A Model
The aim of this study was to assess the influence of abiotic, biotic and movement factors on the spatial distribution of the main forest-forming species in the Caucasus by modeling the geographic expression of their fundamental and realized ecological niches. We revealed a significant effect of topographic conditions and water regime on the potential distribution of the studied species. Our results showed that the acceptable habitats for pure fir forests were relatively gentle slopes (between nearly level and moderately rugged) with loamy soils in humid conditions (Table 4). Pure spruce forests also potentially occurred on relatively gentle slopes in sub-humid and humid conditions. Optimal habitats of both species were mainly located in the middle mountains and highlands of the Western Caucasus and the Georgian part of the Central Greater Caucasus with a humid subtropical and warm summer continental climate (SI Fig. 4a, 5a). These results are in line with Shevchenko and Geraskina (2019), who observed that in the North-Western Greater Caucasus, the modern potential areas of Abies nordmanniana and Picea orientalis almost completely coincided. The authors concluded that the main limiting factors in the distribution of these drought-sensitive species in the region were the precipitation in the driest month, as well as the altitude (Shevchenko and Geraskina 2019). Previous research also revealed a high sensitivity to climate humidity of Abies nordmanniana in the Caucasus (Litvinskaya and Salina 2012) and Picea orientalis in Turkey (Ucarcı and Bilir 2018). According to Akatov et al. (2013), the suitable average annual precipitation for Abies nordmanniana ranged from 700 to 2500 mm. Our result is also consistent with a previous study of fir forests in northwestern Turkey (Coban 2020), which showed that pure fir forests mainly occurred between 1000 and 1600 m above sea level on mountain slopes with a steepness of about 10–20°. Litvinskaya and Salina (2012) observed that in the Western Greater Caucasus, optimal conditions for Abies nordmanniana and Picea orientalis forests were formed at altitudes of 1200–1600 m and up to 1500–1700 m, respectively. Usta and Yılmaz (2020) found that in the Trabzon mountains (northeastern Turkey), slope steepness and altitude positively correlated with the distribution of Picea orientalis. The authors suggested that negative anthropogenic interventions could limit spruce forests to steep slopes unsuitable for agriculture and settlement (Usta and Yılmaz 2020). Our studies of the importance of edaphic factors in fir forest distribution was also supported by Litvinskaya and Salina (2012), who highlighted that Abies nordmanniana is sensitive to deteriorating soil conditions and prefers loamy soils.
In our studies, Pinus sylvestris mainly depended on the topographic factor TRI; the percentage contribution of climatic factors to the species distribution model was relatively low (Table 4). Acceptable habitats of pure pine forests were located in a wide range of mountain slope steepness and altitude from nearly level to highly rugged areas with fairly low mean monthly potential evapotranspiration of the driest quarter. Areas optimal for Pinus sylvestris mainly included the middle mountains and highlands of the Greater Caucasus with warm summer continental, hemiboreal, oceanic or hot summer continental climates (SI Fig. 3a). The wide ecological range of Pinus sylvestris by temperature and humidity gradients, climate continentality, underlying rocks and soil is in line with previous studies of pine forests in the Dagestan Republic (Eastern Greater Caucasus, Russia) by Ermakov et al. (2019). The authors showed that pine forests were distributed in the middle mountains and highlands at an altitude of 1600–2500 m (Ermakov et al. 2019), which is consistent with our results. Researchers also highlighted the drought resistance of Pinus sylvestris (Usta and Yılmaz 2020) and its tolerance to excessive moisture (Rakhmatullina et al. 2017). Rakhmatullina et al. (2017) and Arslan and Örücü (2019) used Maxent models to analyze the contribution of environmental factors to the distribution of pine forests in the Southern Ural (Republic of Bashkortostan, Russia) and Turkey, respectively. They revealed a significant influence of the maximum temperature of the warmest month, which may be due to climatic differences between these regions and the Caucasus.
Our results showed that the potential ranges of Fagus orientalis and Carpinus betulus largely overlapped throughout the study area, while the area of optimal habitats for beech forests was almost twice that for hornbeam forests (Table 5). Optimal areas for both species covered the foothills, low and middle mountains (from level to intermediately rugged areas) of the Western Greater Caucasus and the Black Sea coast of Georgia (Table 4, SI Fig. 1a, 2a). Moreover, the Georgian part of the Central Greater Caucasus, the Eastern Caucasus and the west of the Lesser Caucasus also included optimal sites for beech forests. The low frost resistance of these species (Shevchenko and Geraskina 2019) probably explains the relatively low upper limit of the distribution of beech and hornbeam forests in the Caucasus Mountains. Usta and Yilmaz (2020) also reported that slope steepness and altitude were negatively correlated with the distribution of Carpinus orientalis on the Karadağ Mass, Turkey. Fagus orientalis preferred mainly sub-humid and humid bioclimatic conditions, while Carpinus betulus occurred in conditions with rather low suitable values of mean monthly potential evapotranspiration of the wettest quarter. This result coincided with Jensen et al. (2008), who showed that in central and northern Europe, a drier and warmer climate (annual precipitation of less than 600 mm and mean July temperature above 18°C) favored the distribution of Carpinus betulus, whereas beech forests prevailed in more humid regions. Based on modeling the range of Fagus orientalis with environmental data of the present, past and future climates, Dagtekin et al. (2020) also showed that drier climate and higher temperatures will limit future distribution of this species. Previous studies in the North-Western Greater Caucasus (Shevchenko and Geraskina 2019) and Anatolia, Turkey (Koç et al. 2021) confirmed that the water regime significantly affected the current distribution of beech and hornbeam forests. Shevchenko and Geraskina (2019) reported that beech forests were mainly distributed in areas where the annual precipitation was not less than 600 mm (Shevchenko and Geraskina 2019). According Packham et al. (2012), beech trees have a shallow root system which makes them sensitive to the moisture deficiency during the drought period.
Birch forests of Betula pendula occurred mainly in the sub-humid and humid bioclimatic zones of the Greater and Lesser Caucasus with a warm summer continental or hemiboreal climate, while Betula litwinowii preferred humid habitats in the North-Western and Central Greater Caucasus with a humid subtropical and warm summer continental climate (Table 4, SI Fig. 6a, 7a). Both species were common in the middle mountains and highlands; however, the probability of Betula litwinowii occurrence was higher in more rugged areas. This result supported previous reports that on the northern and southern slopes of the Greater Caucasus, Betula litwinowii usually formed the upper border of the forest belt (1500–2800 m above sea level) on steep slopes (Akhalkatsi et al. 2006; Kessel et al. 2020). In addition, Akatov (2009) and Hansen et al. (2017) concluded that in the Western and Central Greater Caucasus, the upper limits of Betula litwinowii tended to increase in an uphill direction. Beck et al. (2016) associated Betula pendula distribution in southern Europe (mainly in mountain regions) with its sensitivity to summer drought, which did not contradict our conclusion about the importance of the water regime in the distribution of the species.
BA Model
In our study, the contribution of biotic ecological predictors significantly exceeded the contribution of abiotic variables to construction the models of the species distribution in the Caucasus (Table 4). Areas of geographic expression of realized ecological niches of species were 1.2–1.7 times smaller than the areas of geographic expression of their fundamental ecological niches (Table 5). This result is consistent with previous opinions and conclusions (Keane and Crawley 2002; Soberón and Peterson 2005; Peterson et al. 2011; Peterson and Soberón 2012; Atwater et al. 2018; etc.) that positive and negative interactions between species should be considered in ENM or SDM studies if models are to have biological meaning and reality.
Present study revealed that in the Caucasus, Picea orientalis was the main competitor to Abies nordmanniana in the same areas, while the main species limiting the distribution of Picea orientalis was Fagus orientalis (to a lesser extent Abies nordmanniana and Pinus sylvestris) (Table 4). In turn, the main competitor of Fagus orientalis was Carpinus betulus, while the ecological niche of Carpinus betulus was most similar to that of the Pinus sylvestris. The ecological niches of both birch species were similar; Pinus sylvestris was also a competitor species to Betula pendula and B. litwinowii.
Our results (SI Fig. 4b, 5b), as well as species distribution modeling in the North-Western Caucasus (Shevchenko and Geraskina 2019), showed that the potential ranges of Abies nordmanniana and Picea orientalis almost completely coincided. This finding supported previous reports (Nishimura 2006; Litvinskaya and Salina 2012) on the convergence of suitable environmental conditions for spruce and fir. Therefore, Shevchenko and Geraskina (2019) suggested that Abies nordmanniana and Picea orientalis could form mixed communities within the entire range of dark coniferous forests of the North-Western Caucasus. At the same time, in the Caucasus, the areas of pure spruce forests, as well as fir-spruce co-dominated forests, were relatively small (Litvinskaya and Salina 2012; Shevchenko and Geraskina 2019). In our opinion, the similarity and highly competitive nature of the ecological niches of the two species determined the low probability of the occurrence of fir-spruce co-dominated forests (Table 4). Probably, Abies nordmanniana displaced Picea orientalis from territories suitable for both species. Gokturk and Tıraş (2020) also reported that in the mixed stands of Ovacik Forests of Artvin, Turkey, Picea orientalis tended toward a clumped distribution, avoiding a tree-wise mixture with Abies nordmanniana and Pinus sylvestris. Accordingly, in such mixed communities, Picea orientalis was not competitive and could only thrive in groups. Anthropogenic effect and the ability of fir to recover faster after felling and fires could also limit the distribution of fir-spruce forests in the Caucasus (Shevchenko and Geraskina 2019). In addition, according to our data (Table 4), as well as Litvinskaya and Salina (2012) report, the difference in the real ranges of Abies nordmanniana and Picea orientalis was also due to the fact that fir forests prefer wetter habitats. Thus, in BA Models, embergerQ still retained a significant effect on the distribution of pure fir forests.
The distribution of pure spruce forests in the Caucasus was also limited by the presence of pure beech forests in habitats suitable for both species (Table 4). Fagus orientalis is the most widespread shade tolerant deciduous species in the Caucasus. Its potential range covered the area of coniferous-broad leaved forests of the North-Western Caucasus (Shevchenko and Geraskina 2019) and the potential range of dark coniferous forests throughout the Caucasus (SI Fig. 2b, 4b, 5b). The range of embergerQ values in habitats suitable for beech forests was wider than those for spruce and fir forests. At the same time, Fagus orientalis preferred rather gentle slopes located lower in altitude. This was probably why the upper TRI values for pure spruce forests in BA Model shifted to the range of highly rugged areas (Table 4). In our study, despite the similarity of ecological niches, Fagus orientalis was not a competitive species for Abies nordmanniana. This result is consistent with a previous study of fir-beech co-dominated forests in the Northwest of Turkey (Coban 2020), which showed that shade tolerance of both species provided a high degree of their spatial mingling. In the Western Caucasus, beech and fir also formed stable mixed stands in the final stages of forest development (Litvinskaya and Salina 2012; Gornov et al. 2018).
Present study revealed that the most important ecological predictor in the potential distribution of Fagus orientalis was Carpinus betulus. According to Sikkema et al. (2016) and Gornov et al. (2018), the hornbeam is a fast growing tree species with long distance seed distribution that prefers sunny habitats, but at the same time, it is one of the most shade tolerant native trees in Europe. In mixed forests, this species can be a dangerous invader (Sikkema et al. 2016). Jensen et al. (2008) showed that in central and northern Europe, there was a significant negative correlation at the local scale between relative areas of Fagus orientalis and Carpinus betulus due to the competitive relationship between the two species. This finding was also supported by Yakhyayev et al. (2021), who reported that in the northern regions of Azerbaijan, complex cuttings in the secondary hornbeam stands were an effective measure for regenerating the natural beech stands. Carpinus betulus was observed in pure groups in fir-beech co-dominated forest of the northwestern Turkey, where it was not competitive compared to both shade tolerant species (Coban 2020). However, Carpinus betulus replaced beech forests at the felling sites, which caused an increase in the area of hornbeam forests in the Central Caucasus by 6% in the first decade of the 21st century (Tembotova et al. 2012).
At the same time, the ecological niche of Carpinus betulus was most similar to the ecological niche of Pinus sylvestris, which was probably largely due to the relative drought resistance of both species (Table 4). In turn, there were no strong competitors for Pinus sylvestris among the studied species. Significant influence on the species distribution was retained by TRI, the lower values of which shifted to the range of moderately rugged areas. Like spruce forests, according BA Model, pure pine forests were concentrated on steeper slopes at higher altitudes. Based on the studies by Coban (2020) and Gokturk and Tıraş (2020), we assumed that light demanding Pinus sylvestris was able to avoid competition due to concentration in the upper layer of stands and exclusion from suppression by shade tolerant species. Thus, Coban (2020) showed that Pinus sylvestris demonstrated random distribution and spatial association with other species in fir-beech forests of the northwestern Turkey. The author also concluded that the pioneer character of this species allowed its establishment early in the succession stage (Coban 2020). Ecological plasticity of Pinus sylvestris and its ability to occupy habitats unsuitable for other species (Table 4, SI Fig. 3a) were also important in reducing competition with other species.
Similarity of ecological niches of both birch species was due to their similar requirements for relief conditions, temperature and water regimes. These species often form mixed stands of the upper forest belt in the Caucasus Mountains, below which there is a belt of pure pine or birch-pine forests. Accordingly, the influence of the biotic factor caused the displacement of pure birch forests upward and to steeper slopes (Table 4).
BAM Model
According to Peterson et al. (2011), movement factor (M set of environmental conditions) represented the geographic regions accessible for the species for a certain period. Analysis of this factor, along with sets of biotic and abiotic environmental conditions, made it possible to establish the "occupied distributional area" (Soberón and Peterson 2005; Peterson et al. 2011) or the geographic expression of the species realized niches, which is the closest to their real distribution. In our study, we aimed to approximate the final maps of the forest-forming species ranges to their real distribution in the Caucasus with the possibility of practical application. Therefore, we defined the geographic regions accessible for the species as the distances from the sites with the most optimal conditions, where the probability of species occurrence was higher than 0.5. We considered these distances as an indicator of species mobility. Birch forests were the most “mobile” in the Caucasus (0–20 km of accessible areas from optimal habitats), followed by fir, beech and pine forests (0–10 km), and spruce forests (0–6 km). Areas suitable for hornbeam forests were the most compact (only 0–1 km from optimal habitats).
We revealed a significant effect of movement factor on the potential distribution of the main forest-forming species in the Caucasus, with the exception of Betula litwinowii and B. pendula. BAM Models predicted a relatively low contribution of movement factor to the distribution of pure birch forests. Competition from each other and Pinus sylvestris, as well as mountain terrain and water regime, mainly determined the modern ranges of both species in the Caucasus. The acceptable area for Betula pendula exceeded that for B. litwinowii (Table 5, SI Fig. 6c, 7c) due to lesser dependence on the habitat humidity and the slope steepness. At the same time, the area of optimal habitats for Betula litwinowii exceeded that for B. pendula because of the large occupied territories in the relatively humid highlands of the Western and Central Caucasus. Geographic expression of the realized niches of Betula litwinowii and B. pendula, which is the closest to their real distribution, was the upper forest belt in the highlands throughout the Caucasus.
Movement factor significantly limited the areas of suitable habitats of widespread forest-forming species in the Caucasus (Pinus sylvestris, Fagus orientalis and Carpinus betulus) (Table 5, SI Fig. 1c, 2c, 3c). According to A Models, the geographic expression of fundamental ecological niches of these species covered more than 40 thousand km2 throughout the Caucasus, but the influence of biotic and movement factors reduced this area by 2–2.7 times. Pinus sylvestris with a wide ecological range in main environment gradients, spread from nearly level to highly rugged areas with warm summer continental, hemiboreal, oceanic, or hot summer continental climates. Among studied forest-forming species, there were no strong competitors for Pinus sylvestris. However, the complex influence of biotic and movement factors shifted the distribution of pure pine forests to more local areas in the highlands of the Greater and Lesser Caucasus.
Although Fagus orientalis preferred more humid bioclimatic conditions than Carpinus betulus, the potential ranges of these species largely overlapped throughout the Caucasus and there was a competitive relationship between them. Carpinus betulus mainly limited the distribution of Fagus orientalis only in disturbed beech forests (e.g. felling sites) due to its rapid growth and renewal. Nevertheless, the competition from Carpinus betulus, and, to a lesser extent, the species mobility (0–10 km from optimal habitats), limited the real distribution of Fagus orientalis to more compact areas within the boundaries of its potential distribution (foothills, low and middle mountains of the Greater and Lesser Caucasus). To the greatest extent, the movement factor influenced the distribution of Carpinus betulus in the Caucasus. Initially, small optimal area and low mobility of the species (only 0–1 km) significantly limited the geographic expression of the realized niche of Carpinus betulus to relatively small suitable and optimal sites from the foothills to the middle mountains of the Western Greater Caucasus and the Lesser Caucasus.
Due to the dependence of Abies nordmanniana and Picea orientalis on factors of water regime, their predicted ranges in the Caucasus were initially relatively small (Table 5) and almost completely coincided. At the same time, competition from other forest-forming species (Fagus orientalis, Pinus sylvestris, Abies nordmanniana) and relatively low mobility of Picea orientalis (0–6 km) limited its “occupied distributional area” to the small territories in the highlands of the potential range (SI Fig. 5c). Thus, the highlands of the Western Caucasus and the Georgian part of the Central Greater Caucasus, as well as the highlands of the western Lesser Caucasus and the borders of Russia and Azerbaijan can be recommended for conservation and restoration of Picea orientalis in the Caucasus. Abies nordmanniana is able to displace Picea orientalis from territories suitable for both species, especially in disturbed ecosystems. Therefore, the real distribution of Abies nordmanniana in the Caucasus was mainly determined by habitat humidity and species mobility (0–10 km). Areas suitable and optimal for pure fir forests, which can be recommended for conservation and restoration of Abies nordmanniana, were compacted to the middle mountains and highlands throughout the Western Greater Caucasus, Georgian part of the Central Greater Caucasus and the west of the Lesser Caucasus (SI Fig. 4c).