Syntaxonomy and scale-dependent species diversity in chalk outcrops plant communities within Kharkiv region (Ukraine)

Abstract

We studied extremely basiphilous plant communities representing all stages of succession on chalk outcrops (from almost pure chalk with open vegetation to well-developed steppe phytocoenoses) in the Kharkiv region in the valleys of the Oskol and Vovchha rivers. We recorded all vascular plants and bryophytes in eight biodiversity-plots with seven grain sizes (0.0001-100 m2). The vegetation of the studied plots was divided into two groups which were interpreted syntaxonomically as two different classes (Helianthemo-Thymetea and Festuco-Brometea) based on species composition. Comparison of the structure of these two groups of communities, their ecological characteristics, soil analysis and the results of the phytoindicative assessment proved the validity of assigning them to different vegetation classes. Species richness at various spatial scales for extreme basiphilous communities of the chalk outcrops showed that both total species richness and the richness of vascular plants for communities of the Helianthemo- Thymetea class was less than half that of the Festuco-Brometea communities. This can be explained by extreme environmental conditions due to the structure and chemistry of the soil, as well as harsh microclimatic conditions. Within the communities of more advanced stages of succession towards steppe the species richness of all the analyzed taxonomic groups within all spatial scales was higher. Our results indicate that the studied communities have high conservation value (rare and endangered species can reach onethird of their composition) and we propose special conservation measures for them.