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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.

Assessments of climate change impacts on forests and their vitality are essential for semi-arid environments such as Central Asia, where the mountain regions belong to the globally important biodiversity hotspots. Alterations in species distribution or drought-induced tree mortality might not only result in a loss of biodiversity but also in a loss of other ecosystem services. Here, we evaluate spatial trends and patterns of the growth-climate relationship in a tree-ring network comprising 33 juniper sites from the northern Pamir-Alay and Tien Shan mountain ranges in eastern Uzbekistan and across Kyrgyzstan for the common period 1935-2011. Junipers growing at lower elevations are sensitive to summer drought, which has increased in intensity during the studied period. At higher elevations, juniper growth, previously favored by warm summer temperatures, has in the recent few decades become negatively affected by increasing summer aridity. Moreover, response shifts are observed during all seasons. Rising temperatures and alterations in precipitation patterns during the past eight decades can account for the observed increase in drought stress of junipers at all altitudes. The implications of our findings are vital for the application of adequate long-term measures of ecosystem conservation, but also for paleo-climatic approaches and coupled climate-vegetation model simulations for Central Asia.

The increasing carbon dioxide (CO2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land-atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data., (© 2014 John Wiley & Sons Ltd.)

The carbon isotope composition (δ13C) in tree rings were used to derive the intrinsic water-use efficiency (iWUE) of Araucaria araucana trees of northern Patagonia along a strong precipitation gradient. It is well known that climatic and ontogenetic factors affect growth performance of this species but little is known about their influence in the physiological responses, as iWUE. Thus, the main objective of this study was to assess the physiological reactions of young and adult trees from two open xeric and two moderately dense mesic A. araucana forests to the increases in atmospheric CO2 (C a) and air temperature during the 20th century, and to relate these responses with radial tree growth. The results indicated that the iWUE and the intercellular CO2 concentration (C i) increased 33% and 32% in average during the last century, respectively, but carbon isotope discrimination (∆13C) was more variable between sites and age classes. Trees from xeric sites presented greater iWUE and lower ∆13C and C i values than those from mesic sites. In general, iWUE was strongly related with C a and was significantly affected by mean summer maximum temperature. ∆13C from mesic sites seemed to be mainly affected by summer maximum temperature, while trees from xeric conditions did not show any influence. Tree age also presented a significant effect on iWUE. Adult trees showed higher iWUE values than young trees, indicating an incidence of the tree age and/or height, mainly in closed mesic forests. Moreover, some trees presented positive relationships between iWUE and radial tree growth, while others presented negative or no relationships, indicating that other factors may negatively influence tree growth. Broadly, the results demonstrate the incidence of climatic, environmental and ontogenetic variability in the tree responses; however, more studies are needed to better understand which forests will be more affected by actual and future climate changes. [ABSTRACT FROM AUTHOR]

We present new tree-ring width, δC, and δO chronologies from the Koksu site (49°N, 86° E, 2,200 m asl), situated in the Russian Altai. A strong temperature signal is recorded in the tree-ring width (June-July) and stable isotope (July-August) chronologies, a July precipitation signal captured by the stable isotope data. To investigate the nature of common climatic patterns, our new chronologies are compared with previously published tree-ring and stable isotope data from other sites in the Altai region. The temperature signal preserved in the conifer trees is strongly expressed at local and regional scales for all studied sites, resulting in even stronger temperature and precipitation signals in combined average chronologies compared to separate chronologies. This enables the reconstruction of June-July and July-August temperatures for the last 200 years using tree-ring and stable carbon isotopes. A July precipitation reconstruction based on oxygen isotopic variability recorded in tree-rings can potentially improve the understanding of hydrological changes and the occurrence of extreme events in the Russian Altai. [ABSTRACT FROM AUTHOR]

Tree-ring-based temperature reconstructions form a substantial part of the international proxy data base used to examine and model global climate variations of the last Millennium. However, most tree-ring-based reconstructions are derived from study sites in the high latitudes or high altitudes, paying very little attention to low elevation sites. Thus, a large gap in the geographical coverage of climate reconstructions from temperate low elevation sites in central Europe still exists. This motivated us to concentrate our efforts on the European oak ( Quercus robur ) in N-Poland. We developed a new robust tree-ring width chronology (TRW), as well as four wood anatomical chronologies (e.g. average vessel area and number of vessels) from Q. robur for the period 1810 to 2010. The chronologies were examined for their climatological responses. While TRW was found to have weak correlations with climate, the earlywood vessel parameters (EVP), especially average vessel area (AVA), revealed significant positive correlations to minimum winter temperatures. Based on stable climate–growth correlations, a reconstruction of minimum winter temperatures (29th November to 20th January) back to 1810 was performed for north Poland. The reconstruction indicates a promising potential to reveal low-frequency climate information. An additional extreme year analysis suggested that in cold winters, a cold–warm–cold pattern in the minimum temperatures was responsible for the relatively small earlywood vessels. Spatial field correlations imply that our reconstruction is more related to temperature variations towards the east of Europe. The reconstructed temperature compared well with two existing temperature reconstructions, especially during most of the 20th century, even though the temperature reconstructions differ spatially and temporally. Based on these findings, the relatively extensive resource of archeological oak material from this region may be useful to perform multicentennial climate reconstructions in the temperate climate zone. [ABSTRACT FROM AUTHOR]

Palaeoclimate proxies have shown links between climate changes and volcanic activity. However, not much is known about the impact of volcanic outbursts on growth of lowland trees. We investigate the effect of large volcanic eruptions on the growth rate of trees. The study is based on an unexplored comprehensive database with 1128 samples of long tree-ring width (TRW) chronologies of Quercus robur L. and Pinus sylvestris L., correlating with forest net primary production (NPP), originating from three different sites in eastern Germany (Greifswald, Eberswalde and Saxony). This study focuses on trees in rarely examined temperate zones where tree growth is less temperature limited. Growth relationships were compared against 52 large volcanic eruptions known for the last 1000 years. Dendrochronological methods revealed a predominantly negative (60.2%) effect of large volcanic eruptions on the tree-ring chronologies. Nevertheless, also positive (31.7%) and neutral (8.7%) tree growth reactions were detected. In the tree-ring width chronologies of Q. robur and P. sylvestris , we detected a negative influence on tree growth for up to four years after large eruptions. The chronologies of Q. robur revealed a stronger negative response (68.1%) than those of P. sylvestris (53%). However, at the Greifswald site both tree species (79% Q. robur and 73% P. sylvestris ) show a negative response in tree growth after every volcanic eruption. Furthermore, the results suggest that volcanic aerosols originating from the Northern Hemisphere cause a greater reduction in tree growth than aerosols being emitted from Southern Hemisphere volcanoes, which might be related to the long distances between trees and volcanoes, as well as the global atmospheric circulation patterns. This study demonstrates that the effects of major volcanic eruptions are less clear in trees from central European lowlands than in trees growing at the altitudinal or latitudinal timberlines. [ABSTRACT FROM AUTHOR]