Background: Ecological niche modeling of the main forest-forming species within the same geographic range contributes significantly to understanding the coexistence of species and the regularities of formation of their current spatial distribution. The main abiotic and biotic environmental variables, as well as species dispersal capability, affecting the spatial distribution of the main forest-forming species in the Caucasus, have not been sufficiently studied.Methods: We conducted studies within the physiographic boundaries of the Caucasus, including Russian Federation, Georgia, Armenia, and Azerbaijan. Our studies focused on ecological niche modeling of pure fir, spruce, pine, beech, hornbeam, and birch forests through species distribution modeling and the concept of the BAM (Biotic-Abiotic-Movement) diagram. We selected 648 geographic records of pure forests occurrence. ENVIREM and SoilGrids databases, statistical tools in R, Maxent were used to assess the influence of abiotic, biotic, and movement factors on the spatial distribution of the forest-forming species.Results: Geographic expression of fundamental ecological niches of the main forest-forming species depended mainly on topographic conditions and water regime. Competitor influence reduced the potential ranges of the studied species by 1.2–1.7 times to the geographic expression of their realized ecological niches. Movement factor significantly limited the areas suitable for pure forests (by 1.2–1.8 times compared with geographic expression of realized ecological niches), except for birch forests.Conclusion: Distribution maps, modeled by abiotic, biotic variables and movement factor, were the closest to the real distribution of the forest-forming species in the Caucasus. Biotic and movement factors should be considered in modeling studies of forest ecosystems if models are to have biological meaning and reality.
Adaptations of Abies nordmanniana, Picea orientalis, Pinus sylvestris, Fagus orientalis, and Carpinus betulus to the abiotic environmental conditions of the study area largely determined their predicted distribution in the Caucasus. The ecological niches of the species mostly coincided in two analyzed complex environmental factors (characteristics of water regime and topographic parameters). The complex humidity factor was the main factor determining the potential distribution the forest-forming species in the Caucasus (65% of the contribution in the Maxent models). Topographic ENVIREM predictors were also significant in the species distribution (20% of the contribution in the models). Abies nordmanniana and Fagus orientalis were the most sensitive to the humidity factor, while Pinus sylvestris depended largely on the topographic factors. The similarity of the distribution potential of the studied species in the Caucasus was explained largely by a high degree of overlap of ecological niches (Schoener’s D = 0.55-0.79) and their visual overlap in the orthogonal space of the analyzed ecological factors. The largest Schoener’s D indexes were observed for the pairs Pinus sylvestris – Picea orientalis, Abies nordmanniana – Picea orientalis, Fagus orientalis – Picea orientalis, and Fagus orientalis – Carpinus betulus. Carpinus betulus, Fagus orientalis and Pinus sylvestris had the widest ecological niches.
According to the results of the study, it found that in the upper reaches of the Malka River, the most common is the group of birch forests with tallgrass vegetation cover, including Betuletum calamagrostioso-herbosum. Smaller areas occupied by group of complex herbaceous birch forests, represented by Pineto-betuletum calamagrostioso-herbosum and a group of pine forests with herbrich vegetation cover, including Pinetum calamagrostioso-herbosum. Forest stands are mainly singlestorey, medium and low-density, bonitet classes II-IV. The floristic composition of the Malka River basin forests includes 101 species from 68 genera and 38 families. Forest ecosystems of the study area are home to four species of vascular plants included in the list of protected taxa of the KabardinoBalkarian Republic: Betula raddeana Trautv, Vaccinium myrtillus L., V. vitis-idaea L., Allium victorialis L. Among protected lichen species, we found – Leptogium burnetiae C. W. Dodge, Letharia vulpina (L.) Hue, Lobaria pulmonaria (L.) Hoffm.), Lobarina scrobiculata (Scop.) Nyl.), Sticta sylvatica (Huds.) Ach. and Usnea florida (L.) Weber ex F. H. Wigg. The article analyzes the geographic spectrum of the forest flora of vascular plants and lichens, including 12 geographic elements, with the boreal geoelement predominating. The lichen flora dominated by boreal geoelements.
Based on the results of the studies conducted in the forest areas of the basins of the rivers Belaya and Malaya Laba, there has been compiled a typological scheme of the coniferous and mixed forests of North-Western Caucasus (NWC) which included 21 forest types, that are distributed into 12 groups. The forest stands of the studied forests types is predominantly complex in composition: mixed-age groups, multi-tiered, medium to high productivity, medium to high-density. The forest regeneration is weak or very weak, according to the scale proposed by S. M. Bebia. The data obtained during the research can be used for assessing the dynamics of the forest biogeocenoses, biodiversity, as well as successional processes.
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