Sub-Doppler spectroscopy of the near-UV Cs atom 6S1/2_{1/2}-7P1/2_{1/2} transition in a microfabricated vapor cell

We report on the characterization of sub-Doppler resonances detected by probing the 6S$_{1/2}$-7P$_{1/2}$ transition of Cs atom at 459 nm in a microfabricated vapor cell. The dependence of the sub-Doppler resonance (linewidth, amplitude) on some key experimental parameters, including the laser intensity and the cell temperature, is investigated. These narrow atomic resonances are of interest for high-resolution spectroscopy, instrumentation, and may constitute the basis of a near-UV microcell optical standard

An Overview of Extreme Years in Quercus sp. Tree Ring Records from the Northern Moldavian Plateau

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

The first tree-ring reconstrruction of streamflow variability over the last ∼250 years in the Lower Danube

Paleoclimate reconstructions are increasingly used to characterize climate variability and change prior to the instrumental record, in order to improve our estimates of climate extremes and to provide a baseline for climate change projections. Most of these reconstructions are focused on temperature, precipitation, and/or drought indices and, to a lesser extent, reconstruct streamflow variability. In this study, the first regional tree-ring width chronology (i.e. Quercus sp.), from the Caraorman forest (Danube Delta, Romania), was used to reconstruct the last ∼250 years of annual (from November previous year to July of the current year) streamflow of the Lower Danube River. The obtained results indicate a stable and significant correlation between the tree-ring width index from the Caraorman forest and the Danube streamflow at the Ceatal Izmail hydrologic station situated in the southeastern part of Europe. Interannual streamflow variation for the analyzed period indicates 14 extremely high flow years, with streamflow greater than 8780 m3/s (1770, 1771, 1799, 1836, 1838, 1839, 1871, 1876, 1877, 1879, 1940, 1941, 1997 and 2010) and 14 extremely low flow years, with streamflow lower than 5300 m3/s (1741, 1745, 1750, 1753, 1773, 1794, 1812, 1832, 1843, 1882, 1899, 1921, 1964 and 1994). Periods characterized by pluvials in the lower Danube Delta are associated with a low-pressure system centered over Europe, positive sea surface temperature (SST) anomalies over the Atlantic Ocean, and negative SST anomalies over the Baltic, North, and Mediterranean Seas. These large-scale conditions favor the advection of moist air from the Mediterranean and the Black Sea towards the southeastern part of Romania, which in turn leads to high precipitation rates over this region. Opposite to this, low streamflow years are associated with a high-pressure system centered over Europe, characterized by a northward shift of the storm tracks and negative SST anomalies over the Atlantic Ocean, and positive SST anomalies over the Baltic, North, and Mediterranean Seas. Based on our results, we argue that the reconstruction of river streamflow data based on the tree-ring width has important scientific and practical implications for a better understanding of the streamflow variation of the past, necessary for water resource management and environmental-hydrological protection

Network Analysis Based on Important Node Selection and Community Detection

The stability and robustness of a complex network can be significantly improved by determining important nodes and by analyzing their tendency to group into clusters. Several centrality measures for evaluating the importance of a node in a complex network exist in the literature, each one focusing on a different perspective. Community detection algorithms can be used to determine clusters of nodes based on the network structure. This paper shows by empirical means that node importance can be evaluated by a dual perspective—by combining the traditional centrality measures regarding the whole network as one unit, and by analyzing the node clusters yielded by community detection. Not only do these approaches offer overlapping results but also complementary information regarding the top important nodes. To confirm this mechanism, we performed experiments for synthetic and real-world networks and the results indicate the interesting relation between important nodes on community and network level

Different climate response of three tree ring proxies of Pinus sylvestris from the Eastern Carpathians, Romania

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

The Climatic Response of Tree Ring Width Components of Ash (Fraxinus excelsior L.) and Common Oak (Quercus robur L.) from Eastern Europe

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

The Response of Beech (<i>Fagus sylvatica</i> L.) Populations to Climate in the Easternmost Sites of Its European Distribution

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

Mitigation of He permeation in microfabricated vapor cells with alumino-silicate glass and Al2O3 coatings

International audienceWe study helium permeation in micro-fabricated Cs vapor cells built with alumino-silicate glass (ASG) or borosilicate (BSG) windows, coated or not with Al2O3 layers. Gas permeation rate is derived from long-term measurements of the pressure-shifted clock hyperfine transition frequency in a coherent population trapping (CPT) clock setup. We find that the use of ASG windows reduces, in comparison with BSG, He permeation by more than two orders of magnitude whereas cells with Al2O3 coated BSG exhibit permeation rates up to 100 times smaller than those measured with bare BSG

A Permanent Research Platform for Ecological Studies in Intact Temperate Mountainous Forests from Slătioara UNESCO Site and Its Surroundings, Romania

This paper describes a permanent research platform (PRP) designed and implemented in &ldquo;Codrul secular Slătioara&rdquo; and its surroundings (2205.85 ha), having also the role of introductory paper for future research articles based on data collected from this platform. &ldquo;Codrul secular Slătioara&rdquo; is known as one of the largest temperate mountainous intact forests of Europe and, in 2017, it was included in UNESCO World Heritage List, as part of the &ldquo;Ancient and Primeval Beech Forests of the Carpathians and Other Regions of Europe&rdquo;. Moreover, the PRP overlaps other three scientific reserves, the share of strictly protected forests exceeding 70%. This platform has a multiple role, being developed for research, conservation and educational activities. The PRP was designed for an ecological analysis of the intact forest ecosystems. It contains 193 circular sample plots, each of them of 500 m2, and it is structured on two levels. The first level contains 58 sample plots corresponding to a square grid of 500 &times; 500 m, stretching over the entire forested area, and the second level contains 135 plots, placed according to a square grid of 100 &times; 100 m, covering 136 ha within the core area of the UNESCO site. We measured the characteristics of 8296 living trees, 1743 standing dead trees, 1900 dead wood trunks, 3214 saplings, and the abundance&ndash;dominance indices of flora species. Thus, we identified 14 tree species, 17 shrub species, and 248 other cormophyte species forming the herbaceous layer. In terms of volume, the main tree species are Norway spruce, silver fir and European beech. The tallest species are Norway spruce (56 m) and silver fir (51 m). The average volume of living trees is 659 m3&middot;ha&minus;1, with a maximum of 1441 m3&middot;ha&minus;1. The mean total dead wood volume is about 158 m3&middot;ha&minus;1, with sample plots where the total dead wood volume exceeds 600 m3&middot;ha&minus;1. After presenting the results of preliminary data processing, the paper describes the main research topics to be further considered, based on the PRP, and the foresights related to the PRP&rsquo;s monitoring and development