Description: The aim of this study was to compare the climatic responses of three tree rings proxies: tree ring width (TRW), maximum latewood density (MXD), and blue intensity (BI). For this study, 20 cores of Pinus sylvestris covering the period 1886–2015 were extracted from living non-damaged trees from the Eastern Carpathian Mountains (Romania). Each chronology was compared to monthly and daily climate data. All tree ring proxies had a stronger correlation with the daily climate data compared to monthly data. The highest correlation coefficient was obtained between the MXD chronology and daily maximum temperature over the period beginning with the end of July and ending in the middle of September (r = 0.64). The optimal intervals for the temperature signature were 01 Aug – 24 Sept for the MXD chronology, 05 Aug – 25 Aug for the BI chronology, and both 16 Nov of the previous year – 16 March of the current year and 15 Apr – 05 May for the TRW chronology. The results from our study indicate that MXD can be used as a proxy indicator for summer maximum temperature, while TRW can be used as a proxy indicator for just March maximum temperature. The weak and unstable relationship between BI and maximum temperature indicates that BI is not a good proxy indicator for climate reconstructions over the analysed region.

Description: We present the first annual oxygen isotope record (1900–2016) from the latewood (LW) cellulose of oak trees (Quercus robur) from NW Romania. As expected, the results correlate negatively with summer relative humidity, sunshine duration and precipitation and positively with summer maximum temperature. Spatial correlation analysis reveals a clear signal reflecting drought conditions at a European scale. Interannual variability is influenced by large-scale atmospheric circulation and by surface temperatures in the North Atlantic Ocean and the Mediterranean Sea. There is considerable potential to produce long and well-replicated oak tree ring stable isotope chronologies in Romania which would allow reconstructions of both regional drought and large-scale circulation variability over southern and central Europe.

Description: Abstract We analyze annually resolved tree-ring stable carbon (δ13C) and oxygen (δ18O) isotopic chronologies from Swiss stone pine (Pinus cembra L.) in Romania. The chronologies cover the period between 1876 and 2012 and integrate data from four individual trees from the Calimani Mts in the eastern Carpathians where climatic records are scarce and starts only from 1961. Calibration trials show that the δ13C values correlate with local April-May relative humidity and with regional to larger scale (European) summer precipitation. δ18O correlates significantly with local relative humidity, cloud cover, maximum temperature, as well as European scale drought conditions. In all cases, the climate effects on δ13C values are weaker than those recorded in the δ18O data, with the latter revealing a tendency towards higher (lower) values of δ18O during extremely dry (wet) years. The most striking signal, however, is the strong link between the interannual δ18O variability recorded in the Calimani Mts and large-scale circulation patterns associated with North Atlantic and Mediteraneean Sea sea surface temperatures. High (low) values of δ18O occur in association with a high (low) pressure system over the central and eastern part of Europe and with a significantly warmer (colder) Mediterranean Sea surface temperature. These results demonstrate the possibility of using tree ring oxygen isotopes from the eastern Carpathians to reconstruct regional drought conditions in eastern Europe on long-term time scales and larger scale circulation dynamics over the pre-instrumental periods.

Description: This paper aims to develop the first differentiated (earlywood—EW, latewood—LW, and total ring width—RW) dendrochronological series for ash (Fraxinus excelsior L.) and oak (Quercus robur L.) trees from the Republic of Moldova, and to analyze their climatic response and their spatio-temporal stability. For this, 18 ash and 26 oak trees were cored from the DobruÈ�a protected area, Republic of Moldova, Eastern Europe, and new EW, LW, and RW chronologies were developed for ash and oak covering the last century. The obtained results showed that the RW and LW have a similar climatic response for both species, while EW is capturing interannual climate variations and has a different reaction. The analyses performed with monthly climatic data revealed a significant and negative correlation with the mean air temperature and a significant and positive correlation with precipitation and the Standardized Precipitation-Evapotranspiration Index (SPEI) for both ash and oak. The temperature during the vegetation period has a strong influence on all tree-ring components of ash, while for oak the strong correlation was found only for LW. The positive and significant correlation between LW and RW with precipitation for both species, suggests that ash and oak are sensitive to the hydrological component and the precipitation is the main tree growth-limiting factor. Despite the significant correlation with precipitation and temperature for the whole analyzed period, the 25-year moving correlation analyses show that they are not stable in time and can switch from positive to negative or vice versa, while the correlation with SPEI3 drought index, which is a integration of both climatic parameters, is stable in time. By employing the stability map analysis, we show that oak and ash tree ring components, from the eastern part of the Republic of Moldova, have a stable and significant correlation with SPEI3 and scPDSI drought indices from February (January) until September, over the eastern part of Europe.

Description: In this study, we made use of a regional oak tree-ring network from six stands that cover the northern Moldavian Plateau (eastern Europe) to analyze how different tree ring parameters (i.e., early wood tree-ring width, late wood tree-ring width, and total tree-ring width) of Quercus sp. are influenced by the occurrence of extreme climatic events (e.g., long-lasting drought events). In order to explore the influence of extreme hydroclimatic events on tree ring width, we have selected each of the six most extreme positive and negative years of tree growth and addressed the seasonal cycle of tree growth in comparison with the main climatic parameters, then evaluated both the current and lagged consequences of extreme hydroclimatic events on tree ring width and the capacity of trees to recover. Our results indicate that the variability of oak tree ring width from the Moldavian Plateau is mainly influenced by the availability of water resources, and that an important limiting growth factor for Quercus sp. is the occurrence of long-lasting drought events, e.g., at least two years in a row with severe drought conditions.

Description: In the context of forecasted climate change scenarios, the growth of forest tree species at their distribution margin is crucial to adapt current forest management strategies. Analyses of beech (Fagus sylvatica L.) growth have shown high plasticity, but easternmost beech populations have been rarely studied. To describe the response of the marginal beech population to the climate in the far east sites of its distribution, we first compiled new tree ring width chronologies. Then we analyzed climate–growth relationships for three marginal beech populations in the Republic of Moldova. We observed a relatively high growth rate in the marginal populations compared to core distribution sites. Our analyses further revealed a distinct and significant response of beech growth to all climatic variables, assessing for the first time the relationship between growth and vapor pressure deficit (VPD) which described how plant growth responds to drought. These results highlight that accumulated water deficit is an essential limiting factor of beech growth in this region. In conclusion, beech growth in the easternmost marginal population is drought-limited, and the sensitivity to VPD will need to be considered in future studies to update the forest management of other economic and ecologically important species.

Description: Numerical simulations indicate that extreme climate events (e.g., droughts, floods, heat waves) will increase in a warming world, putting enormous pressure on society and political decision-makers. To provide a long-term perspective on the variability of these extreme events, here we use a ~700 years tree-ring oxygen isotope chronology from Eastern Europe, in combination with paleo-reanalysis data, to show that the summer drying over Eastern Europe observed over the last ~150 years is to the best of our knowledge unprecedented over the last 700 years. This drying is driven by a change in the pressure patterns over Europe, characterized by a shift from zonal to a wavier flow around 1850CE, leading to extreme summer droughts and aridification. To our knowledge, this is the first and longest reconstruction of drought variability, based on stable oxygen isotopes in the tree-ring cellulose, for Eastern Europe, helping to fill a gap in the spatial coverage of paleoclimate reconstructions.