Facilitation in subnival vegetation patches

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Abstract
We examined spatial relationships among species in the subnival zone of the central Caucasus. The species composition of 300 vegetation patches was analysed. 144 of them contained only one species, whereas the other 156 contained 2.36 ± 1.31 species, with species numbers distributed as follows: 59 patches with two species, 41 with three, 39 with four and 17 with five species.In the multi‐species patches, the 22 most frequent species were examined and 46 statistically significant species associations, 36 positive and 10 negative, were found. Ten of these 22 species were typical subnival plants very rarely occurring at lower altitudes. The other 12 species are ‘invaders’ as they have broader altitudinal ranges of occurrence and are common in alpine and even in subalpine belts. Contrary to the typical subnival species the invaders were found exclusively in the patches with more than one species. Invader species were significantly more associated with subnival species while they were significantly less associated with other species than expected by chance. Our interpretation is that typical subnival species nurse plants from lower altitudes and facilitate their invasion to more adverse subnival environments.