Syntaxonomy and ecological differentiation of the pioneer vegetation of Ukraine Classes:

The studies of the pioneer vegetation of freshwater shorelines of water bodies are of particular interest owing to the specific ecology of these habitats and the short cycle for their development in which periods of flooding and subsequent drainage alternate. Using the methods of phytosociological classification and cluster analysis based on the interpretation of 414 phytosociological relevés, the syntaxonomic structure of the pioneer vegetation of freshwater shorelines of the water bodies of Ukraine has been established that are represented by the phytosociological classes Isoëto-Nanojuncetea and Bidentetea . The class Isoëto-Nanojuncetea includes 8 associations that belong to 2 alliances and 1 order and the class Bidentetea includes 10 associations belonging to 2 alliances and 1 order. Phytocoenoses of both classes are more typical for the Polissia region and the forest-steppe zone of Ukraine, where there are favourable habitats with a flat relief, low degree of dissection and a high level of soil humidity. Using a DCA ordination analysis of associations their position in ecological space was determined. It was established that the main factors of ecological differentiation for Isoëto-Nanojuncetea habitats are soil humidity, soil aeration, nitrogen content, as well as temperature regime. Differentiation in the hyperspace of abiotic factors of the class Bidentetea occurs mainly along the gradients of soil humidity, salt regime and acidity. The ecological distribution of syntaxa of this class is also significantly influenced by the concentration of mineral nitrogen compounds in the soil. ______________________________________________________________________________________________________________________________________________

The pioneer vegetation of wetland habitats, in particular freshwater shorelines, has great biosphere importance. In particular, its phytocoenoses have anti-erosion, stabilization, water-cleaning, water protection, and other functions. The littoral zones of natural or man-made water bodies, where such vegetation is formed, are habitats for many species of plants and animals and channels for the distribution of their genetic material, in particular, for neophyte migration. Plant development period, short life cycles, and long-term survival in dormant propagules contribute to the uniqueness of these phytocoenoses and their highly dynamic habitats (DEIL, 2005;ŠUMBEROVÁ & HRIVNÁK, 2013). Most of the species and plant communities belonging to this vegetation type, especially the floodplain ephemeretum, are considered to be rare.
In Ukraine, the pioneer vegetation of the littoral zones of freshwater continental water bodies occupy river banks, shorelines of lakes and water reservoirs, the bottoms of periodically draining fish ponds, wet agricultural lands, depressions along roads and forest paths, sand and clay pits, forest glades, and other territories with a specific ecology of these habitats, when periods of flooding and drainage alternate. Vegetation of ephemeral wetlands also form in anthropogenically transformed habitats.
The syntaxonomy of the vegetation of ephemeral wetlands in Europe has been developed in sufficient detail. Classification schemes and characteristics are presented for the territories of Germany  (BORHIDI, 2003;MAKRA, 2006) and some other countries. An overview of the syntaxonomic system of alliances, orders and classes of the annual herb vegetation in Europe is presented by MUCINA .
In Ukraine, the pioneer vegetation of freshwater shorelines is represented by two classes: Isoëto-Nanojuncetea and Bidentetea. Due to the seasonal nature of their development, the syntaxonomy of these plant communities has not previously been sufficiently developed. In Ukraine, coenoses of the class Isoëto-Nanojuncetea were described for the first time by the Czech phytocoenologist J. Vicherek in 1968. For the territory of the Middle Dnieper River he identified a new association Peplido alternifoliae-Juncetum tenageiae. O. Senchylo and I. Goncharenko studied the littoral ecotone of the territory of the Dnieper Valley in the forest-steppe zone using the method of ecological profiling. These authors identified 3 subassociations of the most widespread within the Isoëto-Nanojuncetea class -Cypero fusci-Limoselletum (SENCHYLO ET AL., 1998;SENCHYLO & GONCHARENKO, 2008). Coenoses of this association were also described by V. Shevchyk et al. in the Dnieper Valley for the Kanivsky Nature Reserve . Dichostyli-Gnaphalietum uliginosi communities were recorded in the Danube Estuary within the territory of the Danube Biosphere Reserve (DUBYNA . For the Ukrainian Rostochia SOROKA (2010) marked the distribution of 5 associations: Cyperetum flavescentis, Cypero fusci-Limoselletum, Centunculo-Anthoceretum punctati, Pepli-Agrostietum and Ranunculo-Myosuretum minimi. In the upper part of the Tysa Basin (Ukrainian Carpathians) FELBABA- KLUSHYNA (2010KLUSHYNA ( , 2017 described the Juncetum bufonii and Cyperetum flavescentis associations. The first one is also noted for other regions of the Carpathians, in particular, the Natural Reserves Skolivski Beskydy (SOLOMAKHA  and "Gorgany" (KLIMUK . SHAPOVAL (2006) in the territory of the depressions of the Left Bank of the Lower Dnieper described several syntaxa of the class Isoëto-Nanojuncetea, including new ones for science. In particular, the alliance Myosuro-Beckmannion eruciformis, associations Middendorfio borysthenicae-Crypsietum alopecuroides and Myosuro-Beckmannietum eruciformis. As a result of studies of the Udai valley within the territory of the Pyryatynsky National Natural Park KOVALENKO (2014) identified 7 associations, including 2 which were described for the first time -Psammophiliello-Juncetum nastanthi and Polygono recti-Juncetum juzepczukii. The position of these syntaxa in the Braun-Blanquet system is still insufficiently clear and further research is nessesary. In the Western Bug valley, the distribution of 2 associations were recorded -Juncetum bufonii and Cyperetum micheliani (KUZYARIN ET AL., 2015). 3 assocations have been recorded in the Sluch valley -Cyperetum micheliani, Pulicario vulgaris-Menthetum pulegii and Veronico anagalloidis-Lythretum hyssopifoliae (KOROTKA & PASHKEVYCH, 2017).
An overview of the syntaxonomy of Isoëto-Nanojuncetea has been provided in some generalist publications: for the Northern Black Sea region (DUBYNA ET AL., 2004), Ukrainian Polissia (ONYSHCHENKO, 2006), Polissia subprovince of the mixed forest zone (KONISHCHUK, 2013), water bodies and swamps of the forest-steppe zone of Ukraine (CHORNA, 2013). The summary work was "Prodrome of the Vegetation of Ukraine" (DUBYNA ET AL., 2019).
Creation and development of phytosociological databases will not allow only (i) the clarification of the syntaxonomic structure of pioneer vegetation, but also (ii) the determination of the floristic composition and productivity of phytocoenoses; (iii) the possibility to study the population structure and patterns of the adaptive response of various ecobiomorphs in different ecological and coenotic conditions; (iv) the clarifiction of the dynamics of pioneer vegetation under conditions of increasing adaptive specialization; and (v) an assessment of the changes in communities in the spatio-temporal scale (DUBYNA . Ordination analysis of phytocoenoses that supplements syntaxonomic studies (RAHMAN ET AL., 2017; ÇOBAN & WILLNER, 2019) allows us (a) to identify the leading factors of the plant communities' distribution within ecological space according to the main environmental gradients (KUZEMKO ET AL., 2016; KOROLYUK ET AL., 2018; LASHCHINSKIY ET AL., 2019); (b) to determine the state of ecosystems in terms of their biotic components (DIDUKH, 2012), the features of the spatial distribution of vegetation (TONG ET AL., 2019; ZHIYANG ET AL., 2020), the patterns of ecological and biological organization of the vegetation cover (WILDI, 2018), the relationships of vegetation parameters within local and regional ecological processes (CHATURVEDI & RAGHUBANSHI, 2018); and (c) to identify phytoindicators of climate change (BACHMAIR ET AL., 2016).
Based on the above mentioned features, the aims of this article are 1) to generalize the accumulated phytocoenotic materials on the syntaxonomy of vegetation of the Isoëto-Nanojuncetea and Bidentetea classes in Ukraine; 2) to develop their syntaxonomic structure; 3) to determine the ecological series of syntaxa and 4) to establish the main factors of ecological differentiation of phytocoenoses based on ordination analyses.

Data collection
The phytosociological materials for this research work included 414 relevés from the territory of Ukraine made directly by the authors and documented in the literature. Our own relevés were made between 1989 and 2020 on plots of standard size 4 x 4 m, according to the method of floristic classification by J. Braun-Blanquet (BRAUN-BLANQUET, 1964; WESTHOFF & VAN DER MAAREL, 1973). In some cases, on narrow elongated banks of water bodies, the plots were 1 x 4 m or 2 x 5 m in size. At the same time, the requirements for a homogeneous structure of the vegetation cover were observed. The data set also included relevés given in the publications of VICHEREK (1968),

Data analysis
All 414 relevés were entered into a database created in the TURBOVEG 2.79 format (HENNEKENS & SCHAMINÉE, 2001). They were combined with a database of aquatic, wetland and meadow vegetation, numbering 4932 vegetation plots in general. Relevés of related types of vegetation were included in the analysis to obtain a clearer separation of groups and to avoid subjective assessments. Then the relevés were exported to the JUICE program (TICHÝ, 2002).
The phytosociological data were interpreted using the JUICE 7.0.83 software package (TICHÝ, 2002). The processing of relevés was carried out using the method of two-factor indicator species analysis (TWINSPAN), in particular its modified version (HILL, 1979; ROLEČEK ET AL., 2009), as well as use of the PC-ORD software package (MCCUNE & MEFFORD, 2016). At first, using the modified TWINSPAN, the general database (5346 relevés) was processed in order to divide it into smaller groups based on their floristic differences. The "pseudo-species" cut levels were 0, 5, 15, 30%. The measure of cluster heterogeneity was the of Whittaker's beta (WHITTAKER, 1978) taken as the correlation measure between the total number of species in all relevés of a cluster to the average number per relevé. Then all groups with a list of diagnostic species corresponding to the classes Isoëto-Nanojuncetea and Bidentetea, according to MUCINA ET AL. (2016), were analized separately using the PC-ORD software. Sørensen's coefficient was chosen as a measure of similarity of clusters (SÖRENSEN, 1948), and the grouping was carried out according to the "flexible" beta method at -0.25. This made it possible to obtain phytocoenons that approximately corresponded to the rank of the association.
Identification of diagnostic species of association was carried out in accordance with the fidelity index  The class Isoëto-Nanojuncetea includes plant communities of low-growing annual herbs with an ephemeral life cycle (nanoephemeretum) in shallows of alluvial habitats of continental water bodies that are periodically flooded. They develop in conditions of rapid periodic seasonal changes in surface-soil humidity. The discussed phytocoenoses occupy mainly drying out streams, temporary water bodies, riverbanks, littoral zones of lakes, ponds and reservoirs. In Ukraine, the Isoëto-Nanojuncetea is represented by 8 associations that belong to 2 alliances and 1 order. Class is characterized by a lower level of coenotic diversity on the European scale. The main factors of territorial differentiation of these plant communities, are those that also determine their coenotic diversity, these are the type of relief of newly-formed ecotopes, the composition and thickness of alluvial sediments, as well as the hydrological regime of water bodies (water depth and flooding period). The formation of the coenoses is limited by excessive flooding and drainage, as well as human impact. The communities of the Isoëto-Nanojuncetea are more typical for the Polissia region and the forest-steppe zone of Ukraine, where suitable habitats (with a flat relief, a low degree of dissection and a high soil humidity) are common. In the steppe zone of Ukraine these aforementioned phytocoenoses are rare. Mainly here they occur on depressed landforms with slightly saline soils and flooding .

Classification scheme
The alliance Eleocharition ovatae includes ephemeral coenoses of therophytes and hemicryptophytes on eutrophic, or slightly saline substrates, in the littoral zones of rivers, lakes, ponds, and reservoirs. The alliance Verbenion supinae unites communities of low-growing annual or perennial herbs which have a short period of ontogeny and are of the stress-tolerator strategy type. They occupy sandy and sandy loamy nitrified soils, mainly those that are anthropogenically transformed. Blocks of diagnostic species are quite clear at the levels of associations (Fig. 6, Table 1).
In Europe, the Isoëto-Nanojuncetea is represented by 2 orders: Isoëtetalia Br.- Bl. 1935 and Nanocyperetalia Klika 1935, and 11 alliances. The diversity of climatic and edaphic conditions at the regional scale contributes to the formation of biogeographic differences in the communities of the floodplain nanoephemeretum.   According to the results of the ordination analysis of Isoëto-Nanojuncetea syntaxa (Fig. 7), it was established that the main factors for their ecological differentiation are: degree of soil aeration, variability of damping, as well as crio-and thermal regime of habitats, and concentration of nitrogen and carbonate compounds in the soil. The thermoregime, salt regime and light regime have especially affected the distribution of the associations Myddendorfio borysthenicae-Crypsietum alopecuroides and Pulicario vulgaris-Menthetum pulegii, which are described in the steppe zone of Ukraine and are the most thermophilic of the coenoses of the whole class. The ombroregime determines the ecological distribution of the communities of Cyperetum micheliani and Juncetum bufonii according to the amount of precipitation, which significantly affects the soil humidity. The variability of damping and the carbonate content of edaphotope, the vectors along which are the closest to the second ordination axis, became the additional gradients of syntaxa differentiation within the Isoëto-Nanojuncetea. The range of values for the main ecological factors and other abiotic gradients is presented in Table 2.  The class Bidentetea includes plant communities of tall annual wetland herbs in wet and nutrientrich habitats both natural and man-made. The stands occupy the muddy alluvial sediments of river banks, ponds, reservoirs, canals, wet ditches, wet forest clearings, etc. The plant communities of the class are often present in habitats of forest and foreststeppe zones but are rarely in the steppe zone of Ukraine. In general, the distribution of 10 associations of Bidentetea were detected (Fig. 8-11). The main factors for their territorial differentiation are: the mechanical composition of the soil, the relief of newly-formed ecotopes, soil humidity, the duration of surface flooding, as well as the degree of anthropogenic impact.
The alliance Bidention tripartitae combines coenoses of annual wetland species, mainly belonging to the genera Bidens L., Persicaria Mill., Ranunculus L., Rumex L. and Xanthium L. They occur on wet loamy or clayey nutrient-rich soils. The alliance Chenopodion rubri includes communities of annual species of the genera Atriplex L. and Chenopodium L. on saline nitrified substrata. Blocks of diagnostic species at the level of associations are quite clear (Fig. 12, Table 3).
The class Bidentetea is spread throughout the territory of Europe. It is also represented by one order and two alliances: Bidention tripartitae and Chenopodion rubri, but differs in a large number of associations, which is caused by a variety of habitats as well as physico-geographical conditions of the different regions. For all European regions there are typical associations: Rumici maritimi-Ranunculetum scelerati, Bidentetum cernuae, Bidentetum tripartitae, Polygonetum hydropiperis and Chenopodietum rubri. Other ones are confined only to certain territories.
Polygonum persicaria --6.6 --11.6 -2.1 15.6 14.7 Rumex acetosa  According to the results of the ordination and gradient analysis of Bidentetea plant communities, it was found that their differentiation occurs mainly along gradients of soil acidity and light regime of their habitats (Fig. 13, Table 2). In addition, the ecological distribution of syntaxa of the class is significantly affected by the concentration of mineral nitrogen compounds in the soil, variability of damping and the ombroregime. The associations Rumici maritimi-Ranunculetum scelerathi and Polygonetum hydropiperis are especially sensitive to these factors. In the ecological differentiation of the communities Xanthio riparii-Chenopodietum rubri the gradient of the thermoregime is of significant importance.  (CHYTRÝ, 2011). In the "Vegetation of Europe ..." (MUCINA ET AL., 2016), the class Isoëto-Nanojuncetea is assigned to the vegetation of freshwater springs, shorelines, and swamps. At the same time, the class Bidentetea is included in anthropogenic vegetation. It should be noted that the class Bidentetea are often presented in ruderal vegetation, due to its high level of synanthropization (OSYPENKO & SHEVCHYK, 2001;BORHIDI, 2003;ABRAMOVA & GOLOVANOV, 2016;ERMAKOV, 2012; DUBYNA ET AL., 2019). The last reason is not sufficiently substantiated in our opinion. The phytocoenoses of this mentioned class grow in specific conditions of newly-formed ecotopes, favourable for the invasion of alien species. That's why plant communities of the class Bidentetea have relationships with phytocoenoses of nitrophylic weeds of the order Chenopodietalia albi (that belong to the class Stellarietea mediae), given that the excess nitrogen in the littoral zone is favourable for the distribution of nitrophylic species (BISSELS . This also leads to the uncertain and debatable syntaxonomical status of the alliance Chenopodion rubri, which is sometimes included in the classes Chenopodietea or Stellarietea mediae (GALCHENKO, 2006 A debatable point is also the correlation between the communities of the classes Isoëto-Nanojuncetea and Crypsietea aculeatae. Italian, Spanish, and French phytosociologists consider that the class Crypsietea aculeatae is a syntaxonomic synonym of the class Isoëto-Nanojuncetea (BARDAT ET AL., 2004; BIONDI ET AL., 2014; RIVAS-MARTÍNEZ ET AL., 2001). Bulgarian scientists relate the order Crypsietalia aculeatae to the class Isoëto-Nanojuncetea (TZONEV ET AL., 2009). RODWELL  have the same point of view and distinguish 3 alliances: subsaline vegetation in the class Isoëto-Nanojuncetea: Cypero-Spergularion salinae, Polygono salsuginei-Crypsion aculeatae and Puccinellion peisonis. In our opinion both of the classes mentioned are different syntaxa of the highest rank, firstly, according to the salt regime of the soil. However, in the future, the syntaxonomic position of some syntaxa remains to be clarified, in particular, the association Middendorfio borysthenicae-Crypsietum alopecuroidis (SHAPOVAL, 2006) and the alliance Myosuro-Beckmannion eruciformis (SHAPOVAL 2006), described from the area of depressions within saline soils in the steppe zone of Ukraine. The results of phytosociological analysis show that their floristic composition is closer to the alliance Beckmannion eruciformis Soó 1933. In this regard, further research is necessary in this direction as well as to establish the correct syntaxonomy nomenclature of the class Isoëto-Nanojuncetea in Europe (TOMASELLI ET AL., 2020).
Unlike the Isoëto-Nanojuncetea, the syntaxonomy of the Bidentetea class is more determined and stable. The plant communities of the class, which is widespread throughout Europe, are combined in one order Bidentetalia tripartitae and two alliances: Bidention tripartitae and Chenopodion glauci (MUCINA ET AL., 2016).
In addition to taxonomic and syntaxonomic differentiation, analysis of the structure of the annual herb vegetation of shorelines and littoral zones of water bodies of Ukraine made it possible to identify functional information through the distribution of communities in ecological space. In fact, our results are consistent with previous studies (ŠUMBEROVÁ & HRIVNÁK, 2013; ALTENFELDER ET AL., 2014), demonstrating that the main factor in the ecological differentiation of these phytocoenoses is the soil humidity regime. In addition to soil humidity, the variability of communities is largely determined by the salt regime and soil acidity. Although, by definition of the Bidentetea class coenoses these are confined to nutrient-rich soils, only some of the plant communities demonstrate an increased requirement for the content of assimilated nitrogen compounds. In the ecologicalphytocoenotic series of the nanoephemeral plant communities the arrangement of associations occurs mainly in the direction of the variability of damping, soil aeration, and the nitrogen content of the substrate. In contrast to the countries of Central Europe, the significant length of the territory of Ukraine in the meridional direction determines the variability of the Isoëto-Nanojuncetea communities and this also occurs along a gradient of the temperature regime of the habitats, which confirms the influence of climatic parameters on the distribution and floristic composition of the communities of the aforementioned class (DEIL, 2005).
Our research has shown that the use of generalized, large-scale phytosociological databases can be a valuable tool for identifying the current state of these discussed plant communities on a continental scale. Thus, this knowledge can contribute to the improvement of the European vegetation classification of littoral zones of continental water bodies, providing information on diagnostic species, their syntaxonomic structure, and the geographical and ecological features of phytocoenoses formed in unique habitat conditions. From the point of biodiversity conservation, our research could also be used for monitoring in the context of climate change. Increases in temperature and changes in precipitation can significantly change the composition and abundance of species, which can lead to associated changes in the distribution of local plant communities (MOOMAW ET AL., 2018). Wetlands are particularly vulnerable to changes in climate (FAY ET AL., 2016; GRIEGER ET AL., 2020), as well as to other human impacts on the environment. These discussed vegetation complexes are very sensitive to ecological and human disturbances. That is why they can be good bioindicators for monitoring and preserving newly-formed ecotopes (ERNANDES ET AL., 2017). Also, for the plant communities of shorelines and littoral zones, a significant threat is the invasion of alien species, which, over time can be quite large-scale and even lead to structural transformations and formation of new a syntaxa (HRIVNÁK ET AL., 2016; KOROTKA & PASHKEVYCH, 2017). As has already mentioned by other authors (BAGELLA ET AL., 2016; TOMASELLI ET AL., 2020), these unique plant communities are very vulnerable and easily affected by these different impacts. They occur in habitats № 3130 (oligotrophic to mesotrophic standing waters with vegetation of the Littorelletea uniflorae and / or of the Isoëto-Nanojuncetea) and № 3270 (rivers with muddy banks with Chenopodion rubri p.p. and Bidention p.p. vegetation) that are protected by Council Directive 92/43 / EEC. Therefore, it is necessary to pay more attention to their study and preservation.

Conclusions
The pioneer vegetation of the freshwater shorelines and littoral zones of the water bodies of Ukraine is represented by 2 classes: Isoëto-Nanojuncetea and Bidentetea. The syntaxonomical structure of the Isoëto-Nanojuncetea class includes 1 order, 2 alliances and 8 associations. The level of its coenotic diversity is lower than the European average, which is possibly due to a lack of research into these habitats in our country. The syntaxonomy of the class Bidentetea includes 1 order, 2 alliances, and 10 associations. The level of its coenotic richness is average for European countries. It has an average European level for its coenotic diversity. Plant communities of both classes are more typical for the Polissia region and the forest-steppe zone of Ukraine, where there are favourable habitats with a flat relief, a low degree of dissection and a high level of soil humidity. The coenotic diversity and territorial differentiation of the communities of these discussed classes are mainly caused by the hydrological regime of water bodies, in particular the duration of flooding, and the structure and thickness of alluvial sediments. Further studies will make it possible to supplement and clarify the syntaxonomic structure of the Isoëto-Nanojuncetea class, in particular, the position of the alliance Myosuro-Beckmannion eruciformis which SHAPOVAL (2006) described from the area of depressions with saline soils in the steppe zone of Ukraine.
On the basis of the ordination analysis of the syntaxa, it was established that in the ecological differentiation of the communities of the classes Isoëto-Nanojuncetea and Bidentetea the main factors are the water regime and the concentration of mineral nitrogen compounds in the soil. In the first class, the variability of coenoses is also determined by the degree of soil aeration, as well as by the temperature regime of habitats. Differentiation of the plant communities in the Bidentetea class in ecological space and the formation of their ecological-coenotic series also occur along the gradients of the salt regime and acidity of the substrates.
Our research has shown that ordering, supplementing and combining materials in the form of phytosociological databases can be used for a large-scale analysis of the current state of different plant complexes. Thus, it can contribute to the improvement of the classification of European vegetation. The results of this work will be also useful for preserving the biodiversity of these pioneer phytocoenoses.