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The meteo-climatic arguments for the eastern biogeographic limit of beech in Romania are of particular interest to forestry, but also to geographers and climatologists. This limit marks the transition from the temperate humid climate of Western and Central Europe to the temperate continental climate of Eastern Europe. Our paper provides improved knowledge on climatic conditions that restrict the longitudinal distribution of beech at temperate latitudes in north-eastern Romania. Results show that rainfall scarcity and high temperature in the March-June interval, added to temperature drop below the freezing point in April-June are the main climatic conditions that limit beech expansion eastward of this biogeographic limit. Introduction The transition between Suceava and Bârladului plateaus and Moldova Plain is characterized by a gradual, eastward change from forest to foreststeppe and steppe. In this region, European beech (Fagus sylvatica) occurs at the eastern limit of its natural distribution. Although old and vigorous beech trees are frequently found in the forests of this area (e.g., a 417-year-old beech-tree was identified in the Humosu Iași Reserve; Roibu, 2010, Roibu, et al., 2017), the species is subject to restrictive climatic conditions specific to north-eastern Romania. Temperature drop below the freezing point at the beginning of the annual vegetation growth cycle negatively affects beech growth rings. Department of Geography, “Stefan cel Mare” University of Suceava, Romania Forest Biometrics Laboratory, “Stefan cel Mare” University of Suceava, Romania Regional Meteorological Center of Moldova, National Meteorological Administration, Romania email: dumitrum@atlas.usv.ro; petricabistricean@gmail.com Dumitru Mihăilă, Andrei Emil Briciu, Cătălin Constantin Roibu, Petruţ Ionel Bistricean 18 Furthermore, high temperatures favour increased evapotranspiration and have a negative effect on the growth and expansion of this species. Water scarcity, manifested as drought, raises many economic and ecological issues (Mihăilă, 2006, Nedealcov, 2012). In this part of Europe drought occurs with increased frequency and duration, making the territories vulnerable (Alexandrov et al., 2010). The thermo-pluviometric prognoses show an increase in water shortage particularly during the vegetation season (Potop, 2011, Piticar et al., 2016, Mihăilă et al., 2017). Dry and very hot summers and autumns are more frequent (Boroneanţ et al., 2013) and generally followed by cold, unstable winters with short-lived snow cover. Water shortage during the vegetation season leads to a lower bioaccumulation in forest ecosystems, which also lose their vitality (Dobbertin, 2005). Water shortage acts as a limiting factor on the eastern natural limit of beech. Beech development is optimum under temperate and wet climatic conditions and this species becomes increasingly sensitive to drought, as the climatic conditions at the limit of its eastern distribution area change (Bréda et al., 2006, Nahm et al., 2006, Kramer et al. 2010, Piticar et al., 2016, Mihăilă et al., 2017). Dendrochronological research on beech trees has shown a positive correlation between the annual ring width and precipitation amount of the vegetation season (Dittmar et al., 2003, Fotelli et al., 2009, Roibu, 2010, Roibu et. al, 2017, Scharnweber et al., 2011). The relationship between water shortage and tree radial growth is based on the monthly or seasonal precipitation amount (Michelot et al., 2012, Lebourgeois et al., 2005, Scharnweber et al., 2011). Precipitation variability, characteristic of the areas with a continental transition climate, requires that the analysis of the temporal relationships between tree growth and moisture availability should be based on synthetic climatic indices at different temporal scales (Vicente-Serrano et al., 2012). The Standardized Precipitation Index (SPI) has the ability to highlight the duration and intensity of precipitation deficit at different time scales (McKee et al., 1993). Rainfall deficit and high temperatures accentuate the negative consequences of climate on physiological processes of trees and accumulation of woody biomass. The standardized precipitationevapotranspiration index (SPEI) integrates potential evapotranspiration into rainfall deficit and can be successfully used in the climate-tree relationship analysis (Vicente-Serrano et al., 2010, Popa, Caisîn, 2015). Chira et al. noted in 2003 that beech decline occurred over successive drought years. Analysis of the de Martonne climatic index for the last 40 years Restricting climatic conditions on the Eastern distribution limit of european beech (Fagus sylvatica) 19 indicated an increase in the frequency and duration of drought in the forest belt of the Suceava hilly area. Chira et al. (2005) showed that the 1999-2000 drought was strongly felt in the eastern and north-eastern part of Romania and contributed, along with other climatic factors, to the phenomenon of beech debilitation (drying) in the old-growth (over 100 years old), pure and highly productive forest stands. Other climatic factors including late frosts and hail weaken the resistance of beech (large hail causes growth similar to beech cancer) and favour the occurrence of Nectria infections (Chira et al., 2005). Negative correlations with high temperature during the vegetation season were also noted. Climate warming is considered a promoting factor for beech debilitation (Chira et al., 2005). 1. Study area The eastern limit of the distribution of beech forests in north-eastern Romania overlaps the contact line between the Moldova Plain and the Suceava Plateau in the west, the Bârlad Plateau in the south respectively. Practically, this diffuse or transition area between forest and forest-steppe is largely identified in the physical-geographic contact subunits between the Moldova hilly plain and the two plateau areas (Suceava in the west, Bârlad in the south), which are known in the Romanian geography under the name of cuestas: Ibănești Cuesta (located in the north of the Moldova Plain), Moldavă Cuesta (between Moldova Plain and Suceava Plateau) and Iași Cuesta (between Moldova Plain and Bârlad Plateau). Due to specific physico-geographic and socio-economic conditions, only patches of compact beech forests can still be found today in this area, particularly on higher elevation, more isolated landforms. The mathematical position in terms of latitude (the study area is located between 48°15'N and 47°N) and longitude (25°4'E 27°55'E), along with its geographical position within Europe (in the centre of the continent, with low oceanic influences) and Romania (north-eastern part) shape the climate of the study area with temperate (annual mean global radiation on a horizontal surface is around 115 Kcal/cm; the existence of four seasons) and transition characteristics (from the oceanic climate, which becomes increasingly evident as we move westward from the Eastern Carpathian Mountain range, to the continental climate, typical of Eastern Europe; from the sub-Mediterranean climatic influences frequent in the southern parts of Romania, to the Baltic and boreal characteristics often present in northern Romania). Permanent, semi-permanent or seasonal high-pressure (e.g., Azores High, Siberian High, Scandinavian-Baltic High, Greenland High) and lowDumitru Mihăilă, Andrei Emil Briciu, Cătălin Constantin Roibu, Petruţ Ionel Bistricean 20 pressure atmospheric systems (e.g., Icelandic Low, Mediterranean Low, Eurasian Low) impose a high degree of variability on climatic elements (especially temperature, precipitation, wind parameters) and phenomena, a feature specific to climates with an enhanced degree of continentality. Among the regional and local background features of the studied area, the following are climatically relevant: a) Eastern Carpathians, which form an orographic barrier in the direction of wetter air masses coming from Western Europe; oceanic air masses generate rain on the western slopes of the Eastern Carpathian range and foehn on the eastern slopes towards Suceava Plateau and Moldova Sub-Carpathians (the average difference in elevation between Obcina Mare Peaks and Suceava Plateau is 800 1000 m a.s.l.); b) the study area is widely open towards north, northeast and east, in the direction of Baltic, Nordic and Continental air masses (main river valleys and mountain ranges are arranged northwest southeast, which facilitates the advection of air masses from NW and SE); c) the elevation difference of 150-300 m between Suceava / Bârlad plateaus and the Moldova Plain, evident in the transition area between the two landform types (the northern, western and southern hilly marginal cuestas of the Jijia Plain), which favors a second foehnization (of lower intensity but relevant with regard to rainfall occurrence) of the western air masses in their advection towards eastern Europe. Next to their role in precipitation distribution patterns, the two orographic barriers block winter and spring advections of polar continental air masses towards western Europe, thus enhancing frost in the lowlands of northeastern Moldova and favoring prolonged thermal inversions and a longer persistence of the snow layer. In summer, under relative shelter conditions, temperature in the lower elevation areas of the Moldova Plateau reaches and frequently exceeds the heatwave threshold. All these climatic conditions impose thermal and pluviometric restrictions on beech growth, which lead to a precarious balance in the proper succession of phenophases in the species, with more profound and frequent syncopes towards the east. This leads to increasingly unfavorable conditions for beech expansion in an increasingly continental climate. 2. Data used The data from 13 meteorological stations and 57 pluviometric stations (Figure 1 and Table 1) were used to depict the climatic conditions for beech growth in the study area. Additional wood cores were extr

present environment and sustainable development

RESTRICTING CLIMATIC CONDITIONS ON THE EASTERN DISTRIBUTION LIMIT OF EUROPEAN BEECH (FAGUS SYLVATICA) IN NORTH-EASTERN ROMANIA FOR THE MARCH – JUNE INTERVAL