Syntaxonomy of psammophytic vegetation of the natural monument «Dyakovskiy forest» (Saratov Region)

«Dyakovskiy forest» (Saratov Region) is a unique natural phenomenon. It is located in the transitional zone between dry and desert steppes. Vegetation of this territory is represented by steppes and meadows, as well as small patches of deciduous forests. (Fig. 2) The sand massif is located far from nearest localities of the sandy steppes to the west of the Volga River (Fig. 1). «Dyakovskiy forest» is the unique southern forest outpost in European Russia, supposedly of relict origin (Shilova, Neronov, 2010). Sandy steppes were studied in May 2018. 16 relevés were performed on 10×10 m plots. We compared the communities from «Dyakovskiy forest» with the associations of the class Festucetea vaginatae Soó ex Vicherek 1972 from the southeast of the European Russia and Ukraine. For classification and comparative analysis we used cluster analysis in the PAST software (Hammer et al., 2001). The IBIS 7.2 was used for data storage and processing (Zverev, 2007). Cluster analysis revealed differentiation patterns of psammophytic vegetation (Fig. 3). The central part of the dendrogram is occupied by sandy steppes dominated by perennial grasses (cluster 1). These communities form the prevailing plant communities in the study area; they represent the final stages of sand stabilization. Five species determine the physiognomy of the sandy steppes of the «Dyakovskiy forest»: Artemisia marschalliana, Chamaecytisus borysthenicus, Festuca beckeri, Jurinea polyclonos, Stipa borysthenica. The first cluster was divided into two groups of relevés, representing different stages of sand stabilization. The first group is characterized by high activity of bunchgrasses: Agropyron fragile, Poa bulbosa,and Stipa borysthenica. It is also indicated by the presence of common steppe plants: Allium tulipifolium, Artemisia austriaca, Carex supina, Centaurea pseudomaculosa, Eremogone biebersteinii, Galium ruthenicum, Veronica verna. The second group is distinguished by the high frequency of obligate psammophytes: Tragopogon tanaiticus, Linaria dulcis, Leymus racemosus, and Secale sylvestre. The prevalence of these species allows interpreting the second group as a stage of relatively less fixed sands. Communities with the dominance of annual plants are grouped in the right part of the dendrogram (cluster 2). Now these communities can occur only in disturbed places, for example on a plowed strips protecting «Dyakovskiy forest» from fires. One relevé represents scrub communities that inhabit relatively well-moistened depressions. Thus, the dendrogram structure reflect the main patterns in species composition, associated with sand fixation chronosequence — from stage of annual psammophytes to stage of bunchgrasses and shrubs. To determine the syntaxonomical status of sandy steppes in the system of floristic classification, the earlier described associations of the class Festucetea vaginatae were analyzed. On the dendrogram all associations are united in three groups (Fig. 4). The most numerous first group represents communities from Azov and Black Sea coasts. The second group includes intercontinental sandy steppes. The third group of four associations is transitional, geographically it represents the Aleshkovskiye sands from Don River valley close to its estuary. Steppes from the «Dyakovskiy forest» are the closest to their analogues from Orenburg Region (Dulepova et al., 2018), while a comparison of their species composition shows significant differences (Table 2). Their similarity is determined by a group of common psammophytic species, most of them involved in the diagnosis of the class Festucetea vaginatae, its central order Festucetalia vaginatae Soό 1957 and the alliance Festucion beckeri Vicherek 1972 (Vicherek, 1972). A series of close associations was described from the territory of the Rostov Region (Demina, 2009; Demina et al., 2012). Two associations from the Peskovatskiy sand massif (Rostov Region) were used in the cluster analysis. The level of similarity of these associations with the Saratov and Orenburg associations is low. Thus, the peculiarity of the species composition of the described sandy steppes allows to attribute them to a new association. Association Thymo pallasiani–Festucetum beckeri ass. nov. hoc loco (Table 1: 2–13), holotypus hoc loco: relevé 10 (field number 18-088): Saratov Region (border with Volgograd Region), Krasnokutskiy district, 7 km SW from Dyakovka village, sands, 50.70002º N, 46.68761º E, 21.05.2018, author — A. Yu. Korolyuk. Diagnostic species: Alyssum desertorum, Carex colchica, Chamaecytisus borysthenicus, Chondrilla juncea, Euphorbia seguieriana, Festuca beckeri, Helichrysum arenarium, Jurinea polyclonos, Kochia laniflora, Koeleria sabuletorum, Poa bulbosa, Potentilla incana, Silene borysthenica, Stipa borysthenica, Thymus pallasianus. This association is considered as central in the alliance Festucion beckeri and is diagnosed by differential species of higher-level syntaxa. Communities of associations dominate on territory of the «Dyakovskiy forest» (Fig. 5, 6). Association includes 2 variants representing different stages of sand fixing. The var. Agropyron fragile inhabits more fixed sands. The var. Leymus racemosus represents earlier stages. In the study area we distinguish two communities that belong to the class Festucetea vaginatae. The community Spiraea crenata–Scirpoides holoschoenus is confined to shallow depressions. It is characte­rized by shrub layer with cover up to 20 % and a height of 60–80 cm. It is differentiated from surrounding­ ­ve­getation by high abundance of Scirpoides holo­schoenus and Festuca rupicola. The community Secale sylvestre–Anisantha tectorum is described on a fire protection strips. These are species poor coenoses dominated by annual plants (Fig. 7).