Bioclimatic factors limiting the distribution of Iris sibirica across Eurasia

With a wide range in Eurasia, Iris sibirica has an irregular distribution pattern and is listed in regional and national Red Data Books and on Red Lists of many countries in Europe and Asia. The abiotic causes of the species rarity (bioclimatic limiting factors) and adaptations in different parts of its range are still poorly understood. The formalisation of I. sibirica's ecological niches in the n-dimensional space of environmental factors will clarify these issues. We hypothesised that the ecological niches of I. sibirica were differentiated in the European and Asian parts of its range, allowing the species to adapt under diverse environmental conditions of plain and mountain habitats. This study aimed to (1) identify abiotic boundaries for various parts of the I. sibirica range in Eurasia (i.e. population groups), (2) test whether the ecological niches of Eurasian population groups were differentiated in environmental space, (3) provide information on environmental reasons of the species rarity in various parts of the range. To test whether the ecological niches of I. sibirica overlapped in various parts of its range, we applied the kernel density estimation method for niche visualisation in PCA (Principal Component Analysis) axis space. We have also used quantitative niche overlap metrics such as Schoener's D, Hellinger's I and COUE (Centroid shift, Overlap (Stability), Unfilling, Expansion). WorldClim bioclimatic variables were used to formalise the temperature and precipitation components of ecological niches. By a non-hierarchical iterative k-means clustering of abiotic variables in presence points, West European (463 points), East European-Siberian (186 points) and Mountain (112 points) population groups of I. sibirica were distinguished. In terms of temperature and precipitation parameters, the Mountain population habitats were similar to those of the West European populations (Stability of 0.90–0.99), but clearly differed from habitats of the East European-Siberian populations (Expansion of 0.85–1.00). In both cases, the ecological niche of the Mountain population group was clearly wider in the precipitation component. This is probably due to the relatively humid climate and orographic heterogeneity of mountainous areas, which enables I. sibirica to occupy sites with a wide range of precipitation but suitable moisture availability (e.g. couloirs, microrelief depressions). The ecological niches of the West European and East European-Siberian populations clearly differed by temperature component, but overlapped by precipitation component in the environmental space of PCA axes. Schener's D and Hellinger's I values were ranged from 0.02 to 0.21 for temperature variables and from 0.24 to 0.71 for precipitation variables. For the East European-Siberian population group, Unfilling in the temperature component of niche was 0.84–0.88. Thus, I. sibirica is adaptable to a wide range of temperature conditions in Eurasia, but has a narrow specialisation in moisture availability and is limited in suitable habitats (e.g. wet, swampy and marshy meadows), especially in the more continental and less humid East European-Siberian part of its range.