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Dry spells strongly influence biomass production in forest ecosystems. Their effects may last several years following a drought event, prolonging growth reduction and therefore restricting carbon sequestration. Yet, our understanding of the impact of dry spells on the vitality of trees' above-ground biomass components (e.g., stems and leaves) at a landscape level remains limited. We analyzed the responses of Pinus sylvestris and Picea abies to the four most severe drought years in topographically complex sites. To represent stem growth and canopy greenness, we used chronologies of tree-ring width and time series of the Normalized Difference Vegetation Index (NDVI). We analyzed the responses of radial tree growth and NDVI to dry spells using superposed epoch analysis and further explored this relationship using mixed-effect models. Our results show a stronger and more persistent response of radial growth to dry spells and faster recovery of canopy greenness. Canopy greenness started to recover the year after the dry spell, whereas radial tree growth remained reduced for the two subsequent years and did not recover the pre-drought level until the fourth year after the event. Stem growth and canopy greenness were influenced by climatic conditions during and after drought events, while the effect of topography was marginal. The opposite responses of stem growth and canopy greenness following drought events suggest a different impact of dry spells on trees´ sink and source compartments. These results underscore the crucial importance of understanding the complexities of tree growth as a major sink of atmospheric carbon. [ABSTRACT FROM AUTHOR]

The growth of past, present, and future forests was, is and will be affected by climate variability. This multifaceted relationship has been assessed in several regional studies, but spatially resolved, large-scale analyses are largely missing so far. Here we estimate recent changes in growth of 5800 beech trees (Fagus sylvatica L.) from 324 sites, representing the full geographic and climatic range of species. Future growth trends were predicted considering state-of-the-art climate scenarios. The validated models indicate growth declines across large region of the distribution in recent decades, and project severe future growth declines ranging from −20% to more than −50% by 2090, depending on the region and climate change scenario (i.e. CMIP6 SSP1-2.6 and SSP5-8.5). Forecasted forest productivity losses are most striking towards the southern distribution limit of Fagus sylvatica, in regions where persisting atmospheric high-pressure systems are expected to increase drought severity. The projected 21st century growth changes across Europe indicate serious ecological and economic consequences that require immediate forest adaptation. Tree ring data from a network of beech tree stands across Europe show evidence for a recent growth decline from 1986-2016 and project up to 50% growth reductions in some areas of Europe with future climate change. [ABSTRACT FROM AUTHOR]

Keywords: Aspromonte; conservation biology; habitat tree; life-history trait; longevity; mountain forest; nature conservation; old tree; Quercus; radiocarbon dating; sessile oak; sustainable development; tree dating EN Aspromonte conservation biology habitat tree life-history trait longevity mountain forest nature conservation old tree Quercus radiocarbon dating sessile oak sustainable development tree dating 1 4 4 12/03/20 20201201 NES 201201 The longevity of trees is a major life-history trait that could provide crucial information about ecological history, forest dynamics, and the carbon cycle. Moreover, tree age determination based on SP 14 sp C has been successfully used for dating tree species lacking annual growth rings, such as those from the tropics (Chambers et al. 1998). Dating Aspromonte sessile oaks has proven that radiocarbon dating might be a better approach for accurately estimating tree longevity when the inner part of the trunk is rotten, as has been done previously for other old English oaks in Romania (Appendix S2: Section S2). [Extracted from the article]

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

Understanding how climate has modulated forest growth and composition in the past is necessary to predict the influence of the ongoing climate warming on the dynamics of mountain forests. We studied the past dynamics of subalpine Pyrenean forests during the last 700 years by assessing the relationships between sedimentary pollen and tree-ring records, and their link with climatic drivers. We compared the pollen record and the montane pollen ratio, an integrative index obtained from sedimentary pollen that allows inferring past altitudinal variations in the montane-subalpine ecotone, with tree-ring width from mountain pine (Pinus uncinata) subalpine forests located in Central Pyrenees. To assess climate-growth associations, we related the dendrochronological data with instrumental meteorological records (1901-2010) and temperature reconstructions from the Pyrenees and Northern Hemisphere. Few robust associations were found between arboreal pollen taxa and tree-ring width series of the surrounding forests. However, significant correlations were found between the montane pollen ratio and tree-ring width series from nearby forests (located less than 10 km apart). This relationship could be potentially useful to infer long-term forest growth changes at decadal to centennial scales using the montane pollen ratio. On the contrary, our results show that tree radial growth has mainly been constrained by low temperatures although the growth sensitivity to climate has considerably varied over the last 700 years. Similar results were obtained for the last century as growth variability of these high-elevation forests is still driven by low temperatures, but a relaxation of this constrain in recent decades has been detected. [ABSTRACT FROM AUTHOR]

July-to-October temperature variations are reconstructed for the last 800 years based on tree-ring widths from the Cazorla Range. Annual tree-ring width at this site has been found to be negatively correlated with temperature of the previous summer. This relationship is genuine, metabolically plausible, and cannot be explained as an indirect correlation mediated by hydroclimate. The resulting reconstruction (NCZ) represents the southernmost annually resolved temperature record based on tree-rings in Europe and provides detailed information on the regional climate evolution during the Late Holocene in the southeast of the Iberian Peninsula. The tree-ring based temperature reconstruction of Cazorla Range reveals predominantly warm summer temperatures during the transition between the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) from the 13th to the mid of the sixteenth century. The LIA spanned a slightly longer time (1500-1930 CE) than in other European summer temperature reconstructions from the Alps and Pyrenees. The twentieth century, though warmer than the preceding centuries, does not show unprecedented warmth in the last 800 years. Three ensembles of climate simulations conducted with two global atmosphere-ocean general circulation climate models (GCMs), considering different external forcings, were used for comparison: ECHO-G (Erik) and MPI-ESM (E1 and E2). Additionally, individual simulations were available from GCM included in the fifth Coupled Model Intercomparison Project, as well as single-forcing simulations performed with MPI-ESM. The comparison of the reconstructed and simulated temperatures revealed a close agreement of NCZ with the simulations performed with total solar irradiance forcing with wider amplitude. Furthermore, the correlations with single-forcing simulations suggest volcanism as the main factor controlling preindustrial summer temperature variations in the Cazorla Range over the last five centuries. The persistent anti-correlation between NCZ and simulated temperatures during the MCA-LIA transitional period underlines the current limitations for attributing temperature variations during that period to internal variability or external forcing. [ABSTRACT FROM AUTHOR]

In the eastern Mediterranean in general and in Turkey in particular, temperature reconstructions based on tree rings have not been achieved so far. Furthermore, centennial-long chronologies of stable isotopes are generally also missing. Recent studies have identified the tree species Juniperus excelsa as one of the most promising tree species in Turkey for developing long climate sensitive stable carbon isotope chronologies because this species is long-living and thus has the ability to capture low-frequency climate signals. We were able to develop a statistically robust, precisely dated and annually resolved chronology back to AD 1125. We proved that variability of δC in tree rings of J. excelsa is mainly dependent on winter-to-spring temperatures (January-May). Low-frequency trends, which were associated with the medieval warm period and the little ice age, were identified in the winter-to-spring temperature reconstruction, however, the twentieth century warming trend found elsewhere could not be identified in our proxy record, nor was it found in the corresponding meteorological data used for our study. Comparisons with other northern-hemispherical proxy data showed that similar low-frequency signals are present until the beginning of the twentieth century when the other proxies derived from further north indicate a significant warming while the winter-to-spring temperature proxy from SW-Turkey does not. Correlation analyses including our temperature reconstruction and seven well-known climate indices suggest that various atmospheric oscillation patterns are capable of influencing the temperature variations in SW-Turkey. [ABSTRACT FROM AUTHOR]

Abstract: δ 13C and δ 18O of tree rings contain time integrated information about the environmental conditions weighted by seasonal growth dynamics and are well established as sources of palaeoclimatic and ecophysiological data. Annually resolved isotope chronologies are frequently produced by pooling dated growth rings from several trees prior to the isotopic analyses. This procedure has the advantage of saving time and resources, but precludes from defining the isotopic error or statistical uncertainty related to the inter-tree variability. Up to now only a few studies have compared isotope series from pooled tree rings with isotopic measurements from individual trees. We tested whether or not the δ 13C and the δ 18O chronologies derived from pooled and from individual tree rings display significant differences at two locations from the Iberian Peninsula to assess advantages and constraints of both methodologies. The comparisons along the period 1900–2003 reveal a good agreement between pooled chronologies and the two mean master series which were created by averaging raw individual values (Mean) or by generating a mass calibrated mean (MassC). In most of the cases, pooled chronologies show high synchronicity with averaged individual samples at interannual scale but some differences also show up especially when comparing δ 18O decadal to multi-decadal variations. Moreover, differences in the first order autocorrelation among individuals may be obscured by pooling strategies. The lack of replication of pooled chronologies prevents detection of a bias due to a higher mass contribution of one sample but uncertainties associated with the analytical process itself, as sample inhomogeneity, seems to account for the observed differences. [Copyright &y& Elsevier]

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