Shrub and tree-shrub vegetation of the class Crataego-Prunetea Tx. 1962 nom. conserv. propos. in the Southwest of Russia

This article provides a regional survey of the syntaxa of shrub and tree-shrub vegetation of the class Crataego-Prunetea Tx. 1962 nom. conserv. propos., which represents scrub and mantle vegetation seral or marginal to broad-leaved forests in the nemoral zone and the submediterranean regions of Europe. Until recently, the study of this vegetation in Russia was very fragmentary, and data on the diversity of shrub communities in the country were practically not used in the European Checklist (Mucina et al., 2016). Some problematic issues in the study of the ecology and geography of shrub vegetation, in particular, their differentiation from shrub steppes, successional status, according to the authors, require detailed study. The data for the review was the database including 130 relevés done by the authors in 2005–2023 in the Southwest of Russia (Belgorod, Bryansk, Voronezh, Kursk, Lipetsk, Oryol, Rostov, and Tula Regions). 103 relevés are presented for the first time; although 38 of these were previously published, but after comparative analysis, their syntaxonomic position was changed. For higher units of shrub vegetation, regional diagnostic combinations are established, which included species with constancy of more than 20 % and statistical φ-coefficient of accuracy values more than 20 (p < 0.05). Based on the analysis, the following regional diagnostic combination was identified for the class Crataego-Prunetea: Agrimonia eupatoria s. l., Fragaria viridis, Phlomoides tuberosa, Poa angustifolia, Prunus fruticosa, P. spinosa s. l., P. tenella, Rhamnus cathartica, Rosa canina aggr. Its peculiarity is the absence of shrub species of genera Crataegus, Rubus and Rosa (with the exception of R. canina aggr.), which are traditionally diagnostic for the class in the Central and Southern Europe communities (Mucina et al., 2016). Attention is also drawn to the revealed fidelity of species of meadow steppes, steppe forest edges and meadows to this group of relevés: Agrimonia eupatoria s. l., Fragaria viridis, Phlomoides tuberosa, Poa angustifolia, which corresponds to the pattern of communities on the “forest–steppe” gradient in the Southwest of Russia. In the study area, the class is represented by the only order Prunetalia spinosae Tx. 1952, the diagnostic species of which are accepted within the scope of the class, and 3 alliances in its composition: Berberidion vulgaris Br.-Bl. ex Tx. 1952 nom. conserv. propos., Acerion tatarici Fitsailo 2007 nom. dub. propos., Prunion fruticosae Tx. 1952. It is recommended to change the verbal diagnosis of the alliance Berberidion vulgaris, which, according to as our study shows, unites thermophylous shrub and tree-shrub communities of the temperate and sub-Mediterranean parts of not only Southern and Central, but also Eastern Europe. Regional combination of diagnostic species of the alliance are Agrimonia eupatoria, Campanula rapunculoides, Prunus spinosa s. l., Rhamnus cathartica. The alliance Acerion tatarici unites tree-shrub thermophylous communities with the presence and dominance of Acer tataricum on the slopes of ravines and river valleys, edges of broad-leaved forests and outskirts of steppe areas in the forest-steppe and steppe zones in Eastern Europe. Diagnostic combination of the alliance are Acer tataricum, Alliaria petiolata, Aristolochia clematitis, Euphorbia stepposa, Scutellaria altissima, Ulmus minor. We pre-suggest rejecting the name of the alliance as doubtful (nom. dub. propos.) due to errors in its original establishment. A unit of similar content might be described with the accumulation of geobotanical data. The alliance Prunion fruticosae unites subcontinental and continental shrub communities of the forest-steppe and steppe zones of Central and Eastern Europe. Regional diagnostic combination of the alliance: Adonis vernalis, Chamaecytisus ruthenicus, Coronilla varia, Filipendula vulgaris, Galium verum s. l., Iris aphylla, Phlomoides tuberosa, Prunus fruticosa, Prunus tenella, Salvia nutans, Seseli annuum, Stachys recta, Stipa pennata, Thalictrum minus s. l., Vincetoxicum hirundinaria, Xanthoselinum alsaticum. This combination high constancy of meadow-steppe and steppe herbs reflects significant steppization process. Based on floristic comparison with syntaxa known in Europe, we believe that it is inappropriate to classify communities formed by Prunus spinosa s. l., P. tenella and P. fruticosa in the Southwest of Russia either as widely interpreted syntaxa of Central Europe or as shrub steppes of more easterner regions of Russia. Within the two alliances and one order of the class Crataego-Prunetea, four new associations (Poo angustifoliae–Prunetum spinosae ass. nov., Urtico dioicae–Prunetum spinosae ass. nov., Agrimonio eupatoriae–Prunetum tenellae ass. nov., Phlomoido tuberosae–Prunetum fruticosae ass. nov.) and three subassociations (Poo angustifoliae–Prunetum spinosae typicum subass. nov., P. a.–P. s. galietosum veri subass. nov., P. a.–P. s. festucetosum valesiacae) are established. Some types of shrub communities in the studied area are still known from small relevé set in few sites including phytocoenoses dominated by species common in the communities of the class Crataego-Prunetea, whose position in the syntaxonomic hierarchy is yet not clear. That is the reason of the so far non-rank status of the communities Cornus sanguinea, Rhamnus cathartica, and Lonicera tatarica within this class.