Differentiation of ecological niches of the forest-forming species in the Caucasus

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