Syntaxonomy and ecological differentiation of the pioneer vegetation of Ukraine

Pioneer psammophytic vegetation is usually developed on wind-drift sandy substrates such as arenas, spits, beaches, river terraces, and this vegetation occupies significant areas in all three natural zones of Ukraine. The Koelerio-Corynephoretea canescentis class was represented by 13 associations, 3 alliances and 1 order; Festucetea vaginatae class by 22 associations, 2 alliances and 1 order; Helichryso-Crucianelletea maritimae by 10 associations, 4 alliances and 1 order. The results of cluster analysis and synoptic tables of the classes are presented. 9 alliances are briefly described. Leading factors of territorial and ecological differentiation are identified. It was found that the territorial distribution of plant communities is influenced by the character of ecotope mesorelief, soil composition and humus horizon thickness, as well as the degree of eolian processes development. The main factors of their ecological differentiation are soil acidity, salt regime and ombroregime. Based on the results of DCA-ordination of syntaxa within certain vegetation classes, it was found that their distribution is influenced by factors that correlate with the environment-specific conditions. It has emerged that an ecological differentiation of syntaxa within Festucetea vaginatae is determined by the integrated effect of gradients, and soil salinity is leading among them. Temperature regime and climate continentality are leading factors in the distribution of syntaxa within the Koelerio-Corynephoretea canescentis class. The gradients of ombroregime and soil humidity have a significant impact. The distribution of communities of the Helichryso-Crucianelletea maritimae class in the ecological space is determined mainly by factors of variability of damping, ombroregime and climate continentality. The author’s syntaxonomic concept assumes the independence of the studied classes: Koelerio-Corynephoretea canescentis, Helichryso-Crucianelletea maritimae and Festucetea vaginatae, conside­ring that the leading factors of community differentiation of the Festucetea vaginatae and Koelerio-Corynephoretea canescentis classes are the origin (genesis) of sandy substrates, as well as soil acidity. Phytosociological analysis of a large number of relevés of coastal littoral vegetation also provides support for independence of the Helichryso-Crucianelletea maritimae and Ammophiletea classes diffe­rent floristically and ecologically. A review of the psammophytic vegetation of Ukraine will determine the place of the selected syntaxonomic units in the pan-European system.


Introduction
Pioneer psammophytic vegetation developed on wind-drift sandy substrates such as arenas, spits, beaches, terraces, etc., performs extremely important functions: biological, ecological, regulatory, recreational, and many others. It stabilizes loose scattered soils, creates preconditions for the development of vegetation in subsequent successional stages. The geocomplexes on which such vegetation occurs are sites of biological diversity formation and conservation. They are habitats for many plant and animal species, ways for dispersing their genetic material, and also serve as migration routes by which alien species spread actively. Newformed arena ecotopes are the centers of formation of endemic floristic complexes; they are characterized by generic endemism.
The main areas of arenas in Ukraine are located in Polissia, and the smaller one in the forest-steppe zone. The largest continuous sand massifs in the steppe zone are located in the lower valley of the Dnieper River. This territory is called "Oleshkivsky Sands", and it occupies about 200 thousand hectares. Large areas are occupied by "Pridonetski Sands" (80 thousand ha) and sand areas on coasts, spits and islands of the Black and Azov Sea, Lower Danube and the Southern Bug River.
The macrorelief of sand massifs is mostly flat with small differences in elevation. However, the greatest importance in the distribution of plant communities belongs to the mesorelief of the sands, which is usually characterized by an alternation of high, up to 3-5 m, rounded (hills) or elongated dunes and fairly deep depressions. The topography and vegetation of wind-drift sands are mutually determining factors, since the absence of vegetation contributes to the dune formation, and their subsequent overgrowth promotes the formation of sand dunes, which over time are somewhat leveled in the hilly ones (Gordienko, 1969).
Pioneer psammophytic vegetation is represented in Ukraine by three classes: Festucetea vaginatae, Helichryso-Crucianelletea maritimae and Koelerio-Corynephoretea canescentis. The syntaxonomy of vegetation of Festucetea vaginatae and Koelerio-Corynephoretea canescentis classes in Ukraine has been the subject of studies conducted by many authors. J. Vicherek on the territory of the lower and middle Dnieper River described Festucion beckeri alliance with 4 associations and, in fact, made an attempt to validate Festucetea vaginatae class, first identified by R. von Soó in 1968 (Vicherek, 1972). Secali-Stipetum borysthenicae association and two alliances: Verbascion pinnatifidi and Cynodonto-Teucrion polii were described for the coastal dune area in the Crimean Peninsula (Korzhenevsky, 1986;Korzhenevsky & Klyukin, 1990). Arena landscapes of the "Biruchiy Island" spit in the Azov Sea were studied by Dubyna et al. (1995). On the territory of the spit, they identified 8 lowerrank syntaxa belonging to Festucetea vaginatae class, and two syntaxa attributed by the authors to Chenopodietea class. Three associations were described as new for science. Psammophytic vegetation of the Bilosaraiska Spit and the sea coast near Mariupol city was studied by Androsova & Solomakha (1996). Didukh & Korotchenko (1996) recorded a Centaureo borysthenicae-Festucetum beckeri association in the southern part of the forest-steppe zone of Ukraine, near the mouth of the Vorskla River and on islands of the Dniprodzerzhinsky and Pechenezhsky reservoirs. Umanets & Solomakha (1999a, 1999b conducted their research in the 1990s in the Central Black Sea region. In the Black Sea Biosphere Reserve, the vegetation syntaxonomy included without limitation psammophytic coenoses of Festucetea vaginatae class, within which the authors distinguished the Medicagini tenderiensis-Seselietalia tenderiensi order containing two alliances: Medicagini tenderiensis-Seselion tenderiensi and Melico chrysolepi-Ephedrion distachyae. On the territory of the Danube Biosphere Reserve communities of 19 associations of the Festucetea vaginatae class were described (Dubyna et al., 1996(Dubyna et al., , 2003. 11 associations of this class were identified on the territory of the National Nature Park "Dzharylhatskyi" (Dubyna & Dziuba, 2005a;Davydova, 2019). In the Pryazovia spits: Obitochna, Berdianska, Kryva, Bezymenna, Shyrokinska, Bilosaraiska, Stepanivska and Fedotova, Tyshchenko (2006) recorded the distribution of communities of 9 associations belonging to the Festucetea vaginatae class; while three associations were newly identified in the course of surveys conducted by the author. In the estuarine region of the Dnieper River, Chynkina (2003) identified communities of one association. In addition to the southern regions, coenoses of the class were also observed in the Middle Dnieper River region, on islands of the Dnieper River within Kyiv city (Tsukanova, 2005;Aleshkina, 2011), as well as on the territory of the Kaniv Nature Reserve  where the authors identified a Artemisio dniproicae-Salicion acutifoliae alliance. Its syntaxonomic status is debatable, since such communities involving shrubs are more often classified as Salicetea purpureae Moor 1958 (Mucina et al., 2016).
Phytocoenoses of sandy steppes were also observed in the northern steppe and forest-steppe zone of Ukraine, on the left bank of the Dnieper River (Bairak, 1998;Gomlya, 2005;Dziuba et al., 2010;Chusova, 2019); there the phytocoenoses occupy much smaller areas and are not characterized by coenotic diversity.
Within the scope of final surveys on psammophytic vegetation in the Black Sea region, D. Dubyna and T. Dziuba in the "Prodrome of the vegetation of Ukraine" identified a new alliance Artemisio arenariae-Festucion beckeri in the Festucetea vaginatae class and isolated from one the class Helichryso-Crucianelletea maritimae, in which Ephedro distachyae-Medicaginetalia romanicae order was allocated for the Ukraine territory together with the alliance Ephedro distachyae-Medicaginion romanicae (Dubyna et al., 2019a). Unfortunately, the attempt of Korzhenevsky & Kvitnitskaya (2014) to validate the alliances Verbascion pinnatifidi Korzhenevsky et Klyukin 1990 and Cynodonto-Teucrion polii Korzhenevsky et Klyukin 1990, as well as the vegetation associations of aeolian landforms in the Crimea, were not completely successful since expressis verbis "typus" should have been used to indi-cate the type of syntaxon name, according to article 5 of the ICPN (Weber et al., 2000).
Coenoses of the Koelerio-Corynephoretea canescentis class were first studied in Ukraine by Vicherek (1972) using the Braun-Blanquet approach on riverside sands of the Middle Dnieper River, where he described the Veronico dillenii-Corynephoretum association with its two subassociations. Later, coenoses belonging to this class (within the boundaries of Sedo-Scleranthetea and Festucetea vaginatae) were described for the Kaniv Nature Reserve area . Didukh & Korotchenko (1996) noted the distribution of plant communities belonging to the same association in the southern part of the left-bank forest-steppe zone of Ukraine, in the lower valley of the Vorskla River, in the upper valley of the Berestova River, and on islands of the Dniprodzerzhinsky and Pechenizhsky reservoirs. On left bank of the Dnieper River, coenoses belonging to this class were recorded by Bayrak (1998) and Galchenko (2006). V. Shevchyk and V. Solomakha, and later V. Shevchyk with O. Polishko described the communities of Thymo pallasiani-Centauretum sumensis and Cladonietum associations as part of the vegetation on the Dnieper River pine-forest terrace which grows along the bank of the Kaniv Reservoir Shevchyk & Polishko, 2000;Polishko, 2001Polishko, , 2005. On the Zhytomyr Polissia area, Yakushenko (2004) allocated the Artemisio campestris-Dianthetum borbasii association. Plant communities of the two associations were identified in area of the planned Korostyshivsky National Nature Park (Orlov & Yakushenko, 2005). Coenoses of the Helichryso-Jasionetum association were described on the territory of the Polissky Nature Reserve (Vorobyov et al., 1998), and of Thymetum pulegioido-serpylli association were identified in the Cheremsky Nature Reserve (Konishchuk, 2006). Senchylo (2010) analyzed mainly coenoses of the Koelerio-Corynephoretea canescentis class; the scientist performed phytocoenotic studies on 7 following plant associations in the Dnieper River floodplain area within the forest-steppe zone: Artemisio dniproicae-Sedetum sexangularis, Centaureo borysthenicae-Festucetum beckeri, Veronico dillenii-Secaletum sylvestri, Chamaecytiso ruthenici-Festucetum beckeri, Thymo pallasiani-Centauretum sumensis, Sedo sexangulare-Festucetum beckeri, Diantho borbasii-Agrostietum syreistschikovii. Among the syntaxonomic diversity of vegetation in the Pyryatinsky National Natural Park, Kovalenko (2016) identified communities of Veronico dillenii-Secalietum sylvestris, Linario odorae-Agropyretum dasyanthi and Chamaecytiso ruthenicae-Festucetum beckeri associations, which were assigned to the Festucetea vaginatae class. In Ukrainian Roztochia, Soroka (2008) identified Spergulo vernalis-Corynephoretum and Diantho-Armerietum associations.
One of the generalizing works on classified syntaxa of xerophytic psammophytic vegetation in the forest and forest-steppe zones of the plain area of Ukraine was Kuzemko (2009) which includes syntaxonomic processing of 391 relevés. On the basis of cluster analysis and comparison of synoptic tables, the author identified 12 association-level syntaxa within Koelerio-Corynephoretea canescentis class (incl. Sedo-Scleranthetea and Festucetea vaginatae) and two associations within Molinio-Arrhenatheretea class. Phytosociological materials from Ukraine consisting of 179 relevés were included in the large-scale analysis of coastal dune vegetation in the Baltic-Atlantic, Mediterranean and Black Sea regions (Marcenò et al., 2018). The authors analyzed a large database (11,769 vegetation plots) and created the first formal classification of European coastal dune vegetation, accompanied by an expert system. This allowed them to identify 18 middle-rank syntaxa (alliances) which belong to the classes Ammophiletea, Honckenyo-Elymetea arenarii and Koelerio-Corynephoretea canescentis, and providing a critical revision of the concept of the class Ammophiletea used in EuroVegChecklist.
The analysis of significant databases of phytosociological relevés allows one to solve many issues on construction and effectiveness of vegetation classification (Lengyel et al., 2018;Willner et al., 2019;Bondareva et al., 2019;Landucci et al., 2020), biodiversity definition (Sabatini et al., 2018), biogeographic distribution of vegetation and its ecological affinity , and others. Currently, we have collected more than 1700 phytosociological relevés of psammophytic vegetation in Ukraine which includes more than 1000 relevés belonging to the classes Festucetea vaginatae, Helichryso-Crucianelletea maritimae, Koelerio-Corynephoretea canescentis, syntaxonomic processing of which, in our opinion, will make a certain contribution to studying of psammophytic vegetation in Europe and supplement its syntaxonomy.
In recent years, the ecological component of phytocoenoses has become increasingly used as an object of study and in classification proving (Çoban & Willner, 2019;Zhou et al., 2019;Willner et al., 2019), including for the study of vegetation dynamics (Cao et al., 2019), identifying the main ecological factors of plant communities' differentiation (Kuzemko et al., 2016;Korolyuk et al., 2018), assessing the impact of environment on the floristic richness in phytocoenoses (Jansons et al., 2016;Yousaf et al., 2016;Slezák et al., 2017). The method developed by Didukh (2012) for determining the main eco-factors of the distribution of plant communities by 12 gradients completes phytosociological studies of vegetation and allows us to determine the ecosystem state by indicators of their biotic components.
The purpose of the paper was to generalize the accumulated phytocoenotic materials and existing data on syntaxonomy of the pioneer psammophytic vegetation in Ukraine, to clarify its current state and to develop a syntaxonomic structure, to establish the leading factors of ecological differentiation of phytocoenoses on the basis of ordination analysis, as well as to discuss controversial issues related to the syntaxonomic content of the classes Festucetea vaginatae, Koelerio-Corynephoretea canescentis, Helichryso-Crucianelletea maritimae.

Materials and methods
The work is based on the materials of field studies carried out by the authors during 1984-2018 according to the Braun-Blanquet approach to floristic classification (Braun-Blanquet, 1964;Westhoff & van der Maarel, 1973). The standard size of plots was 4 × 4 m. Sometimes, in particular on elongated coastal zones and dunes, the plots have size 1 × 4 or 2 × 5 m. At the same time, requirements for structure uniformity of vegetation cover were met. Phytosociological analysis also included the relevés presented in abovementioned publications of J. Vicherek, O. Tyshchenko, Y. Didukh, I. Korotchenko, O. Senchylo, O. Umanets, I. Solomakha, V. Solomakha, A. Androsova, O. Orlova, D. Iakushenko, V. Shevchyk, O. Polishko, O. Bayrak, N. Galchenko, M. Soroka, A. Davydova, L. Gomlya, T. Chinkina, V. Korzhenevsky and A. Klyukin Khomyak. The size of plots was not always specified in the relevés, but we used all the available ones.
The materials were ordered by development of a database of geobotanical relevés in TURBOVEG 2.79 (Hennekens & Schaminée, 2001). Interpretation of the phytosociological material was carried out in several steps. At the initial one, the entire database of geobotanical relevés of halophytic, psammophytic and littoral vegetation in Ukraine (7388 relevés) was processed and divided into smaller groups based on their floristic differences using the method of two-way indicator species analysis (TWINSPAN), in particular its modified algorithm (Hill, 1979;Tichý, 2002;Roleček et al., 2009) implemented in the JUICE 7.0 software package. "Pseudospecies" cut level was 0%, 5%, 15% and 30%. The Whittaker's beta was chosen as cluster heterogeneity measure (Whittaker, 1978). Further, the clusters were selected which corresponded to the classes of psammophytic vegetation, based on diagnostic species characteristics.
Since a significant number of relevés included only vascular plants, or when mosses and lichens were identidied only up to their genus, authors excluded mosses, lichens and algae species from processing in order to "align" the relevés. These species were added again at the final stages of compiling the phytocoenotic tables. Cluster analysis was carried out using the PC-ORD program after the rejection of a certain part of relevés related to Ammophiletea Br.-Bl. et Tx. ex Westhoff et al. 1946, Salicetea purpureae Moor 1958 (Artemisio dniproicae-Salicetum acutifoliae , Pyrolo-Pinetea sylvestris Korneck 1974, Sedo-Scleranthetea Br.-Bl. 1955, relevés of ruderal vegetation, relevés attributed by the authors to communities, and also not containing dominant species at all. The Sörensen coefficient (Sörensen, 1948) was chosen as the similarity measure and grouping was performed using the "flexible" beta method at -0.25. This resulted in finding smaller groups approximately corresponding to the association's rank. Relevés of Festucetea vaginatae class were grouped according to Ward's method (Ward, 1963). Identification of diagnostic species in syntaxa was carried out in accordance with the fidelity index -the Phi coefficient (Willner et al., 2009), the threshold values of which were assumed at the level of 25. All relevé groups were standardized to equal size, and non-essential fidelity values were removed based on the Fisher exact test.
Identification of the obtained phytocoenoses was carried out on the basis of their diagnostic species and floristic composition by comparison with foreign and Ukrainian publications (Demina, 2009(Demina, , 2015Mucina et al., 2016;Bulokhov, 2019;Dubyna et al., 2019a).
At the next stage of processing, after exclusion of relevés related to Ammophiletea, Salicetea purpureae, Pyrolo-Pinetea sylvestris, relevés of disturbed habitats, as well as those that do not contain dominant species, phytocoenons with the studied classes remained: Helichryso-Crucianelletea maritimae, Koelerio-Corynephoretea canescentis and Festucetea vaginatae -955 relevés (Fig. 2). Then we performed cluster analysis separately per classes. It should be noted that a significant number of sample sites within the studied classes were identified by us as agglomerative groups representing the initial stages of sand overgrowth, or as rankless (basal/derrivate) communities (a total of 258 relevés), which were not included by the authors at this stage in the classification scheme. Based on processing of phytocoenotic tables and cluster analysis results, a refined classification scheme of psammophytic vegetation of Ukraine was obtained. Koelerio-Corynephoretea canescentis class includes pioneer communities on fluvioglacial and riverine sandy, shallow, weakly mobile soils, mostly dry and poor in nutrients, having acidic reaction. According to our results, the class in Ukraine include 1 order, 3 alliances and 13 associations (Fig. 3, Table 1).
According to the results of ordination analysis of communities in the class Koelerio-Corynephoretea canescentis, it was found that temperature regime and climate continentality play a leading role in their ecological differentiation (Fig. 4). These parameters are most important for coenoses within Corynephorion canescentis. Also, the distribution of communities is significantly affected by the gradients of ombroregime and soil humidity. Communities of Corynephoro-Silenetum tataricae, Thymo angustifolii-Festucetum beckeri, Jasiono montanae-Thymetum serpylli and Jasiono montanae-Festucetum ovinae associations are more "sensitive" to changes in these parameters. Communities of the last two syntaxa will also respond to concentration of available nitrogen forms in soil substrate. Variability of damping (fH) can be considered the leading ecological factor for the coenoses Diantho borbasii-Agrostietum syreistschikovii, Veronico dillenii-Secaletum sylvestris and Chamaecytiso ruthenicae-Festucetun beckeri. Communities of the Diantho deltoidis-Armerietum elongatae association were described on the territory of the Ukrainian Roztochia on specific moraine deposits, and their distribution in the ecological space determines content of carbonates and other salts in the soil. Diantho deltoidis-Armerietum elongatae, Corynephoro-Silenetum tataricae, Artemisio dniproicae-Sedetum sexangularis and Thymo angustifolii-Festucetum beckeri communities differ significantly in their ecological amplitude (Fig. 4).    The class Festucetea vaginatae combines communities of sandy steppes and sandy meadows. In Ukraine, these coenoses are common in the steppe zone on the lower Dnieper River arenas, in the Black Sea region and the Azov Sea regions. These coenoses are less common on riverine sands. According to our study, in Ukraine, the class includes one order, two alliances and 22 associations. Cluster analysis suggests that Artemisio arenariae-Festucion beckeri alliance is comprised of 4-5 sub-alliances, but it is not still possible to identify clear diagnostic blocks (Fig. 5, Table 2). The leading factors of territorial differentiation of phytocoenoses also determining their coenotic diversity are the mesorelief patterns (elements of dunes, interdunal depressions, flat areas in arenas), soil type, its humus content and density, as well as the degree of aeolian processes influence. The hyperspace of ecological conditions from humid (deep depressions) to semi-desert (dune tops) contributes to their significant coenotic richness and diversity. The alliance Festucion beckeri includes coenoses occurring on sandy areas of low hilly coastal and riverine dunes which consist of non-carbonate soils. The alliance Artemisio arenariae-Festucion beckeri includes steppe plant communities occurring on sandy and sandy-shell soils of the Black Sea and the Azov Sea coastal areas.
Coenoses belonging to Helichryso-Crucianelletea maritimae class occupy specific ecotopes in stabilized overgrown (grey) dunes and overgrown uprush berm crest areas on the coasts of the Black Sea and Azov Sea. In Ukraine, the class includes 1 order, 4 alliances and 10 associations (Fig. 7, Table 3).     The Ephedro distachyae-Medicaginion romanicae alliance includes plant communities of stabilized coastal dunes and uprush berm crest areas covered with dense sand-shell soils in the Black Sea and Azov Sea. The Medicagini tenderiensis-Seselion tenderiensi alliance is presented by coenoses of mesophytic sand steppes on accumulative sand-shell macroforms in the northwest coast of the Black Sea. The Scabiosion ucranicae alliance is presented by coenoses of stabilized coastal dunes in the Western Black Sea Region. The Cynodonto-Teucrion polii alliance includes plant communities on young dunes, as well as on uprush limit parts of berm crest areas growing on washed sandy and shelly soils, enriched with detritus, on the Azov-Black Sea coast of Crimea. The leading factors of territorial differentiation of these communities are the landforms and their relative age, groundwater level, substrate density and type (sandy, shelly), as well as the degree of dune geomorphostructures stabilization depending on the activity of the sea and aeolian processes. Within the ecological space, distribution of communities is mainly determined by the variability of damping, ombroregime and climate continentality factors (Fig. 8). They have a greater influence on the coenoses in Anisantho tectorum-Helichrysetum arenariae, Scabioso ucranicae-Caricetum ligericae and Secali sylvestri-Alyssetum borzaeani associations. In addition to these factors, Anisantho tectorum-Medicaginetum kotovii, Ephedro-Caricetum colchicae and Medicagini tenderiensis-Seselietum tenderiensi communities also depend on the degree of soil aeration, soil humidity and the content of various salts. Anisantho tectorum-Helichrysetum arenariae, Scabioso ucranicae-Caricetum ligericae and Ephedro-Caricetum colchicae syntaxa have greater stenotopy, while coenoses of Cynodonto-Teucrion polii alliance developing in the Crimea demonstrate stenotopy by most ecological parameters.

Discussion
Studies on the vegetation cover of natural zones in Eurasia are important for understanding the processes of their development and functioning at the continental scale (Erdős et al., 2018). Therefore, a critical analysis of the accumulated phytocoenological materials against the background of modern knowledge is important. At the present time, the syntaxonomic content of the classes Festucetea vaginatae and Koelerio-Corynephoretea canescentis is still a debatable issue, which is facilitated by the transitional position of their syntaxa and the belonging of certain diagnostic species of the highest classification ranks to the same genus, in particular, coenose-forming species Festuca psammophila, F. polesica, F. vaginata, and F. beckeri. As already noted, a number of European authors classified phytocoenoses of overgrown sands as belonging to the order Festucetalia vaginatae Soó 1957 in the class Koelerio-Corynephoretea canescentis Borhidi, 2003;Biondi et al., 2014), when others classified them as entering into the class Festuco-Brometea Tzonev et al., 2009). Complementary to Ukraine, scientists from the Czech Republic (Chytrý, 2007), Romania (Sanda et al., 2008) and Russia (Demina, 2009(Demina, , 2011Ermakov, 2012) recognized the class independence, where syntaxonomic diversity of sandy steppes is higher than that of Central Europe. According to the authors, the leading factors of differentiation of communities in Festucetea vaginatae and Koelerio-Corynephoretea canescentis classes are the origin (genesis) of sandy substrates, as well as a reaction of soil solution. In the habitats of coenoses of the class Festucetea vaginatae, sandy soils were developed by marine and river sediments and have an alkaline reaction. Communities of Koelerio-Corynephoretea canescentis class were developed mainly on fluvioglacial sands having acidic reaction. This was confirmed by the ordination analysis results of alliances in these classes (Fig. 9). Climatic conditions of their distribution are also important in the steppe (Festucetea vaginatae) and forest and forest-steppe zones (Koelerio-Corynephoretea canescentis), respectively.
Ordination analysis of psammophytic vegetation alliances allowed us to determine the main factors of ecological differentiation of the communities (Fig. 9). They are primarily soil acidity (Rc) and salt regime (Sl). Most of the alliances are located along the DCA1 axis, i.e. along these vectors. According to soil pH gradient, coenoses of the classes Koelerio-Corynephoretea canescentis and Helichryso-Crucianelletea maritimae occupy the opposite positions. Communities of Festucetea vaginatae are located in the center, occupying an intermediate position and overlapping most of the Helichryso-Crucianelletea maritimae syntaxa, which indicates that they occur under similar environmental conditions. However, as the graph shows, one of the main parameters for the latter class is the content of carbonates (Ca) and nitrogen (Nt) in the soil. It is reasonable to assume an influence of ombroregime (Om) on distribution of Koelerio-Corynephoretea canescentis communities associated with forest and forest-steppe zones.
The syntaxonomic position of psammophytic communities with the participation of the shrub Salix rosmarinifolia (association Salici rosmarinifoliae-Holoschoenetum vulgaris Mititelu et al. 1973 and subassociation Centaureo brevicepsis-Festucetum beckeri salicetosum rosmarinifoliae Vicherek 1972) has to be clarified in the future. This is due to the fact that such coenoses were classified as Salicetea arenariae Weber 1999 on the Atlantic coast area of Western Europe (Mucina et al., 2016), but in Ukraine some authors classified it to the class  (Solomakha et al., 2015), and others classified it to the class Festucetea vaginatae (Dubyna et al., 2003(Dubyna et al., , 2019a. It should be noted that the position of Jasiono montanae-Festucetum ovinae Klika 1941 within Koelerion glaucae is obviously somewhat ambiguous; since there is clearly a transitional position of its communities to meso-xerophytic vegetation on shell skeletal silicate soils of the Sedo-Scleranthetea class. Czech scientists attributed this association to the alliance Hyperico perforati-Scleranthion perennis Moravec 1967 within the Sedo-Scleranthetea class (Sádlo & Chytrý, 2007), but in Ukraine they are described on sandy habitats having a weak acidic reaction of soil solution; so, the authors previously assigned these communities to the Koelerio-Corynephoretea canescentis class. Coenoflora composition and syntaxonomic position of the association Festuco psammophilae-Koelerietum glaucae sensu Gal'chenko 2006, non Klika 1931 remain unclear, since the diagnostic species given by the author was Festuca valesiaca (Galchenko, 2006), and the floristic composition differs significantly from the protolog. Further conducting of phytosociological studies of communities with the participation of such psammophytic coenose-forming species as Festuca psammophila and F. polesica are required, in particular, on the territory of the Ukrainian Polesie, which in this regard has not been sufficiently studied. Presumably, phytocoenoses of Festuco psammophilae-Koelerietum glaucae Klika 1931, Spergulo-Festucetum psammophilae Passarge 1960, Diantho arenarii-Festucetum polesicae R. Tx. 1937 and other associations should be distributed on the territory of Ukraine.
Probably, further research on communities in the little-known class Pyrolo-Pinetea sylvestris in Ukraine will allow us to determine the syntaxonomic position of associations Thymo pallasiani-Centauretum sumensis Shevchyk et Solomakha in Shevchyk, Solomakha et Voytuk 1996 andCladonietum Shevchyk et Polishko 2000 nom. inval. (art. 3b) originally identified by the authors as part of Koelerion glaucae alliance Shevchyk & Polishko, 2000). Previously, according to results obtained from the analysis of psammophytic vegetation, the associations in their floral composition were closer to native dry pine forests on sandy soils developed on the sandy terraces of large rivers (Koelerio glaucae-Pinion sylvestris alliance Ermakov 1999, Pyrolo-Pinetea sylvestris class).
Class Helichryso-Crucianelletea maritimae has been allocated recently on the territory of Ukraine (Dubyna et al., 2019a). Previously, its communities were considered within the class Festucetea vaginatae. There are currently two orders were allocated in Western Europe: Artemisio-Koelerietalia Sissingh 1974 (sandy meadows and shrubs on the rich stabilized grey dunes on the coasts of the Atlantic Ocean and of the Northern, the Ligurian and the Adriatic seas) and Crucianelletalia maritimae Sissingh 1974 (Mediterranean, Cantabro-Francoatlantic dwarf shrubs and Xerothermic meadows on stabilized coastal grey dunes). Their plant communities differ significantly from those of the Black Sea region; because of this a separate order was proposed in the class for the territory of Ukraine: Ephedro distachyae-Medicaginetalia romanicae (Dubyna et al., 2019a). Although some researchers see no rationale for separation of this class and consider it as a synonym of Ammophiletea (Marcenò et al., 2018), the results obtained allow us to maintain the position of its ecological and floral isolation. Despite the fact that both of these high-rank syntaxa include several common species (Eryngium maritimum, Leymus racemosus ssp. sabulosus, Ephedra distachya), the class Helichryso-Crucianelletea maritimae is characterized by the presence of a block of diagnostic species that distinguish it from the complex of diagnostic species Ammophiletea and Festucetea vaginatae, at least on the territory of Ukraine. These species are the following: Artemisia tschernieviana, Asparagus maritimus, Astragalus onobrychis, Centaurea apiculata ssp. adpressa, Erodium cicutarium, Medicago romanica (~ M. sativa ssp. falcata), Melica transsilvanica ssp. klokovii, Scabiosa argentea, Tamarix gracilis, Teucrium polium, Trachomitum sarmatiense, Verbascum pinnatifidum. Ecologically stable dune communities also are distinct in that they do not experience periodic effects of sea waves, since they are protected by the berm crest, and their habitats have morphological differences. Most often, these are gently sloping uprush parts of berm crest and, to some extent, elevated sand deposits (dunes) anchored mostly by perennial vegetation. According to our data, communities of Helichryso-Crucianelletea maritimae class were even closer to Festucetea vaginatae than to Ammophiletea.
Worldwide, coastal dunes and other psammophytic habitats are recognized as highly vulnerable and dynamic ecosystems (Rannow & Neubert, 2014;Acosta & Ercole, 2015;Sarmati et al., 2019); in Ukraine and other countries they are systematically over-used. Inadequate land management contributes to the fact that these habitats are threatened with extinction, especially in the Mediterranean region (Sabatini et al., 2018). Frequent invasions of alien and atypical species into the structure of communities have been recorded; it resulted in their transformation and degradation (Dubyna et al., 2019b). All of this indicates the need for urgent actions to protect and preserve them.

Conclusion
Pioneer psammophytic vegetation of Ukraine is represented by 45 associations, 9 alliances and 3 orders belonging to 3 classes (Koelerio-Corynephoretea canescentis, Helichryso-Crucianelletea maritimae and Festucetea vaginatae). The main factors of territorial and ecological differentiation of pioneer psammophytic vegetation in Ukraine were identified. It was found that the territorial distribution of communities of Koelerio-Corynephoretea canescentis, Festucetea vaginatae and Helichryso-Crucianelletea maritimae were mainly influenced by ecotope mesorelief pattern, soil composition and the thickness of humic horizon, as well as the degree of aeolian processes development. The main factors of their ecological differentiation are soil acidity, salt content and ombroregime. Based on the results of DCA-ordination of syntaxa within certain vegetation classes, it was found that their distribution is influenced by factors that correlate with the environment-specific conditions. It was found that an ecological differentiation of syntaxa within Festucetea vaginatae is determined by the integrated effect of gradients, and soil salinity can be distinguished among these. The main factors in syntaxa distribution within the Koelerio-Corynephoretea canescentis class were temperature regime and climate continentality, as well as the gradients of ombroregime and soil humidity. The distribution of communities of Helichryso-Crucianelletea maritimae class in the ecological space is determined mainly by factors of variability of damping, ombroregime and climate continentality. Coenoses of Secaletum sylvestris, Diantho deltoidis-Armerietum elongatae, Corynephoro-Silenetum tataricae, Artemisio dniproicae-Sedetum sexangularis, Thymo angustifolii-Festucetum beckeri, Anisantho tectorum-Helichrysetum arenariae, Scabioso ucranicae-Caricetum ligericae and Ephedro-Caricetum colchicae associations were characterized by the widest ecological amplitude.
The authors adhere to the position of independence of the studied classes: Koelerio-Corynephoretea canescentis, Helichryso-Crucianelletea maritimae and Festucetea vaginatae, considering that the main factors of differentiation of communities in the classes Festucetea vaginatae and Koelerio-Corynephoretea canescentis were the origin (genesis) of sandy substrates, as well as the reaction of soil solution. Phytosociological analysis of a large number of coastal littoral vegetation relevés also provide support for independence of Helichryso-Crucianelletea maritimae and Ammophiletea classes based on their floristic and ecological differences.
The conducted research will allow the place of selected syntaxonomic units of the pioneer psammophytic vegetation of Ukraine to be determined in the pan-European system. The results of the work will be suitable for development of the Nature Reserve network and Eco-network of Ukraine, maintaining Prodrome of the vegetation of Ukraine, Prodrome of the vegetation of Europe, preparing the next volume of the publication "Vegetation of Ukraine" and "Green Data Book of Ukraine", for further study of successional and adaptation processes in extreme environmental conditions of pioneer habitats, establishing patterns of regenerative geosystems' development and finding out the ways to manage them. The ability of plant communities to indicate the environment state (phytoindication of environmental conditions), which in recent years has become an urgent sociological necessity, will allow the environmental situation in the regions to be monitored, as well help in as determining trends in their further changes.