Gradients, species richness and biogeographical links of steppe grasslands in Western Podolia (Ukraine).

Aims: Western Podolia is one of several regions in the Eurasian forest-steppe zone where diverse steppe vegetation has been relatively well preserved. Our aims were to describe compositional turnover of steppe vegetation along different environmental gradients, to identify the patterns of species richness and evenness across complete compositional gradients and to outline biogeographical links of Western Podolian forest-steppe, with the emphasis on the generally rare mesic steppe types. Study area: Western Podolia and adjacent regions of Western Ukraine. Methods: Two independent datasets of vegetation plots and environmental variables were classified using modified TWINSPAN and ordinated using DCA and CCA. Diagnostic, constant and dominant species were identified. Ecological elements were quantified using Didukh indicator values. Biogeographical analysis was based on an extensive review of literature and NMDS of species frequency data sourced from studies covering a large part of Central and Eastern Europe. Results: The main gradients in species composition correspond to ecological gradients of 1) topographically controlled moisture and nutrient availability and 2) disturbance and human impact. The first gradient ranges from low-productive dry stands dominated by narrow-leaved tufted grasses, through dry-mesic stands co-dominated by broad-leaved grasses and herbs, to highproductive dry-mesic to mesic stands co-dominated by tall herbs and broad-leaved grasses. The second gradient tends towards communities altered by intensive human use. Species richness of steppe grasslands may be extremely high and has a unimodal relationship with estimated productivity. It peaks at mesic sites, in places with high evenness of species abundances. Conclusions: The remaining steppe fragments in Western Podolia are nowadays partly burnt, partly grazed, rarely mown and partly unmanaged. They harbour a number of rare light-demanding species and species reaching their distributional limits within the study area. Exceptional diversity of the grasslands may originate from Early Holocene or Pleistocene forest-steppe communities. [ABSTRACT FROM AUTHOR]