Impact of dominants on the species richness and compositional dissimilarity of high mountain plant communities of the Western Caucasus (Caucasian State Nature Reserve, Russia)

Climate warming may cause not only a reduction in the area of high-mountain vegetation, but also a change in the abundance of many species, including dominant ones, which may have additionally negative consequences for plant communities. Therefore, our knowledge how dominants affect high-mountain plant communities at different spatial scales is important to predict changes in their species richness in the future. In this study, we aimed to answer the following questions: i) How does the species richness of high-mountain plant communities of the Western Caucasus depend on the participation of dominant species? ii) Can this relationship be explained on the basis of the «energy diversity» hypothesis? iii) Do dominants affect the degree of similarity (or difference) in the species composition of plant communities located in different habitats? The research was carried out in the Caucasian State Nature Reserve, Russia. The objects of study were seven homogeneous sites of plant communities dominated by Alchemilla retinervis, A. oxysepala, Geranium gymnocaulon, Carex capillifolia, Inula grandiflora, Calamagrostis arundinacea, Epilobium angustifolium. Within each of them, biomass samples were taken from 25–30 plots of 0.25 m2, which were then disassembled by species and weighed. The participation of dominants (the degree of dominance) was assessed through the ratio of their biomass and biomass of samples as a whole. The results show that, first, most of the studied high-mountain plant communities are characterised by a close relationship between the relative participation of the dominant and the species richness in small plots. Secondly, this connection can be explained on the basis of the «energy-diversity» hypothesis. This means that the size of the species pool of plant communities with different degrees of dominance should be approximately the same. Thirdly, the plant communities with relatively high participation of dominants are characterised by a slightly higher degree of compositional dissimilarity than plant communities with low participation. We have concluded that dominants have a predominantly local effect on the species richness of high-mountain plant communities in the Western Caucasus. However, an increase in their participation leads to a decrease in the occurrence of many accompanying species, and, accordingly, can make these species more vulnerable to the effects of other factors