Сalcareous steppes of Rostov region

The paper presents the syntaxonomical analysis of calcareous steppes of the Don Basin (Rostov Region, Russian Federation), that are formed on chalky soils and carbonate rocks. The species composition of communities are characterized by combination of widespread steppe xerophytes and calciphytes, which are common in thyme coenoses with the dominance of dwarf semishrubs and in bunchgrass steppes. Within the system of phylocoenogenetic classification (Kamelin, 2005) such conenoses are regarded as a distinct steppe subtype (Steppae creta-petrophile).

Cluster analysis revealed three community types with: the dominance of dwarf semishrubs, prevailing xerophytic bunch grasses, mixed coenoses. These types are characterized by different combinations of six ecological species groups, fr om obligate calciphytes to common steppe xerophytes.

High activity of steppe plants (that are mainly diagnostic of the class Festuco-Brometea) let us to consider thyme calcareous steppes as a part of this class. These steppes with bunch grasses and dwarf semishrubs dominance were found on primitive soils formed on chalk, marl and limestone at the southern edge of the Central Russian Upland, Donets Ridge and the Azov Lowland. Communities dominated by obligate calciphyte semishrubs belong to the class Helianthemo–Thymetea.

Northern calcareous steppes within the Central Russian Upland (Don Chalk Ridge) is remarkably different from southern analogues (Donetsk Ridge and Azov Lowland) in structure and species composition that is determined by climatic and edaphic factors, as well as by paleogeographic history of these territories. Syntaxonomicaly they belong to the order Festucetalia valesiacae. The alliance Carici humilis–Thymion calcarei (Averinova, 2014) includes calcareous steppes in the northern part of the Rostov region, which is the southern lim it of their distribution. The new alliance Jurineo stoechadifoliae–Cleistogenion bulgaricae unites the southern thyme steppes.

Communities of the alliance Carici humilis–Thymion calcarei are widely distributed in forest-steppe landscapes at the southern edge of the Central Russian Upland. This alliance is presented by the ass. Centaureo marschallianae–Poetum compressae and the community Agropyron pectinatum–Echinops ritro in Rostov Region These two syntaxa are differentiated by presence of such species as Medicago falcata, Stipa pennata, Polygala sibirica, P. cretacea, Viola ambigua, Onosma simplicissima (O. tanaitica), Pimpinella tragium, Teucrium polium, Astragalus albicaulis.

The new alliance Jurineo stoechadifoliae–Cleistogenion bulgaricae includes four new asso­ciations (Poo comressae–Cleistogenetum bulgaricae, Convolvulo lineati–Vincetoxietum maeotici, Xeran­themo annui–Botrichloetum ischaemi, Hedysaro grandiflori–Genistetum scythicae) and two comm­unities (Gagea pusilla–Jurinea stoechadifolia and Stipa adoxa). The described syntaxa are common in the southwest of Rostov Region (Azov Lowland, the southern part of Donetsk Ridge). The peculiarity of these steppes is the presence of endemic species (Jurinea stoechadifolia, Linum tenuifolium L. czernjajevii, Caragana scythica, Thymus di­morphus, Genista scythica, Gypsophila glomerata, Hyacinthella pallasiana).

In previous studies (Demina, 2011) we placed calcareous steppes of the Don Basin within the forest-steppe suballiance Bupleuro falcati–Gypsophilenion altissimae that represents northern meadow steppes and is included into the alliance Festucion valesiacae. Recent studies in Central Russian Upland were mostly concentrated in the north of the area of our study (Averinova, 2011, 2014; Poluyanov, Averinova, 2012). Comparison of these data with our data allowed assigning Rostov communities to the alliances Carici humilis–Thymion calcarei and Jurineo stoechadifoliae–Cleistogenion bulgaricae.

Ordination scatter plot reveals good differentiation of the steppe syntaxa. Mesophytic steppes related to the suballiance Bupleuro falcati–Gypsophilenion altissimae are mostly grouped on the left side of the ordination scheme, while the xerophytic Jurineo stoechadifoliae–Cleistogenion bulgaricae occupies the right one. The central position of the alliance Carici humilis–Thymion calcarei reflects its transitional position between xerophyte calcareous steppes and more mesic communities.