Climatic Relationship of Vegetation in Forest Stands in the Mediterranean Vegetation Belt of the Eastern Adriatic

: The Mediterranean vegetation belt on the eastern Adriatic covers an area of nearly 15,000 km 2 . It is comprised of forest stands that can be divided into three vegetation zones based on the presence of certain plant species within each: sub-Mediterranean, eu-Mediterranean, and steno-Mediterranean. The dominant ecological factors result in the domination of speciﬁc tree species within the ﬂoral composition between these vegetation zones. The aim of this study was to collect climate data from 38 weather stations over a 30-year period to compare climate data and bioclimate properties in the area of these three vegetation zones. The results conﬁrmed statistically signiﬁcant differences between the main climatic elements and most bioclimatic indices between the vegetation zones. Cooler and more humid conditions were found in the sub-Mediterranean zone, warmer and somewhat drier conditions in the eu-Mediterranean zone, and particularly pronounced warm and dry conditions in the steno-Mediterranean zone. However, the analysis of the main components for researching climate parameters showed that the mean annual air temperature, average minimum air temperature of the coldest month of the year and continentality index, length of the dry season, and de Martonne aridity index contribute the most to the grouping of vegetation in forest stands in the Mediterranean vegetation belt of the eastern Adriatic.


Introduction
The Mediterranean region, situated between Europe, Asia, and Africa, is one of the world's largest centres of biological diversity [1]. According to Hure et al. [2], the Adriatic Sea is an elongated and relatively shallow sea, and it stretches in the southeast direction for 800 km 2 from the highest Mediterranean latitude (45 • 47 N). The eastern coast of the Adriatic Sea, though spatially small, is one of the most biologically diverse regions in the Mediterranean basin, and is considered a biodiversity hotspot [3,4]. For example, the total floral biodiversity in the Mediterranean basin is estimated at some 25,000 plant species [5], while over 10% of this (2797 species) is contained within the wealth of flora of the Mediterranean part of the eastern Adriatic coast [6]. The presence of vegetation in a given area is the result of interrelations between climate, geological substrate, and soil. The Mediterranean climate is characterised by mild and rainy winters, and warm and dry summers [7,8], which in addition to the presence of extended dry periods significantly characterises the Mediterranean climate [9].
The relationship between climate and vegetation determines the interrelations of climatic elements and the distribution range of plant species, plant communities, and entire ecosystems [10]. This is the focus of bioclimatology, as one of the geobotanical sciences. The fundamental climatic elements are expressed in the form of climate indices and are considered useful in describing the relations between the climate and vegetation of a given the vegetation zones, which in turn contribute to the protection and conservation of areas.

Study Area
This study encompasses the Mediterranean vegetation belt along the eastern Ad coast in the Republic of Croatia (Figure 1), covering an area of 15,389 km 2 [27]. In vertical range, the Mediterranean vegetation belt covers an area up to an elevation o m in the northern part, and up to 600 m in the southern part. A detailed description o spatial distribution of the Mediterranean vegetation belt, with sub-Mediterranean Mediterranean and steno-Mediterranean vegetation zones, was performed by Trin [23,24]. The limits of the Mediterranean region and different vegetation zones in Republic of Croatia are taken from the Bioportal-geoportal of the Croatian N Protection Information system [28]. In the sub-Mediterranean vegetation zone along the eastern Adriatic coast, pube oak is the dominant tree species in two associations of the order Quercetalia pubesc [29].
Forests and thickets of pubescent oak described within the association Os Carpinion orientalis are adjacent to the Mediterranean coniferous community from Ist the Dubrovnik inland. In the northern Adriatic coastal zone, it is found at elevation to 350 m and in the lower areas of the Dalmatian continental inland up to 600 m. I lower, warmer part, pubescent oak is accompanied by Oriental hornbeam (Car orientalis Mill.), in the boundary area towards holm oak forests the coniferous speci holm oak associations succeed, though rarely holm oak. Pubescent oak is often foun combination with other deciduous species, such as oriental hornbeam (Carpinus orie Mill.), nettle trees (Celtis spp.), manna ash (Fraxinus ornus L.) and hop hornbeam (O carpinifolia Scop.) [30].
The eu-Mediterranean vegetation zone in Croatia encompasses forest stands w two associations of the order Quercetalia ilicis [29]. Spatially, it includes the significant and largest forest communities of the coastal vegetation belt, in the north the western part of Istria, over part of the islands in Kvarner Bay to Zadar, and fu south along a narrow coastal belt towards the far south, including the islands. dominant species is holm oak, which in most of the southern part forms stands coniferous species, while in the wetter parts of the Adriatic coast forms mixed stands deciduous species. Holm oak is often found with other species such as olive (Olea eur L.), carob (Ceratonia siliqua L.), strawberry tree (Arbutus unedo L.), mock privet (Phi In the sub-Mediterranean vegetation zone along the eastern Adriatic coast, pubescent oak is the dominant tree species in two associations of the order Quercetalia pubescentis [29]. Forests and thickets of pubescent oak described within the association Ostryo-Carpinion orientalis are adjacent to the Mediterranean coniferous community from Istria to the Dubrovnik inland. In the northern Adriatic coastal zone, it is found at elevations up to 350 m and in the lower areas of the Dalmatian continental inland up to 600 m. In the lower, warmer part, pubescent oak is accompanied by Oriental hornbeam (Carpinus orientalis Mill.), in the boundary area towards holm oak forests the coniferous species of holm oak associations succeed, though rarely holm oak. Pubescent oak is often found in combination with other deciduous species, such as oriental hornbeam (Carpinus orientalis Mill.), nettle trees (Celtis spp.), manna ash (Fraxinus ornus L.) and hop hornbeam (Ostrya carpinifolia Scop.) [30].
The eu-Mediterranean vegetation zone in Croatia encompasses forest stands within two associations of the order Quercetalia ilicis [29]. Spatially, it includes the most significant and largest forest communities of the coastal vegetation belt, in the north from the western part of Istria, over part of the islands in Kvarner Bay to Zadar, and further south along a narrow coastal belt towards the far south, including the islands. The dominant species is holm oak, which in most of the southern part forms stands with coniferous species, while in the wetter parts of the Adriatic coast forms mixed stands with deciduous species. Holm oak is often found with other species such as olive (Olea europaea L.), carob (Ceratonia siliqua L.), strawberry tree (Arbutus unedo L.), mock privet (Phillyrea latifolia L.), Mediterranean buckthorn (Rhamus alaternus L.) and terebinth (Pistacia terebinthus L.) [31][32][33]. Holm oak forests also include laurel (Laurus nobilis L.), and tree heath (Erica arborea L.) [32,34].
The steno-Mediterranean vegetation zone includes the association Oleo-Ceratonion with Aleppo pine as the dominant species. This zone is found in the warmest part of the outermost southern and central Dalmatian islands. Aleppo pine is a xerophilic species that is highly resistant to drought, and thrives in the warmest parts of the Mediterranean where forest fires are common [35,36]. Other than occasional forest stands, degradation stages of macchia and garrigues are common, and depending on the level of degradation and the ecological factors within the Aleppo pine stands, holm oak can also be present.

Selection of Weather Stations
The study area includes a total of 38 weather stations (Table 1, Figure 1) found within the areas of the vegetation zones according to Trinajstić [23,24]. Fourteen stations were in the sub-Mediterranean, seventeen in the eu-Mediterranean and seven in the steno-Mediterranean zone. Data collected include air temperature ( • C) and precipitation (mm) for a 30-year time period (from 1981 to 2010) of the Croatian Hydrological and Meteorological Service.

Bioclimatic Indices
In order to show the climate trends during the research period, 12 bioclimatic indices were calculated for use in this study (Table 2). These bioclimatic indices were calculated using the data on mean monthly air temperature and precipitation obtained from the weather stations. Most of the bioclimatic indices are used to calculate the relations of temperature and precipitation during the time period, indicating their robustness and comparability over longer time series [37].  [47] Tmax-mean air temperature of the hottest month ( • C); Tmin-mean air temperature of the coldest month ( • C); P-annual precipitation (mm); Pp-yearly positive precipitation (mm) (total average precipitation of those; months whose average temperature is higher than 0 • C); Tp-yearly positive air temperature ( • C) (sum of the monthly average temperature of those months whose average temperature is higher than 0 • C); m-average minimum air temperature of the coldest month of the year ( • C); M-average maximum air temperature of the warmest month of the year ( • C); Ti-mean temperature of the given month ( • C); Pi-precipitation of the given month (mm); Nm-monthly adjustment factor related to hours of daylight (-); I-heat index for the year (-); PET-annual potential evapotranspiration: sum of the monthly potential evapotranspiration (mm); PETi-potential evapotranspiration of the given month (mm); T-mean annual air temperature ( • C).
Annual potential evapotranspiration (PET) is calculated as the sum of the monthly potential evapotranspiration (PETi) in mm according to Thornthwaite's formula [38].
The dry season water deficit (DSWD) is expressed in mm, and is calculated as the difference in precipitation in the given month (Pi) and the potential evapotranspiration of the given month (PETi) [39].
Length of the dry season (LDS) is calculated according to the method of Gaussen [40] and UNESCO/FAO [41], and according to Blondel and Aronson [48] is considered one of the most useful methods for describing the Mediterranean bioclimates. According to this method, a month is "dry" when the amount of monthly precipitation (P) in mm is less than or equal to twice the amount of the average air temperature (T) in • C, or P ≤ 2T.
The Ombro-evapotranspiration index (Ioe) is calculated as the quotient of the mean annual precipitation (P) and the annual potential evapotranspiration (PET) for an area [42].
Lang's rain factor (LRF) is calculated as the quotient of the mean annual precipitation (P) and mean annual temperature (T). Values lower than 30 indicate arid conditions [43].
The De Martonne index (DMI) is calculated by using the mean annual precipitation (P) and mean annual temperature (T), in which lower values indicate more arid conditions and higher values indicated more humid conditions [44].
The Continentality index (CONTINENTY) is the only bioclimatic index to use air temperature by subtracting the mean air temperature of the coldest month from the mean air temperature of the hottest month. The calculated values pertain to the annual values expressed in degrees Celsius, and the higher the difference, the higher the continentality level [42]. For the study area, the highest air temperatures were measured in August. The Ellenberg Index (EQ) is calculated as the quotient of the mean air temperature of the hottest month (Tmax) in • C and annual precipitation (P) in mm [45].
The Ombrothermic Index (Io) is calculated at the annual level as the quotient of the yearly positive precipitation (in mm) which is the total average precipitation of those months whose average temperature is higher than 0 • C, and the yearly positive air temperature (in • C) which represents the sum of the monthly average temperature of those months whose average temperature is higher than 0 • C. In line with the obtained values, the area is classified according to the Ombric horizons [42] from Lower ultrahyperarid (those with the lowest values) to Ultrahyperhumid (those with the highest values).
The Ombrothermic index of the summer quarter (Iosq) is calculated as the quotient of the monthly precipitation and temperature for June, July and August [46].
The Thermicity Index (It) is calculated as the sum of the mean annual air temperature (T), average minimum air temperature of the coldest month of the year (m) and average maximum air temperature of the warmest month of the year (M) in • C [46].
Emberger [47] developed an approach to explain the distribution of species in the Mediterranean, on the basis of the minimum temperature in the coldest month (m) and the pluviothermic coefficient (Q 2 ). According to Daget [13], this system is best for defining a Mediterranean climate, and the approach has been widely accepted [49].
All climate data were processed using the software package KlimaSoft 2.1. [50].

Statistical Analysis of Data
The statistical analysis of the bioclimatic index of different vegetation zones (descriptive statistics, one-way analysis of variance ANOVA, Kruskal-Wallis test and principal components analysis (PCA) were performed in the software package Statistica 13.4.0 [51]. For ANOVA, the post hoc Fisher LSD test was used, with a significant limit set at p < 0.05. The Shapiro-Wilk test was used to test the normality of data. Where Shapiro-Wilk and Leven's test of homogeneity of variance were statistically significant (p < 0.05), data were then tested with the non-parametric Kruskal-Wallis test.
Spatial interpolation for bioclimatic indices within the researched area was performed using QGIS [52] and GRASS [53] software. For that purpose, inverse distance weighted (IDW) with nearest neighbour searching interpolation was used (GDAL/OGR). This algorithm computes the inverse distance to power gridding combined with the nearest neighbour method using default settings (Weighting power = 4; Smoothing = 0; The radius of the search circle = 1; Maximum number of data points to use = 12; Minimum number of data points to use = 0, Resolution = 100 m). IDW interpolation was used for generating spatial maps of those bioclimatic indices which do not depend on altitude (e.g., m, M, P, LDS and Q2). Additionally, in cases where a good correlation (R > 0.5) was present between altitude and bioclimatic indices (e.g., dM, LKF, T, PET, DSWD, C, Io), linear regression was used to create a spatial raster layers of bioclimatic indices using altitude as an independent variable. Average values of the derived regression model and IDW interpolation were then used for producing maps of the spatial distribution of these bioclimatic indices.
The spatial distribution of vegetation zones was obtained by logistic regression. We defined the vegetation zones for each meteorological station according to Trinajstić [23,24]. The dependent variable was vegetation zones, while the independent variables were altitude and climate elements (average annual air temperature and annual precipitation). Logistic regression equations and coefficients were obtained for each of the three zones. Spatial maps of zones were created based on the logistic regression and cut-off value (c = 0.5). However, the generated boundaries between zones are not in completely in line and they are spatially deviating. Due to those overlaps and/or gaps between zones, the centerline is created at an equal distance from two neighbour zones. This centerline represents the boundary between vegetation zones.
Prior to the principal component (PC) analysis, the correlations among all 16 bioclimatic variables for all weather stations were calculated to exclude the highly correlated ones (r > 0.85). Finally, the PC analysis was performed on eight bioclimatic variables and biplots with the first and second principal component (PC) and with the first and third principal components constructed showing the studied weather stations and bioclimatic variables (as vectors).

Descriptive Statistics and Comparison of Climatic Elements with Bioclimatic Indices
The absolute minimum air temperature in the sub-Mediterranean vegetation zone was −11.4 • C, with an absolute maximum air temperature of 38.1 • C. In the eu-Mediterranean zone, the absolute air temperature variation ranged from the absolute minimum of −5.7 • C to the absolute maximum of 38.1 • C. The range of the absolute minimum and maximum air temperature in the steno-Mediterranean zone was from −4.7 • C to 37.5 • C. Table 3 shows the descriptive statistics of the minimum and maximum values and comparison of mean values of the bioclimatic parameters and indices in the sub-Mediterranean, eu-Mediterranean and steno-Mediterranean vegetation zones.

Spatial Overview of the Bioclimatic Indices
According to the results of logistic regression of spatial distribution (       Figure 3 shows the spatial distribution of the de Martonne aridity index (DMI), and Lang's rain factor (LRF) for the research area. The largest part of the eastern Adriatic Sea, which is occupied by the sub-Mediterranean vegetation zone, has a humid climate according to the aridity index (DMI) (43.75%). According to Lang's rain factor (LRF) a semihumid climate dominates in 68.26% of the land area in a sub-Mediterranean vegetation zone.  The spatial distribution of the Emberger pluviothermic quotient (Q 2 ) and the ombrothermic index (Io) is shown in Figure 4a,b. According to the Emberger pluviothermic quotient (Q 2 ), most of the area in the sub-Mediterranean and eu-Mediterranean vegetation zone can be described as humid, while the steno-Mediterranean was sub-humid. In examining the values of the Ombrothermic index (Io), most of the area is found within the values that correspond to a Mediterranean climate, while in a sub-Mediterranean vegetation zone nearly equal land area has a Mediterranean and a temperate climate, 38.12% and 38.34% respectively.
Atmosphere 2022, 13, x FOR PEER REVIEW 10 of 18 The spatial distribution of the Emberger pluviothermic quotient (Q2) and the ombrothermic index (Io) is shown in Figure 4a

Principal Component Analysis of Climatic Relationships in Vegetation Zones
Sixteen bioclimatic variables were used to describe environmental differences between the 38 weather stations included in the study. In general, bioclimatic variables were highly correlated. Among the 120 pairs examined, a strong positive correlation (r > 0.85) was found in 14 pairs, and a strong negative correlation (r < −0.85) in seven pairs (Supplementary Materials Table S2). Finally, eight environmental variables were selected for the PC analysis. The variables that were highly correlated were not included in the analysis because of redundancy. The first principal component explained 51.7% of the total variance, with a strong negative correlation found for three bioclimatic variables (LDS, m and T), and a strong positive correlation for two bioclimatic variables (DMI and CONTINENTY) (Supplementary Materials Table S3). The first principal component separated the sub-Mediterranean localities (Imotski, Sinj, Knin, Crikvenica, Rijeka, Senj, Krk, Malinska, Kukuljanovo, Karlobag, Pazin, Omišalj, Bakar, Botonega) characterized by lower temperatures and a shorter dry season and higher de Martonne aridity index and continentality index, from the eu-Mediterranean (Dubrovnik, Ston, Orebić, Opuzen, Makarska, Split, Šibenik, Biograd, Zadar, Rab, Mali Lošinj, Cres, Rovinj, Poreč, Pag, Pula, Novigrad) and steno-Mediterranean localities (Lastovo, Mljet, Korčula, Vela Luka, Komiža, Hvar), where higher temperatures and longer dry season and lower de Martonne aridity index and continentality index were recorded ( Figure 6). Within the same principal component, some overlap was observed between the eu-Mediterranean and steno-Mediterranean localities. In addition, one locality from the eu-Mediterranean was grouped with localities from the sub-Mediterranean. The second PC axis explained 19.1% of the variability (Supplementary Materials Table S3). Along the same PC axis, a notable sub-structure within the sub-Mediterranean vegetation zone was observed (Figure 6), where each of the localities was clustered along the average maximum temperature of the warmest month. Although the third PC axis had the Eigenvalue of 1.07 and explained 13.4% of the variability (Supplementary Materials Table S3), along the same axis no separation or grouping of localities was observed along the studied vegetation zones, nor was a substructure observed within individual vegetation zones (Supplementary Materials Figure S1). Moreover, only weak correlations of bioclimatic variables with the third PC axis were recorded. In view of that, the third variable can be disregarded.

Principal Component Analysis of Climatic Relationships in Vegetation Zones
Sixteen bioclimatic variables were used to describe environmental differences between the 38 weather stations included in the study. In general, bioclimatic variables were highly correlated. Among the 120 pairs examined, a strong positive correlation (r > 0.85) was found in 14 pairs, and a strong negative correlation (r < −0.85) in seven pairs (Supplementary Materials Table S2). Finally, eight environmental variables were selected for the PC analysis. The variables that were highly correlated were not included in the analysis because of redundancy. The first principal component explained 51.7% of the total variance, with a strong negative correlation found for three bioclimatic variables (LDS, m and T), and a strong positive correlation for two bioclimatic variables (DMI and CONTINENTY) (Supplementary Materials Table S3). The first principal component separated the sub-Mediterranean localities (Imotski, Sinj, Knin, Crikvenica, Rijeka, Senj, Krk, Malinska, Kukuljanovo, Karlobag, Pazin, Omišalj, Bakar, Botonega) characterized by lower temperatures and a shorter dry season and higher de Martonne aridity index and continentality index, from the eu-Mediterranean (Dubrovnik, Ston, Orebić, Opuzen, Makarska, Split, Šibenik, Biograd, Zadar, Rab, Mali Lošinj, Cres, Rovinj, Poreč, Pag, Pula, Novigrad) and steno-Mediterranean localities (Lastovo, Mljet, Korčula, Vela Luka, Komiža, Hvar), where higher temperatures and longer dry season and lower de Martonne aridity index and continentality index were recorded ( Figure 6). Within the same principal component, some overlap was observed between the eu-Mediterranean and steno-Mediterranean localities. In addition, one locality from the eu-Mediterranean was grouped with localities from the sub-Mediterranean. The second PC axis explained 19.1% of the variability (Supplementary Materials Table S3). Along the same PC axis, a notable sub-structure within the sub-Mediterranean vegetation zone was observed (Figure 6), where each of the localities was clustered along the average maximum temperature of the warmest month. Although the third PC axis had the Eigenvalue of 1.07 and explained 13.4% of the variability (Supplementary Materials Table S3), along the same axis no separation or grouping of localities was observed along the studied vegetation zones, nor was a substructure observed within individual vegetation zones (Supplementary Materials Figure S1). Moreover, only

Discussion
Air temperature and precipitation are the two most significant climatic elements to express certain climatic relations, and to calculate the bioclimatic indices. U bioclimatic indices, it is possible to conduct a detailed analysis of the relations between climate and vegetation. It is well known that climate and vegetation patter the Mediterranean region are highly correlated [7]. Within an individual vegetation z there may be one or more vegetation zones due to the influence of relief factors, prim altitude. The ecological limits defined by climate and vegetation are less abrupt in the of the Tyrrhenian Sea than in the area of the Adriatic Sea [54]. The ecological condi of the forest tree species that dominate in the vegetation zones in the area o Mediterranean vegetation belt are closely related to the values of the bioclim parameters [55]. Five out of eight bioclimatic variables (mean annual air temperatur average minimum temperature of the coldest month (m), length of the dry season (L de Martonne aridity index (DMI), continentality index (CONTINENTY)) that included in the PC analysis along the first PC axis separated the studied localities, a with a small overlap, into three groups that correspond with predefined vegetation z From the PC graph showing the first and second principal component, it is clearly vi that the range of variation in the examined bioclimatic variables appears to be m pronounced between the sub-Mediterranean localities than among the eu-Mediterra and steno-Mediterranean localities.

Sub-Mediterranean Vegetation Zone
This vegetation zone occupies the largest part of the eastern Adriatic and is influenced by altitude. The lowest mean annual air temperature (T) was 13.9 °C recorded in the sub-Mediterranean zone; this was substantially lower than temperatures in the Mediterranean parts of Italy (16.1 °C) and Spain (16.3 °C) According to Canu et al. [15], the sub-Mediterranean climate type is a transitional towards a Mediterranean macro-bioclimate. In this vegetation zone, the abs minimum air temperature was recorded, however, the absolute maximum temperatures in this vegetation zone were indeed high, as also seen by the presen

Discussion
Air temperature and precipitation are the two most significant climatic elements used to express certain climatic relations, and to calculate the bioclimatic indices. Using bioclimatic indices, it is possible to conduct a detailed analysis of the relationships between climate and vegetation. It is well known that climate and vegetation patterns in the Mediterranean region are highly correlated [7]. Within an individual vegetation zone, there may be one or more vegetation zones due to the influence of relief factors, primarily altitude. The ecological limits defined by climate and vegetation are less abrupt in the area of the Tyrrhenian Sea than in the area of the Adriatic Sea [54]. The ecological conditions of the forest tree species that dominate in the vegetation zones in the area of the Mediterranean vegetation belt are closely related to the values of the bioclimatic parameters [55]. Five out of eight bioclimatic variables (mean annual air temperature (T), average minimum temperature of the coldest month (m), length of the dry season (LDS), de Martonne aridity index (DMI), continentality index (CONTINENTY)) that were included in the PC analysis along the first PC axis separated the studied localities, albeit with a small overlap, into three groups that correspond with predefined vegetation zones. From the PC graph showing the first and second principal component, it is clearly visible that the range of variation in the examined bioclimatic variables appears to be more pronounced between the sub-Mediterranean localities than among the eu-Mediterranean and steno-Mediterranean localities.

Sub-Mediterranean Vegetation Zone
This vegetation zone occupies the largest part of the eastern Adriatic and is most influenced by altitude. The lowest mean annual air temperature (T) was 13.9 • C and recorded in the sub-Mediterranean zone; this was substantially lower than the temperatures in the Mediterranean parts of Italy (16.1 • C) and Spain (16.3 • C) [56]. According to Canu et al. [15], the sub-Mediterranean climate type is a transitional zone towards a Mediterranean macro-bioclimate. In this vegetation zone, the absolute minimum air temperature was recorded, however, the absolute maximum air temperatures in this vegetation zone were indeed high, as also seen by the presence of thermophilic species such as pubescent oak. Based on the mean minimum temperature of the coldest month (m), this zone can be considered temperate.
In terms of its geographic position, Croatia is situated between central Europe and the Mediterranean regions, where the interaction between the atmosphere and complex local geophysical properties can cause large spatial and temporal variations in the distribution of precipitation [18]. The mean annual precipitation (P) was highest in the sub-Mediterranean vegetation zone, especially in its northern parts, with an average of 1233.4 mm of precipitation. This amount is far higher than in the Mediterranean parts of Spain (408 mm) and Italy (720 mm) [56]. These results are similar to reports by Raicich [57] and Penzar et al. [21], who explained that the Alps are the cause for the high levels of precipitation in the northern Adriatic. The sub-Mediterranean vegetation zone is situated at a higher elevation, and therefore relief elements result in higher amounts of precipitation [58] and lower values of air temperature [59].
The northern Adriatic in the sub-Mediterranean zone also lacks water in the summer quarter, and towards the south and on the islands, this deficit is larger and lasts for longer, which corroborates the findings of Penzar et al. [21] and Gajić-Čapka and Zaninović [60]. However, in certain parts of this vegetation zone (stations Sub1-Sub3), there is no dry period and according to Breckle [61], this can be considered a temperate climate. According to Lang's rain factor (LRF), the sub-Mediterranean climate zone is semi-humid, while according to the de Martonne index (DMI), it is a very humid climate. According to the ombrothermic index (Io), it is humid. The values of the Emberger pluviothermic quotient (Q 2 ) place the sub-Mediterranean zone in the per-humid climate. Months with a water deficit in the northern part of the sub-Mediterranean were July and August, while in the southern part of the zone, this extended from May to August. However, the length of the dry season (LDS) was lowest in the sub-Mediterranean zone, with an average of one dry month with a water deficit of −215.50 mm. These results are also in line with the claims that the number of dry days and the maximum number of consecutive dry days were lower in the northern part than elsewhere in the Croatian coastal region [62]. Though some authors [15] consider the sub-Mediterranean to be a transitional zone towards the Mediterranean macro-bioclimate, others [63,64] define the Mediterranean climate as having a dry period of at least one month, which is the average of this vegetation zone. There are also more restrictive climatic classifications and definitions, where it is considered that at least two consecutive dry months should not be sufficient to classify a climate as the Mediterranean if the precipitation of the preceding two months can compensate for the deficit of the two dry months [42,65].

Eu-Mediterranean Vegetation Zone
In terms of air temperature, this vegetation zone was significantly warmer than the sub-Mediterranean zone. The mean annual air temperature (T) was from 0.9 • C to 1.1 • C lower than in the Mediterranean parts of Italy and Spain [56]. This vegetation zone has a very high-temperature variation in terms of the mean minimum temperature of the coldest month (m). Since the increase in air temperature also affects the increase in the amount of potential evapotranspiration [66], this vegetation zone also has significantly higher values of potential evapotranspiration compared to the sub-Mediterranean (PET). Spatially, the coastal area of the Republic of Croatia receives more precipitation than the islands, especially in the coastal area near the Dinarides mountain range, which agrees with reports of Branković et al. [67]. According to the ratio of annual precipitation and air temperature (LRF), the climate of this zone is semi-humid, while according to the de Martonne index (DMI), the eu-Mediterranean has a moderately humid climate. The quantity of precipitation (933.8 mm) is substantially higher than in the neighbouring parts of Mediterranean Italy, where annual precipitation was 720 mm [56].
In the northern part of the eu-Mediterranean, months with a water deficit were from May to August, while in the southern area of the eu-Mediterranean, from May to September. A widely distributed tree species in this vegetation zone is the xerophyte holm oak [23,29].
These results are in line with Timbal and Aussenac [68], who state that holm oak is drought resistant. The maximum duration of the dry period in one year was seven months, while in the steno-Mediterranean was up to eight months, which can reduce the vitality of holm oak and result in die-offs. The average water deficit was two dry months, with a value of −325.47 mm. This zone was sub-humid according to the values of the ombrothermic index (Io) and humid according to the Emberger pluviothermic quotient (Q 2 ). Holm oak is able to grow in different Mediterranean climates, from semi-arid to very humid, depending on the amount of precipitation, and from warm to very cold depending on air temperature [31,69]. The grouping of weather stations in this vegetation zone is not completely homogeneous. Namely, the meteorological station Novigrad, which is located in the eu-Mediterranean vegetation zone, was grouped into the sub-Mediterranean zone. This weather station is situated in the western part of the Istrian peninsula, where there is a narrow coastal belt of holm oak forests, which could be influenced by the climate of the continental part of the Istrian peninsula that is in the sub-Mediterranean vegetation zone.
The research results indicated that there were no significant differences in the values of the pluviothermic quotient (Q 2 ) and in the mean minimum temperatures of the coldest month of the year (m) between this vegetation zone and the steno-Mediterranean zone. Based on the Emberger climate diagram [25], the eu-Mediterranean and steno-Mediterranean vegetation zones overlapped to form one vegetation zone.

Steno-Mediterranean Vegetation Zone
The warmest vegetation zone, the steno-Mediterranean, occupies the smallest part of the eastern Adriatic Sea. It is located in its southern island part and narrow coastal strip. The reason for this is the position of the Adriatic Sea, which is located further north compared to other seas in the Mediterranean area. The results showed that the mean annual air temperature in the steno-Mediterranean zone corroborated reports of Seletković et al. [25] and Penzar et al. [21] that the Adriatic Sea is warmest in the offshore areas of the southern Adriatic, where the dominant species is Aleppo pine. It is well known that large water bodies are heat reservoirs and have a strong effect on temperature [7]. The mean annual air temperature (T) in this vegetation zone was 16.1 • C, which is similar to the temperature values of the Mediterranean parts of Italy and Spain [56]. Aleppo pine is a highly drought-tolerant species, a thermophyte that grows well in the warmer parts of the Mediterranean where forest fires are very common [70]. In this vegetation zone, the lowest absolute minimum air temperature was recorded. In the steno-Mediterranean, the absolute minimum air temperature was −4.7 • C. Aleppo pine prefers an absolute minimum temperature of −2 to 10 • C [34,35]. In terms of temperature, the Adriatic Sea represents the cooler part of the distribution range of Aleppo pine. In terms of the winter variation of the mean minimum temperature of the coldest month, this zone is a very hot region.
Precipitation levels are low on the Adriatic islands, and exceptionally low on the islands in the southern part of the Adriatic Sea [71,72]. According to Penzar et al. [21], this is because airflow is horizontal off-shore and on the islands. Considering the values of the Lang rain factor (LRF), this zone has a semi-arid climate, and according to the de Martonne index (DMI) has a moderately-humid climate. Months with a water deficit were from April to October, which is highly xerophilic conditions. According to our analyses, the largest water deficit was in the area of the southern Adriatic, which corresponds to the findings of Perčec Tadić et al. [73], who showed that the period of precipitation deficit was longer in the southern Adriatic. The maximum duration of the dry period within one year was eight months. According to the results of research in this vegetation zone, the average was three dry months (LDS), which is in line with data on the drought tolerance of Aleppo pine [74]. Lower values of the ratio of precipitation and potential evapotranspiration (PET) correspond to dry areas, and the lowest amount of this index was in the area dominated by Aleppo pine, or the steno-Mediterranean. The water deficit in the dry season (DSWD) was −440.22 mm. The steno-Mediterranean is a sub-humid area according to the ombrothermic index (Io) and humid according to the Emberger pluviothermic quotient (Q 2 ).
Aleppo pine is a xerophyte and is more drought resilient than pedunculate oak and holm oak [75,76] and in the area where it is the dominant species and the annual precipitation levels are lowest. Aleppo pine is found in habitats ranging from less arid or semi-arid to humid, and the precipitation level is between 350 and 700 mm [77,78], while in the steno-Mediterranean area in the Republic of Croatia, the quantity of precipitation is on average 721.22 mm, which was nearly the same as in Italy (720 mm) but far more than in the Mediterranean part of Spain, where only 408 mm of precipitation falls [56].

Conclusions
There is a statistically significant difference between all three vegetation zones in the mean annual air temperature, the quantity of precipitation, duration of the dry period, dry season water deficit, Lang rain factor, ombro-evapotranspiration index, aridity index, continentality index, Ellenberg index, ombrothermic index, and ombrothermic index in the summer period. The mean annual air temperature significantly increased from the sub-Mediterranean vegetation zone, to the eu-Mediterranean zone and finally the steno-Mediterranean vegetation zone. On the eastern Adriatic coast, the quantity of precipitation significantly declined from the northern part of the Adriatic Sea in the sub-Mediterranean towards the western part of the Istrian Peninsula, then the islands and the central and southern part of the Croatian Mediterranean areas that belong to the eu-Mediterranean, dominated by holm oak, and was lowest on the southern Adriatic islands in the steno-Mediterranean zone dominated by Aleppo pine. The number of dry months increased from the northern Adriatic coast in the sub-Mediterranean towards the south, i.e., towards the eu-Mediterranean and steno-Mediterranean zones. Through this research, it was found that the mean annual air temperature, average minimum air temperature of the coldest month of the year and bioclimatic indices, continentality index, length of the dry season and de Martonne aridity index contribute the most to the grouping of vegetation in forest stands in the Mediterranean vegetation belt of the eastern Adriatic.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/atmos13101709/s1, Table S1: Logistic regression statistics for spatial distribution of tree vegetation zones; Table S2: Correlations among studied bioclimatic variables at 38 sites from the east Adriatic coast, Table S3: Correlations between 16 bioclimatic variables and the first three principal components, Figure S1: Biplot of the principal component analysis based on bioclimatic variables PC1-PC3.

Data Availability Statement:
The climatic data that support the findings of this study are available on request from the Croatian Hydrological and Meteorological Service. Terms of use, data availability, and contact available at: https://klima.hr/razno/katalog_i_cjenikDHMZ.pdf (accessed on 9 August 2022).