Vegetation of chalk outcrops of Sub-Ural plateau and adjacent territories
The results of long-term studies of the vegetation of chalk outcrops of the Orenburg region (Russian Federation) and North-West Kazakhstan on Sub-Ural plateau and adjacent territories are presented. Chalk outcrops are unique botanical-geographical sites located in steppe and desert zones of Eurasia. Specific communities of calcephyte plant species have spread in these areas, in places of outcrops or close occurrence from the surface of upper-Cretaceous carbonate rocks. The flora of chalk outcrops is characterized by a great amount of rare species, mainly endemic, associated with peculiar substrates, the locality of habitats, and the historical past of the area of outcrops location (Matyshenko, 1985) The history of the study of flora and vegetation of chalk outcrops is given. Synthaxonomic studies of chalk vegetation as part of the ecological-floristic approach cover only territories west of the Volga river (Poluyanov, 2009; Averinova, 2011, 2016; Demina, 2014; Demina et al., 2016; Didukh et al., 2018). Chalk highlands of the North-West Kazakhstan and adjacent regions of the Russian Federation occupy quite large areas. However, up to date, there is no data on the vegetation diversity of these territories based on complete geobotanical relevés, that is why their synthaxonomy remains undeveloped. The study area with 15 massifs of chalk outcrops (Fig. 1) includes the Orenburg region (Novosergievs-ky, Perevolotsky, Sol-Iletskiy, Akbulak and Gaysky districts), and Aktobe (Hobdinsky, Uilsky and Bayga-ninsky district) and Atyrau (Zhylyoysky district) regions of the Republic of Kazakhstan. The largest mas sifs in the Orenburg region of the Russian Federation are: Starobelogorskie (Fig. 2), Chesnokovskie (Fig. 3), Verkhnechibendinskie (Fig. 6), Troitskie (Fig. 7), Pokrovskie Chalk Mountains (Fig. 4) and Durtel mountain (Fig. 5). Chalk massif Akshatau (Fig. 8) and the range Aktolagai (Fig. 9) are the largest within Aktobe region. The investigated sites are mostly located on the Sub-Ural Plateau, which extended from the southern regions of the Orenburg region to the Emba River in the territory of Aktobe region. They are less common within the Obschiy Syrt and sporadic in the Guberlinskie mountains. The study area covers a wide range of zonal vegetation from dry steppes in the northern part of the gradient to northern deserts in the southern one. The dataset includes 270 relevés of chalk outcrops communities performed by the authors in 2014-2019. The primary classification was carried out using TWINSPAN algorithm. As a result three groups of communities are established. The first group is communities of the Emben Plateau, the most southern area; second is communities on relatively developed soils in the slopes bases, depressions between chalk ridges and on their flat tops; third is widespread communities on most of the Podural Plateau and Obschy Syrt, excluding the Emben Plateau. Comparison with associations of calcephyte, semidesert and steppe vegetation (Golub, 1994; Kolomiychuk, Vynokurov, 2016; Lysenko, Yamalov, 2017; Didukh et al., 2018; Korolyuk, 2017) was made to determine the position of studied communities in the system of ecological-floristic classification of the herbasceous vegetation of Eurasia. Cluster analysis results (Fig. 10) revealed the significant specificity the chalk outcrops of the SubUral Plateau in comparison with calciphytic communities of Eastern Europe, as well as with deserts and steppes zonal vegetation. That was the reason to describe a new class for vegetation of the studied chalk outcrops. The class Anabasietea cretaceae Golovanov class nov. hoc loco. Diagnostic species: Anabasis cretacea, Anthemis trotzkiana, Artemisia salsoloides, Atraphaxis decipiens, Crambe aspera, Echinops meyeri, Jurinea kir-ghisorum, Hedysarum tscherkassovae, Lepidium mey-eri, Limonium cretaceum, Linaria cretacea, Matthiola fragrans, Nanophyton erinaceum, Seseli glabratum, Zygophyllum pinnatum; holotypus is order Anabasie-talia creta