges. Trees depend on such groundwater resources, particularly in dry climates. Hence, we expect contrasting responses of trees after earthquakes: at higher elevations, access to groundwater may be impeded, but enabled in the valley bottoms. Thus, earthquake‐enhanced tree growth should be pronounced only on valley floors, with opposite responses happening along ridges. We test this hypothesis in pine forest plantations that were affected by the 2010 Maule earthquake, Chile. We find that tree growth increased following the earthquake because of enhanced photosynthesis on valley floors, but decreased on upper hillslopes due to increased water stress. Overall, these responses are small but measurable. Our study is the first to combine state‐of‐the‐art isotopic and wood anatomic proxies that we quantified at the cellular scale. Our results provide novel insights into the impacts of earthquakes on soil water and tree growth at an unprecedented daily to weekly resolution. Key Points: Earthquakes may stimulate tree growth by promoting photosynthesisDirection of tree growth change depends on local topographic positionFirst dendroecohydrological study to explore earthquake‐water‐vegetation interactions at scale of cells [ABSTRACT FROM AUTHOR]

The African baobab, Adansonia digitata L., has great paleoclimatological potential because of its wide distributional range and millennial length life span. However, dendroclimatological approaches are hampered by dating uncertainties due to its unique, parenchyma-dominated stem anatomy. Here, securely dated time series of annual wood increment growth and intra-ring stable isotopes of carbon and oxygen of cellulose for a baobab tree from Oman covering 1941 to 2005 were established and tested for relationships to hydroclimate variability. Precise dating with the atomic bomb peak (ABP) using highly resolved 14 C measurements confirmed the annual character of the baobab's growth rings. F 14 C values of tree-ring cellulose were found up to 8.8 % lower than in the corresponding atmospheric CO 2 for the period around the ABP, which in conjunction with a considerable autocorrelation of the δ13 C series points to the incorporation of previous year's carbon contributing to the average age of intra-ring wood samples. F 14 C of terminal parenchyma bands, marking the tree-ring boundaries, were found to be considerably younger than their corresponding tree ring, indicating that parenchyma tissue is alive for many years, probably undergoing cell division and structural reorganization and contributing to secondary growth. In contrast to the δ13 C time series, no significant autocorrelation was found in the δ18 O series of tree-ring cellulose despite the enormous water storage potential of this stem-succulent tree species. Year-to-year variability in tree-ring width and stable isotope ratios revealed radial stem growth and the geochemistry of wood cellulose are influenced by fluctuations in the hydroclimate. In particular, δ18 O was found to be a good climate proxy, followed by tree-ring width and δ13 C. Tree-ring width and intra-ring δ18 O min correlated well with each other and with precipitation amount for the period from pre-monsoon May to the end of the monsoon season in September/October. Intra-annual stable isotope courses were found to be rather similar for both δ13 C and δ18 O. Years with particularly low monsoon rain were reflected by increased stable isotope values in the mid-section of intra-annual courses. Distinct patterns with low subseasonal isotope values seem indicative for years with heavy rainfall events from pre-monsoonal cyclones. Rain events from post-monsoonal cyclones may also be recorded; however, only 2 years of observation prevented a more conclusive evaluation. [ABSTRACT FROM AUTHOR]

We investigate the climate signature of δ18O tree-ring records from sites distributed all over Europe covering the last 400 years. An empirical orthogonal function (EOF) analysis reveals two distinct modes of variability on the basis of the existing δ18O tree-ring records. The first mode is associated with anomaly patterns projecting onto the El Niño–Southern Oscillation (ENSO) and reflects a multi-seasonal climatic signal. The ENSO link is pronounced for the last 130 years, but it is found to be weak over the period from 1600 to 1850, suggesting that the relationship between ENSO and the European climate may not be stable over time. The second mode of δ18O variability, which captures a north–south dipole in the European δ18O tree-ring records, is related to a regional summer atmospheric circulation pattern, revealing a pronounced centre over the North Sea. Locally, the δ18O anomalies associated with this mode show the same (opposite) sign with temperature (precipitation). Based on the oxygen isotopic signature derived from tree rings, we argue that the prevailing large-scale atmospheric circulation patterns and the related teleconnections can be analysed beyond instrumental records. [ABSTRACT FROM AUTHOR]

Purification protocols to extract pollen from lake sediments contain chemicals that alter the carbon and oxygen pollen‐isotope values according to pollen characteristics and family affiliation. Modern (raw) pollen of broad‐leaved (Alnus glutinosa, Betula pendula, Carpinus betulus, Corylus avellana, Fagus sylvatica and Quercus robur) and coniferous tree species (Picea abies and Pinus sylvestris) were treated with potassium hydroxide (KOH), hydrofluoric acid (HF), sodium hypochlorite (NaClO) and sulphuric acid (H2SO4) to test the impact on δ13Cpollen and δ18Opollen and assess the applicability in purification protocols. Pollen of broad‐leaved and coniferous trees reacted differently to chemical exposure, but response patterns are generally alike. Alterations of δ13Cpollen values vary between + 1.0‰ (B. pendula, NaClO‐treatment) and −5.0‰ (P. sylvestris, H2SO4‐treatment). The δ13Cpollen values of raw and chemically treated samples seem to be related after treatments with KOH, NaClO and HF, whereas the application of H2SO4 led to inconsistent changes among species. The impact of chemicals on δ18Opollen are more diverse and offsets range between +1.1‰ (C. avellana, NaClO‐treatment) and −17.9‰ (P. sylvestris, H2SO4‐treatment). In general, the use of isotope‐altering chemicals in purification protocols should be brought to a minimum, but the application of KOH and NaClO seems mostly unproblematic before δ13Cpollen and δ18Opollen analysis. [ABSTRACT FROM AUTHOR]

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]

Advances in accelerator mass spectrometry have resulted in an unprecedented amount of new high-precision radiocarbon (14C) -dates, some of which will redefine the international 14C calibration curves (IntCal and SHCal). Often these datasets are unaccompanied by detailed quality insurances in place at the laboratory, questioning whether the 14C structure is real, a result of a laboratory variation or measurement-scatter. A handful of intercomparison studies attempt to elucidate laboratory offsets but may fail to identify measurement-scatter and are often financially constrained. Here we introduce a protocol, called Quality Dating, implemented at ETH-Zürich to ensure reproducible and accurate high-precision 14C-dates. The protocol highlights the importance of the continuous measurements and evaluation of blanks, standards, references and replicates. This protocol is tested on an absolutely dated German Late Glacial tree-ring chronology, part of which is intercompared with the Curt Engelhorn-Center for Archaeometry, Mannheim, Germany (CEZA). The combined dataset contains 170 highly resolved, highly precise 14C-dates that supplement three decadal dates spanning 280 cal. years in IntCal, and provides detailed 14C structure for this interval. [ABSTRACT FROM AUTHOR]

Stable carbon and oxygen isotope ratios of raw pollen sampled from nine abundant tree species growing in natural habitats of central and northern Europe were investigated to understand the intra- and inter-specific variability of pollen-isotope values. All species yielded specific δ13Cpollen and δ18Opollen values and patterns, which can be ascribed to their physiology and habitat preferences. Broad-leaved trees flowering early in the year before leaf proliferation (Alnus glutinosa and Corylus avellana) exhibited on average 2.6‰ lower δ13Cpollen and 3.1‰ lower δ18Opollen values than broad-leaved and coniferous trees flowering during mid and late spring (Acer pseudoplatanus, Betula pendula, Carpinus betulus, Fagus sylvatica, Picea abies, Pinus sylvestris and Quercus robur). Mean species-specific δ13Cpollen values did not change markedly over time, whereas δ18Opollen values of two consecutive years were often statistically distinct. An intra-annual analysis of B. pendula and P. sylvestris pollen revealed increasing δ18Opollen values during the final weeks of pollen development. However, the δ13Cpollen values remained consistent throughout the pollen-maturation process. Detailed intra-individual analysis yielded circumferential and height-dependent variations within carbon and oxygen pollen-isotopes and the sampling position on a tree accounted for differences of up to 3.5‰ for δ13Cpollen and 2.1‰ for δ18Opollen. A comparison of isotope ranges from different geographic settings revealed gradients between maritime and continental as well as between high and low altitudinal study sites. The results of stepwise regression analysis demonstrated, that carbon and oxygen pollen-isotopes also reflect local non-climate environmental conditions. A detailed understanding of isotope patterns and ranges in modern pollen is necessary to enhance the accuracy of palaeoclimate investigations on δ13C and δ18O of fossil pollen. Furthermore, pollen-isotope values are species-specific and the analysis of species growing during different phenophases may be valuable for palaeoweather reconstructions of different seasons. [ABSTRACT FROM AUTHOR]

We investigate the annual variability of δ18O tree ring records from sites distributed all over Europe covering the last 400 years. An Empirical Orthogonal Function (EOF) analysis reveals two distinct modes of variability on the basis of the existing δ18O tree ring records. The first mode of δ18O variability is associated with anomaly patterns of the El Niño-Southern Oscillation (ENSO) and reflects a multi-seasonal climatic signal. The ENSO signal is visible for the last 130 years, but is found weak during the period 1600 to 1850 suggesting that the relationship between ENSO and the European climate may not stable over time. The second mode of δ18O variability, which captures an out-of-phase variability between northwestern and southeastern European δ18O tree ring records, is related to a regional summer atmospheric circulation pattern revealing a pronounced centre over the North Sea. Locally, the δ18O anomalies associated with this mode show the same (opposite) sign with temperature (precipitation). We infer that the investigation of large-scale atmospheric circulation patterns and related teleconnections far beyond instrumental records can be done with oxygen isotopic signature derived from tree rings. However, the European δ18Ocel tree network needs to be consolidated and updated, as well as additional research on the stationarity of reconstructed climate signals and the stationarity of teleconnections is advisable. [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]

The African baobab, Adansonia digitata, has great paleoclimatological potential because of its wide distributional range and millennial lifespan. However, dendroclimatological approaches are hampered by dating uncertainties due to its parenchyma-dominated wood anatomy. Here, securely-dated time series of annual wood increment growth and intra-ring stable isotopes of carbon and oxygen of cellulose for a baobab tree from Oman covering 1941 to 2005 were established. Precise dating with the bomb peak by highly resolved 14C measurements proved the annual character of the baobab's growth rings. But, F14C values of tree-ring cellulose were found up to 8.8 % lower than in the corresponding atmospheric CO2 for the period around the bomb peak. In conjunction with a considerable autocorrelation of the δ13C series this points to the incorporation of previous years' carbon significantly affecting the average age of derived wood. Terminal parenchyma bands, marking the tree-ring boundaries, were found to be significantly younger than their corresponding tree ring indicating that parenchyma tissue is alive for many years undergoing cell division, reorganization and potential growth. No autocorrelation was found in the δ18O series of tree-ring cellulose despite the huge water-storage potential of this stem-succulent tree species. Tree-ring width and stable isotope ratios have revealed significant climate sensitivity. δ18O was found to be a good climate proxy followed by tree-ring width and δ13C. Tree-ring width and intra-ring δ18Omin correlated well with each other and with precipitation amount for the period from pre-monsoon May to the end of the monsoon season in September/October. Intra-annual stable isotope courses were found rather similar for both δ13C and δ18O. Years with particularly low monsoon rain were reflected by increased stable isotope values in the mid-section of intra-annual courses. Distinct patterns with low subseasonal isotope values seem indicative for years with heavy rainfall events from pre-monsoonal cyclones. Rain events from post-monsoonal cyclones may also be recorded, however, only two years of observation prevent from a conclusive evaluation. [ABSTRACT FROM AUTHOR]

Scots pine (Pinus sylvestris L.) is a widely used tree species in European dendroclimatology studies due to its common distribution across much of the continent. Almost all studies find radial growth strongly related to summer temperature, a result reflecting site selection at high elevation/latitude environments where trees grow at their ecophysiological limits. Due to the amount of attention spent on these sites there is a geographical and seasonal bias in temperature reconstructions based upon tree‐ring proxies in Europe. To overcome the limited availability of tree‐ring data in temperate lowlands, we present a northern Poland ring‐width chronology developed from living and historic Scots pine material with a strong common growth signal going back to AD 1200. Investigations into climate‐growth relationships found year‐to‐year ring‐width variability to be more strongly correlated to cold season temperature (November to April) prior to the growing season than summer temperatures during tree‐ring formation. Based on this relationship it was possible to reconstruct cold season temperature conditions for the last 810 years. Spatial field correlations with gridded instrumental records indicated that the reconstruction provides relevant cold season temperature information across the land regions bordering the North Atlantic Ocean and Baltic Sea, lowlands and uplands of western and central Europe, and the eastern and central interior of Russia. Despite an unsuccessful attempt to find a stationary relationship with the North Atlantic Oscillation, comparisons with several cold season temperature reconstructions confirmed the long‐term connection between our reconstructed temperature series for northern Poland and the wider area. [ABSTRACT FROM AUTHOR]

Abstract: In forests, the increase in 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 10 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. [ABSTRACT FROM AUTHOR]