Syntaxonomy of steppe depression vegetation of Ukraine

Aims: To revise the syntaxonomy of the vegetation of steppe depressions (pody), in particular (1) to identify the associations and to reveal their environmental, structural and compositional peculiarities; (2) to assign the associations to higher syntaxa; and (3) to correct nomenclatural aspects according to the ICPN. Study area: Steppe zone of Ukraine, Left-Bank of the Lower Dnieper basin. Methods: 641 relevés were included in the final analysis in the PCOrd program integrated into Juice software. Two expert systems (EVC and EUNIS-ESy) were used to assign relevés to vegetation classes and to EUNIS units. Results: The analysis resulted in nine clusters, which were interpreted as Festuco-Brometea (two units), Molinio-Arrhenatheretea (three units), Isoёto-Nanojuncetea (three units) and one derivate community of the Festuco-Puccinellietea. Detailed characteristics of the species composition, structure, distribution, and environmental conditions are provided for each unit. According to the DCA ordination, the leading factors of the syntaxa differentiation are soil moisture and fluctuating water level. Conclusions: We could clarify the placement of steppe depression vegetation in the system of syntaxonomic units of Europe. The previously described syntaxa of the rank of alliance (Myosuro-Beckmannion eruciformis), suballiance (Galio ruthenici-Caricenion praecocis), and six associations are validated. Two associations and two subassociations are described as a new to science. Taxonomic references: Euro+Med PlantBase (https://www.emplantbase.org), except Mosyakin and Fedoronchuk (1999) for Phlomis scythica Klokov & Des.-Shost. and Tulipa scythica Klokov & Zoz. Syntaxonomic references: Mucina et al. (2016) for syntaxa from alliance to class level; Dubyna et al. (2019) for associations. Abbreviations: DCA = Detrended Correspondence Analysis; DES = Didukh Ecological Scales; EUNIS = European Nature Information System; EVC = EuroVegChecklist; GIVD = Global Index of Vegetation-Plot Databases; ICPN = International Code of Phytosociological Nomenclature.


Introduction
Steppe depressions (pody in Ukrainian) are large closed depressions, up to 16,000 ha in area, elliptical or round in shape with gentle slopes and flat bottoms, periodically flooded by meltwater and characterized by Planosol soils and peculiar ephemeral mesic to wet grassland phytocenoses. These depressions accumulate natural runoff in poorly drained steppe plains within the periglacial area of the Quaternary glaciation. In Ukraine, the largest depressions are concentrated on the Left Bank of the Lower Dnieper (Kherson and Zaporizhia administrative oblasts), while sporadic, smaller depressions and steppe "saucers" occur on the Right Bank of the Dnieper (Kherson, Mykolaiv, rarely Odessa oblasts). In the Russian Federation, similar depressions are common in the Lower Don River and Lower Volga River regions (Molodykh 1982;Evdokimova and Bykovskaya 1985;Marinich et al. 1985;Shapoval 2007;Zakharov 2018).
Following the flooding of depressions, over the entire area of the shallow basin, there is an "explosive" formation of ephemeral hydrophilic cenoses. They exist for a short period, being rapidly replaced by xeromorphic flora and finally become steppic when the depression dries. The average duration of the period between severe floods is, according to various estimates, from 7 to 12 years (Shapoval and Zvegintsov 2010). During periods of flooding and subsequent drying, distinctive alternating phytocenoses with wide ecological amplitude are observed, which consist of plants that withstand drought well and «explosively» increase in number during floods, i.e. are adapted to significant fluctuations in water levels. During short-term floods, the vegetation of depressions is characterized by high values of aboveground phytomass. For example, after the floods of 2003, the average values on the hayfields of the «Black Valley» depression was 12892 ± 518.0 kg/ha in the dry state. However, these values decline rapidly during periods of drought. Also, their productivity decreases due to overgrazing. In particular, in the post-flood period, the value of aboveground phytomass of the adjacent intensively grazed «Sugakli» depression was only 912 ± 239.2 kg/ha, which is significantly less than similar values of hayfields with better moderate grazing management. In general, the stocks of aboveground phytomass in the studied pody under different landuse regimes vary in a wide range from 588 to 14788 kg/ha in the dry state (Shapoval 2004). During the latter, the dominant species become low, sparse, some hydrophytes disappear from the phytocenosis, enduring a prolonged drought in a latent state (seeds formed under a favorable moisture regime, or underground perennial organs such as caudex, rhizomes, etc.).
Vegetation types of depressions are separated in time and space, as actual phytocenoses are scattered territorially (some are confined to the deepest, wettest areas of a depression bottom, others tend to its dry periphery), and they are delimited in time (open water surface overgrown with wetland vegetation, which is later replaced by mesic and semi-dry grasslands). At the same time, the bounda-ries between these phytocenoses are often blurred, and the spatial transitions among them are very gradual.
The problem of the origin of the depressions still has no unambiguous solution; many issues remain problematic and debatable. During the long history of studying the loess cover of the lowland steppes of the Southern Ukraine, many hypotheses and theories of the origin of steppe pody have been put forward. They were considered as remnants of the ancient hydrographic network (Krokos 1927;Lichkov 1927;Zamoriy 1934;Sambur et al. 1956;Mulika 1961;Bulavin 1972) or relict elements of the periglacial area of the Quaternary glaciation (Dokuchaev 1892;Dostovalov 1952;Velichko 1965;Molodykh 1982). According to the results of the recent studies of the morphology and genesis of the large depression relief of the Eastern Azov Sea region (Zakharov 2018) it is established that the existing pody lie in the thickness of loess sediments and do not affect the underlying sediments of sea and river terraces, therefore, they are of aeolian origin and are large deflationary basins, which was assumed earlier (Tutkovskiy 1910;Levengaupt 1932). However, it seems most probable that these geomorphological structures represent a polygenetic group, and their development is caused by a complex of subsidence-suffusion, fluvial and aeolian transformations.
Unfortunately, in Ukraine most of the steppe depressions are plowed, and the surviving remnants are exploited, mainly as hayfields and pastures without compliance with rational management standards, including nature conservation. The only steppe depression that has a national conservation status is the Great Chapelsky pid, as part of the natural core of the Biosphere Reserve «Askania-Nova» (2,376 hectares). Steppe depressions are the sole localities of local and regional endemics in the region of the Left Bank of the Lower Dnieper (Elytrigia repens subsp. pseudocaesia, Phlomis scythica, Tulipa scythica).
The syntaxonomy of these unique complexes is still poorly known and needs to be thoroughly revised. The first attempt to develop a classification of the steppe depression vegetation was made by a team led by Solomakha (Solomakha et al. 2005) in the study of coenotic affinity of Allium regelianum and Ferula orientalis. It was proposed to include such communities in a new alliance Carici praecocis-Elytrigion pseudocaesiae of the new order Carici praecocis-Elytrigietalia pseudocaesiae, which was assigned to the class Festuco-Limonietea (= Festuco-Puccinellietea). In this case, the dataset used for the analysis was only 34 relevés, selected by the criterion of the presence of two target species. The following year, a study on the syntaxonomy of the steppe depression vegetation based on 367 relevés was published (Shapoval 2006). In this article, the author proposed another syntaxonomic solution: the wettest communities are classified within the class Isoёto-Nanojuncetea, order Nanocyperetalia and two alliances -Eleocharition ovatae and newly described Myosuro-Beckmannion eruciformis. Mesic communities of depressions were included in the class Molinio-Arrhenatheretea, order Molinietalia and a new alliance Lythro virgati-Elytrigion pseudocaesiae. Xero-mesic communities, common in small, shallow depressions, were included in the class Festuco-Brometea, order Festucetalia valesiacae, alliances Amygdalion nanae and Festucion valesiacae. However, given the distinctiveness of the depression vegetation, it was proposed to distinguish two suballiances -Cerastio ucrainici-Festucenion valesiacae and Galio ruthenici-Caricenion praecocis within the alliance Festucion valesiacae. All the associations described by Shapoval (2006) were new to science. To date, the latter work remains the most complete overview of the vegetation and syntaxonomic interpretation of the phytocenotic diversity of steppe depressions of the Left Bank of Ukraine. However, the status of many syntaxa remains controversial. Thus, from the above new syntaxa of alliance rank, only the Myosuro-Beckmannion eruciformis is accepted in Mucina et al. (2016). Also, Mucina et al. (2016) mention the order «Myosuro-Beckmannietalia eruciformis Shapoval 2006 (2b, 5)» as synonymous of the Nanocyperetalia. However, the Myosuro-Beckmannion eruciformis with the single association Myosuro-Beckmannietum eruciformis from the beginning was assigned to the classical order Nanocyperetalia, and the order Myosuro-Beckmannietalia eruciformis was not described by Shapoval (2006) and is not mentioned in any other sources, except in Mucina et al. (2016); therefore it should obviously be considered as a phantom name. Finally, the order Carici praecocis-Elytrigietalia pseudocaesiae is considered by Mucina et al. (2016) as a syntaxonomic synonym of the Galietalia veri, and the alliances Carici praecocis-Elytrigion pseudocaesiae and Lythro virgati-Elytrigion pseudocaesiae are considered as synonyms of the Agrostion vinealis. The latter decision seems insufficiently justified because the alliance Agrostion vinealis is described from the forest zone of Ukraine with completely different climatic conditions (Sypailova et al. 1985), and practically none of its diagnostic species, except the widespread Poa angustifolia and Carex praecox, have been found in the steppe depression communities.
The above review has shown that many questions remain unresolved in the syntaxonomy of the steppe depression vegetation. And the biggest, quite objective problem of syntaxonomic analysis of pody vegetation is the availability of representative data because the object of study is quite ephemeral. The precondition for its occurrence is a flood. Due to the exceptional rarity of this phenomenon, it is possible to observe and describe the pody phytocenoses in very limited periods of time, and the interval between the favorable seasons for the mentioned ephemeral vegetation can be decades. Only after the major flooding in 2010 was sufficiently representative data for the current analysis available for collection.
Given this, our aim was to revise the syntaxonomy of the steppe depressions (pody) vegetation, in particular (1) to identify the associations and to reveal their environmental, structural and compositional peculiarities; (2) to assign the associations to higher syntaxa; and (3) to correct nomenclatural aspects according to the ICPN.

Study area
In accordance with the modern administrative-territorial structure of Ukraine, the studied pody are located within Kakhovka and Henichesk districts of Kherson oblast and Melitopol district of Zaporizhia oblast. Great Chapelskyi pid, as well as Staryi pid and a number of small depressions within "Southern" site are components of the natural core of the Askania-Nova Biosphere Reserve ( Figure 1, Table 1). The altitudinal range of the studied pody is from 10 m (Novotroitsky and Syvasky) to 45 m (Garbuzy).
In accordance with the Worldwide Bioclimatic Classification System the study area is located on the border of Temperate xeric steppic and Mediterranean pluviseasonal continental steppic variants, Supra-submediterranean and Supramediterranean variants within the Dobrujo-Crimean subregion of the Eurosiberian biogeographic region (Rivas-Martínez et al. 2004). The climate is characterized as aride, steppe, cold (Beck et al. 2018).
According to the agro-meteorological station Askania-Nova, the average annual temperature is 11.3°С. The average annual precipitation is 400 mm. Most precipitation (37% of the annual amount) falls in the summer in a form of showers and short-term rains. During the period of moisture accumulation (November-March) the amount of precipitation does not exceed 100 mm. Evaporation is 900-1000 mm, and in the summer months it exceeds precipitation by 5-7 times ( Figure 2).   Depressions in lowland steppes are represented by two structural and genetic forms -steppe saucers and pody.
Steppe saucers are small, with depth up to 0.5 m and diameter 2-150 (up to 600) m. Their density is 30-120 saucers per 1 km 2 , depending on erosional dissection and inclination of the terrain. Almost all of them are plowed today. Depressions with a depth of 3-5 (sometimes 10-15) m and a total area of more than 1 ha (up to 16,000 ha), with erosive slopes, catchment basins and flat bottoms represent the second group of depressions -pody. In the interfluve of the Dnieper and Molochna rivers, small depressions with a diameter of up to 1000 m and a depth of about 0.5-3 m are common. Most of depressions are plowed due to their easy accessibility; pristine vegetation is preserved only in the small depressions within the territory of the Biosphere Reserve «Askania-Nova». Other interfluve pody have significant size (see Table 1). The depths of these depressions (relative elevations of watersheds above the bottoms) vary from 1.5-2 m (Small Chapelsky) to 10-15 m (Agaymansky, Great Chapelsky, Sivashsky, Domuzlynsky). The slopes and periphery of the bottoms of these large depressions are plowed, with the exception of the Great Chapelsky. Some depressions (Sugakli, Mustapa, Oleksandrivsky, Rubanovsky, Timoshivsky, etc.) are completely plowed.
In general, pody is a key typological unit of macro-and mesorelief forms of the Steppe zone, and expresses the geomorphological, hydrographic, edaphic, and biotic identity of the whole catchment. The actual concept of steppe depressions (pody) means a complex formation, which includes the following elements: a bottom (perfectly flat surface delineated by the lowest closed horizontal), the slopes, which form a closed depression bowl (its sides) and, finally, the estuaries of a ravine catchment, cut into the general slopes (Shapoval and Zvegintsov 2010) (Figure 3). Only a few depressions have a circle shape, the rest are more or less ellipsoidal, elongated from north to south. The average inclination of slopes is about 2°. The slopes of southern and eastern exposures are steeper (up to 4-6°) and have more pronounced excess of a depression edge over its bottom. This kind of asymmetry of pody is due to the general tendency of lowering the relief in the direction to the Black Sea. In large depressions, slopes are complicated by catchment hollows, and temporary watercourses have produced erosive leaks where these depressions occur in floodplains. The width of such catchment hollows is 500-1000 m, and the length is 7-9 km. Deeper ravines can reach more than 60 km in length (Chekmenchi ravine, which flows into the Ahaimansky pid). In places of transition from a hollow to a bottom, the soil deposits brought by water are formed. These are peculiar deltas that are clearly identified by the steppe nature of vegetation. The slopes of some depressions (Ahaimansky and Sivashsky pody) are terraced. Sometimes there are several bottoms within the large depression, due to generalization of a series of smaller depressions.
The most common and typical soils of the studied region are Luvic Planosol or gleyosolod in the traditional Ukraininan soil classification (Polupan et al. 2005). Their formation is determined by periodic stagnation of melt and rainwater, processes of gleying and sweetening (hydrolysis). This soil type is well diagnosed by numerous iron-manganese nodules. In general, soil varieties in the pody are localized by strips with concentrically closed contours. The width of the strips is determined by an exposure of the slope, a depth of depression, an intensity and nature of moistening, and so on (Anon 1984).
There are two seasonal types of depression flooding: winter-spring, caused by melting snow during thaw, and extremely rare summer-autumn -caused by heavy rains (Drohobych and Polishchuk 2003). A key role in winter-spring floods is played by the snow factor, which accumulates and retains water reserves until the melting period. In addition, heavy rainfall in the previous moisture accumulation period, deep freezing of the soil and the formation of a "frost lock" that prevents infiltration of water; crust and rapid warming are also the key to severe flooding. According to the analysis of well-known dates of flooding in 19-21 centuries, the average duration of the period between severe floods is 7-12 years (Shapoval and Zvegintsov 2010). Occasionally flooding is observed for two or three years in a row, much more often with intervals of 15-17 years or more. In the past, the flooding of the depressions of the Black Sea steppe was much larger (Shalyt 1930) and therefore on old maps they were marked as lakes.
Currently, due to the over-regulation of the catchment area, with much plowing and crossing by various communications (water supply canals, highways, etc.), the frequency and duration of floods have decreased significantly, causing xerophytization of these habitats. Modern heavy floods begin in February and last until the beginning of June (the last small puddles in the depths of the bottom may last until the end of July). The area of flooding can reach 3-4 thousand hectares with the water depth up to 20-40 cm in the center of the depression.
Polygenetics, different sizes, differentiation of microrelief and soil cover of depressions together with sporadic hydrogenic fluctuations, historical and current management determine the nature and dynamics of their vegetation. In fact, it is a unique dynamic complex of hydro-, meso-and xeromorphic communities, which, of course, complicates its study.  Figure 1). The relevés were made according to the standard method of the Braun-Blanquet school on plots from 9 and 16 m 2 (relevés of small spots of hydrophilic vegetation in 2010 and some relevés of 2019) to 100 m 2 (the rest of relevés). Different plot sizes are due to the specifics of spatial differentiation of pody vegetation. "Small" plots (9-16 m²) are mostly timed to small microrelief forms (saucer depths, road tracks, trampled cattle tracks, shores of the arches, etc.) with different moisture conditions and small sizes of vegetation contours. All "large" plots have a standard area of 100 m² and characterize relatively homogeneous vege-tation. The vast majority of the relevés did not include cryptogam species, which are very poorly represented in steppe depressions and mostly have no diagnostic value. For historical relevés, georeferences were determined by the original characteristics of their location in the quarter network of the natural core of the Askania-Nova Biosphere Reserve, corrals of the Great Chapelsky pid or other landmarks -position in relief, adjacency with settlements or economic objects. The new relevés were georeferenced with GPS-navigators Lowrance iFinder and Garmin eTrex 20X, coordinate system WGS-84. A graphical summary of the catena of depression vegetation was completed in the form of an idealized transect, which was constructed based on the results of generalized analysis of vegetation plots and visualization of the results of ordination and territorial differentiation of syntaxa. Images of typical plants were obtained by scanning herbarium specimens of plants collected directly in steppe depressions.
Since the aim of our work was the syntaxonomic analysis of mesic and wet communities of steppe depressions, we deliberately removed from the analysis all vegetation plots of typical steppes, which according to a preliminary phytoindication assessment received an average score 7 or less on the moisture scale based on the DES (Didukh 2011). We also removed from the analysis vegetation plots with cover of shrub layer more than 15%. All taxa identified to the genus level were removed from the species list. The resulting dataset of 641 vegetation plots containing 261 species was analyzed in the Juice software (Tichý 2002). We tested several variants of cluster analysis (both divisive and agglomerative), but the best results in terms of separation and sharpness of vegetation units were obtained with the agglomerative cluster analysis in PCOrd (McCune and Mefford 2006) with the following parameters: square root transformation of species data, Relative Sørensen index as distance measure, flexible Beta -0.25 as group linkage method. Phytoindication assessment of syntaxa was performed using DES for flora of Ukraine (Didukh 2011) in the Juice program. In one case, we rearranged the plots manually between units 7 and 8, for a clearer separation of the two subassociations, moving all plots with presence of Damasonium alisma to a cluster where this species had a much greater frequency. Diagnostic taxa for vegetation units were determined based on their fidelity values calculated with phi coefficient (Chytrý et al. 2002) with Fisher's exact test at p > 0.001 and standardisation of relevé groups to equal size. The threshold value of the phi coefficient for diagnostic species for syntaxa of all ranks was 0.3. For the assignment of communities to syntaxonomical classes and to EUNIS units we used two expert systems: EVC, which allows with a fairly high degree of reliability to determine the affiliation of vegetation plots to vegetation classes and is based on a recent review of the European vegetation (Mucina et al. 2016) and EUNIS-ESy (Chytrý et al. 2020). Both expert systems were used in the Juice program environment.

Description of vegetation units
As a result of the classification, we obtained nine units (Table 1, Suppl. materials 1, 2). Below we provide characteristics of their distribution, environmental conditions, structure and composition.
Environmental conditions. Communities characterized by clear signs of succession with accumulation of a thick litter. The territory is kept in a completely protected regime ('absolut zapovednost"). Here, the ecosystem is not grazed by wild ungulates which contributes to growth of vegetative-mobile mesophytic species and impoverishment of phytodiversity. Soils are meadow-chestnut gleyed sweetened and gley-sweet Planosol. These small depressions are almost not flooded, although they usually have better moisture conditions compared to the adjacent steppe. Sometimes during snowy winters, there may be short-term puddles on the bottoms in February-March, but heavy floods are not observed and the water completely disappears before the period of active vegetation.
Structure and composition. Total cover varies in a wide range -from 19 to 100%, an average of 75,3%, litter -from 5 to 70%. In general, phytocenoses are quite dynamic and are characterized by various combinations of mesomorphic rhizome species and rotations of their coenotic positions depending on different changes in the environment. Dominant species are Bromopsis inermis, Elytrigia repens, Carex praecox, Poa angustifolia, rarely Bromopsis riparia ( Figure 4). Elytrigia repens subsp. pseudocaesia, Alopecurus pratensis and Carex melanostachya, which are the most mesophytic components, occur sporadically. Turf-forming xeromorphic species (Stipa capillata and Agropyron cristatum subsp. pectinatum) are rare. The herb layer has clear vertical differentiation. The first layer is formed by tall forbs (Ferula euxina, Peucedanum ruthenicum, Asparagus officinalis) and grasses -Bromopsis inermis and Elytrigia repens, sporadically Stipa capillata, Rumex crispus, Sisymbrium altissimum. In the second layer, Carex praecox and Poa angustifolia dominate, Falcaria vulgaris, Galium ruthenicum, Vicia villosa are common. The third layer is formed by Viola kitaibeliana, Lamium amplexicaule var. orientale, Cruciata pedemontana, Veronica arvensis. Some synanthropic plants are present in the floristic composition, even among the characteristic species of the syntaxon, due to sporadic zoogenic soil disturbances -anthills (Lasius) or vole's colonies (Microtus), which are optimal stations for weeds. Sisymbrium altissimum and Salsola tragus spread in bulk after fires; Falcaria vulgaris, Eryngium campestre, Atriplex oblongifolia, Lactuca serriola are also common.  Distribution. Small depressions of the natural core of the Askania-Nova Biosphere Reserve and sporadically on the slopes and dry bottom of the Great Chapelsky pid. Environmental conditions. Communities are mostly localized along the bottom edge and at lower slopes (on the verge of flooding) or in local depressions, surrounded by more xerophytic phytocenoses, so they occur in depressions with preserved slopes and adjacent pristine steppe. During strong floods they give way to more hydrophytic communities; during severe droughts they are in a depressed state, lose hygromesophytic elements, and are replaced by more dry communities. The conditions of this association are perfectly suited to Carex melanostachya, which can resist extreme changes in moisture conditions, growing both in a dry steppe and among ephemeral shallow-water vegetation.
Structure and composition. The total cover varies in a wide range from 40 to 100%, occasionally 10-25%, on average 73%. Communities are more mesophytic than the Ferulo-Caricetum praecocis, which is manifested primarily in the strong phytocenotic position of the dominant Carex melanostachya and Elytrigia repens subsp. pseudocaesia, increase in the occurrence and total proportion of Alopecurus pratensis, presence of Eryngium planum (which tends in the Ascanian steppe to depressions with saline soils and sufficient moisture) as well as Hypericum perforatum, Veronica spicata, V. barrelieri, Gagea transversalis, Euphorbia esula subsp. tommasiniana, Ferula euxina and Rumex crispus, and sometimes a significant admixture of annual plants, confined to short-term wetlands ("saucers", puddles), namely Gypsophila muralis, Cyperus flavescens, Myosurus minimus and Rorippa brachycarpa and Phalacrachena inuloides as characteristic element of the mesophytic forbs of steppe depressions. Another typical mesophytic species of these communities is Sibbaldianthe bifurca subsp. orientalis, which is found in watershed hollows and depressions with semi-dry or mesic grassland vegetation. Thus, the phytocenoses of this unit show a more mesomorphic character, although they are accompanied by many xerophytic steppe elements (Seseli tortuosum, Euphorbia seguierana, Sisymbrium polymorphum, Festuca valesiaca, F. pseudovina, Agropyron cristatum subsp. pectinatum, Phlomis herba-venti subsp. pungens, and very rarely Stipa capillata and S. ucrainica), which generally reveals the mixed, transition nature of these communities.
Environmental conditions. The territory is grazed by wild ungulates, mostly in a state of modest overgrazing.
Structure and composition. Litter is almost absent. Sometimes, where there is considerable aboveground phytomass, strands of coarse dry biomass from common rhizome grasses can be present. Total cover of herb layer is 70-100% (average 80.3%). Phytocenoses are characterized by an absolute dominance of rhizome-turf mesophytic grass Alopecurus pratensis ( Figure 5) Long-term grazing regime of this community leaves an imprint on the structure of herb layer and is marked by a significant participation of Artemisia austriaca (the number of individuals increases markedly in dry periods with increasing grazing pressure), Poa bulbosa, Capsella bursa-pastoris, Cardaria draba, Polygonum aviculare, Senecio vernalis, Lactuca serriola, L. tatarica, Lamium amplexicaule, Erodium cicutarium, Euphorbia esula subsp. tommasiniana, Taraxacum sect. Taraxacum etc. However, trampling and fragmentary exposure of soil contributes to spreading of many annual plants including Trifolium retusum, Arenaria leptoclados, Cerastium pumilum, Crepis ramosissima, Cruciata pedemontana, Draba verna, Medicago minima, Myosotis stricta, Veronica arvensis etc. In general, these phytocenoses are characterized by low floristic richness and insignificant physiognomic variability due to an admixture of meadow forbs, and dominance of Alopecurus pratensis.

Cluster 4 "Herniario glabrae-Poetum angustifoliaе" (Table 2, column 4)
Distribution. Slopes and dry bottoms of Zeleny, "Black Valley", Ahaimansky, Garbuzy, Small Chapelsky pody, nameless depressions from the outskirts of the village Podivka and the village Novotroyitske, on the slopes of the Great Chapelsky pid, as well as known from old relevés (1970s) from the natural core of the Biosphere Reserve «Askania-Nova» («Southern» site). Today, due to reservogenic succession (i.e. succession caused by the protected regime of the territory, with an unbalanced or incomplete structure), accompanied by the accumulation of abundant litter, these phytocenoses have disappeared from the «Southern» site and are replaced mainly by monodominant communities of Poa angustifolia belonging to cluster 1.
Environmental conditions. This vegetation unit includes the most common phytocenoses, distributed in dry small depressions and in concentric strips on non-flooded edges of major depressions, which are used as pastures and periodic hayfields (under favorable vegetation conditions). Communities are confined to meadow-chestnut residual saline sweetened gley heavy loam soils. At the same time, they are characterized by a relatively stable floristic composition, which in general is maintained in scattered depressions with a similar landuse regime.
Structure and composition. Total cover varies from 25 to 95%, averaging 78.4%. Dominants are Poa angustifolia, Elytrigia repens subsp. pseudocaesia, Ventenata dubia, Artemisia santonicum and A. austriaca, in some places Festuca valesiaca, Alopecurus pratensis, Carex praecox and C. melanostachya. Extremely bright and colorful aspects are formed by the large and coenotically strong contribution of forbs (Figure 6)    In general, these phytocenoses are relatively open, so in between beds of grasses, it is easy to see the whitish-dusty dried soil with iron-manganese nodules (beans) common on the surface, sometimes quite large (up to 1.5-2 cm in diameter, 20-30 pcs./m 2 ).

Cluster 5 «Lathyro nissoliae-Phalacrachenetum inuloidis» (Table 2, column 5)
Distribution. Along the edge of Ahaimanskyi pid bottom, including the old fallows, which were plowed in inter-flood periods. Sporadic spots and rather large closed massifs are observed in the lower part of the catchment basins and in the northern part of the Great Chapelsky pid bottom.
Environmental conditions. These communities have a fluctuating nature. The ecological optimum is realized during severe floods and in the short post-hydrogenous period.
Structure and composition. Phytocenoses are formed by polycarpic biomorphs and hemicryptophytes, which are dominants (predominate numerically or by mass) and edificators (determine the structure and functioning of the community, form a specific environment); namely, Beckmannia eruciformis, Gratiola officinalis, Elytrigia repens subsp. pseudocaesia, Lythrum virgatum etc. The proportion of therophytes is 60-80%. These syntaxa are related to the previous cluster 5, but are more hydrophilic and tend to more wet habitats.
The total cover varies in the range of 65-97%, averaging 82.2%. Litter is not developed -up to 4%, sometimes 10-20%, due to soaked strands of the previous year's vegetation that floated with the flowing water. Phytocenoses are distributed sporadically in local concavities of the bottom, sometimes merging into large integral massifs, characterized by distinct layers and sparse synusia. The first layer is dominated by perennial hemicryptophytes and cryptophytes: the characteristic dominant Beckmannia eruciformis (cover up to 80%), Elytrigia repens subsp. pseudocaesia, Lythrum virgatum, Schoenoplectus lacustris, occasionally Alopecurus pratensis (Figure 8)
Environmental conditions. Hydrophilous coenoses formed during heavy flooding. Concentrated in local concavities and furrows, or occurs sporadically in the depression bottoms.

Cluster 8 «Elatinо-Butometum umbellatі damasonietosum alismae» (Table 2, column 8)
Distribution. Phytocenoses of the Great Chapelsky pid with the presence of rare species Damasonium alisma ( Figure 10). Outside this depression, D. alisma grows only near the village of Sofiyivka, Novotroitske district, Kherson oblast, in a gulley that connects the basins of the Barnashivka site and the Ahaimansky pid, on both sides of the former sewage sump, near the Kherson -Henichesk highway (Shapoval 2012). In other depressions, no specimen of D. alisma was found, despite the similar ecological and coenotic parameters and related floristic composition of these habitats.
Environmental conditions. Phytocenoses of the subassociation tend to occur in shallow water, often with open water gaps. In general, the described phytocenoses are extremely rare and exist ephemerally, with an exceptionally favorable flooding regime. In insufficiently wet seasons, such hydrophilic communities are transformed into mesic grasslands, preserving the core of dominant plants that are able to resist of moisture deficiency. But a whole complex of water demanding ephemeral species of depression disappear and are replaced by the more resistant mesophytic species.
Structure and composition. Total cover varies in the range of 65-97%, averaging 87.5%. The first herbal layer is formed by tall dominants and edificators, generally typical for bottom of depressions during periods of flooding: Elytrigia repens subsp. pseudocaesia and Lythrum virgatum with an admixture of Beckmannia eruciformis, Alopecurus pratensis, Butomus umbellatus, Rumex crispus, Poa angustifolia and Juncus atratus. The second layer is composed of dominants Eleocharis palustris, Carex melanostachya and Gratiola officinalis, with a significant proportion of Euphorbia esula subsp. tommasiniana, Phlomis scythica and sporadically Inula britannica, Rorippa austriaca, Phalacrachena inuloides.

Ordination and territorial differentiation of vegetation units
The DCA ordination of the identified units ( Figure 11) showed that they are distributed along the first ordination axis from the driest (cluster 1) to the wettest (cluster 9). Xerophytic and mesoxerophytic units 1-3 are located in the right part of the ordination diagram and units 4-9, which are characteristic for wetter conditions, are located in the left part of the diagram. Clusters 3-5 are concentrated in the central part, which indicates their mesic nature, not only by moisture, but also by other closely correlated edaphic factors, including soil aeration, fluctuating water level, nitrogen content in soil and salt regime of the soil. Units 1 and 9 are located at the extremes of the first ordination axis, while the remaining units are separated into two rows along the second ordination axis. In the lower part of the diagram are units 3, 4 and 6, and in the upper part are units 2, 5 and 7. Probably the leading factors of differentiation along the second axis are climatic -first of all, thermal regime and light. Almost all units are well separated from each other, with the exception of units 7 and 8, which we have interpreted as subassociations of one association. Regardless of the number of vegetation plots in these units, which varies widely, the amplitude of the units is approximately the same.
Peculiarities of ecological differentiation of steppe depression syntaxa can be traced on the transect across the conditional (model) depression, which has well-preserved natural slopes and bottom and is periodically flooded (Figure 12). Xero-mesophytic and mesic communities of syntaxa 1, 2 and 4 are formed at the edges of the depression, its slopes are occupied by communities belonging to units 3 (upper part of a slope) and 5 (lower part of a slope), and communities of units 6, 7 and 8 at the bottom as well as unit 9 (the latter in the presence of a shallow artificial watercourse constantly filled with artesian water). The abrupt change of ecological values on the slopes and especially on the bottom of a depression are clearly visible. In addition to a sharp increase of moisture, there is an increase in the variability of dampness, soil aeration, soil pH and salt regime and a decrease in the carbonates content of the soil. At the same time indicators of climatic factors do not change.

Identification of vegetation units by expert systems
The classification of vegetation plots by the expert system EVC (Suppl. material 3: Fig. A) showed a predominance of plots belonging to the class Festuco-Brometea within units 1-2, although a significant portion of the plots also belonged to the Molinio-Arrhenatheretea class. In addition, the plots assigned to the class Molinio-Arrhenatheretea represented a significant portion in cluster 3, although the predominant portion of the plots assigned in that cluster by the expert system belonged to  the class Sedo-Scleranthetea. In the clusters 4-8 there was a clear predominance of plots assigned to the class Molinio-Arrhenatheretea, although in cluster 7 there was also a significant portion of plots assigned to the classes Isoёto-Nanojuncetea and Phragmito-Magnocaricetea. Cluster 9 clearly shows the predominance of plots assigned by the expert system to Festuco-Puccinellietea class.
The interpretation of vegetation plots by the expert system EUNIS-ESy in units of the EUNIS habitat classification (Suppl. material 3: Fig. B) showed that most plots of unit 1 were classified as anthropogenic habitat, which can probably be explained by the large number of therophytes in xerophytic communities of the steppe depressions, which are also characteristic for xerophytic anthropogenic vegetation. Within the units 2-6 the plots assigned to grassland habitats prevailed. A significant part of those units was identified only to the first level of the hierarchy (R). Clusters 2 and 3 contained a considerable proportion of plots of dry and mesic grasslands, cluster 5 largely contained plots of wet and subhalophytic meadows, and plots in cluster 4 were distributed evenly to grassland habitats and anthropogenic habitats, and somewhat less commonly to wetlands. The latter clearly predominated in clusters 7-9. Cluster 8 also showed a high proportion of plots as-signed to freshwater habitats, in particular to type C35b (periodically exposed shore with stable mesotrophic sediments with pioneer vegetation).

Syntaxonomy
The obtained results of the vegetation classification, in particular the list of diagnostic, constant and dominant species of the syntaxa (Suppl. material 4), supported by the results of their phytoindication analysis, distribution in relief, as well as the interpretation by two expert systems, allowed us to develop an ecologically sound syntaxonomic system of the steppe depression vegetation of Ukraine. We then attempted to fit these units into the existing system of syntaxa in Europe (Mucina et al. 2016). Cluster 1 (Ferulo euxinae-Caricetum praecocis) occupies an intermediate position between the classes Festuco-Brometea and Artemisietea vulgaris (Agropyretalia intermedio-repentis). Communities of this association are characterized by a significant participation of synanthropic species. However, these species do not form clear diagnostic blocks, and secondly, the communities are formed naturally, not due to human activities, which does not allow them to be classified within synanthropic vegetation syntaxa e.g., to assign them to the Agropyretalia intermedio-repentis order. Therefore, at this stage, we assign these communities, as in the original publication (Shapoval 2006), to the class Festuco-Brometea, order Festucetalia valesiacae and alliance Festucion valesiacae. Whereas these comunities are somewhat different from the typical communities of the alliance, we consider them as a separate suballiance Galio ruthenici-Caricenion praecocis. It is quite possible that in the future this suballiance will get the rank of alliance, but so far the lack of their own character species does not allow to consider them in the rank of a separate alliance. Cluster 2 (Diantho guttati-Caricetum melanostachyae) can be included in the same suballiance, although this association is slightly more mesophytic according to the results of phytoindication assessment, but according to the expert systems, it contains the most plots of the Festuco-Brometea class and true steppe habitat type -R1B. In addition, its floristic composition is quite similar to the previous association. Earlier these coenoses were described as association Potentillo orientalis-Caricetum melanostachyae; however, a significant increase in the plots used in our dataset revealed the sporadic nature of Sibbaldianthe bifurca subsp. orientalis (syn. Potentilla orientalis) in this syntaxon. Instead, Dianthus guttatus has a higher diagnostic value for this association (see Table 1). These features of the floristic composition, as well as nomenclature changes in relation to Sibbaldianthe bifurca subsp. orientalis, prompted us to reject the previous invalid name and describe these communities as a new association.
Units 3-5 obviously represent mesic grasslands and their mesophytic character was shown by the results of phytoindication. According to the results of the analysis using the EVC expert system, a significant number of plots are assigned to the class Molinio-Arrhenatheretea, which is also confirmed by the results of the analysis using the expert system EUNIS-ESy, which assigned these plots to mesic grassland habitats. Therefore, we classify them within the Molinio-Arrhenatheretea class. Among the higher-ranking syntaxa recognized in EuroVegCheklist, these communities are the most similar to the order Althaeetalia officinalis and its alliance Althaeion officinalis. Although the diagnosis of the order and alliance in the original publication (Golub 1995) is not clearly defined, its definition as "Tall-herb periodically flooded meadows of the steppe and semi-desert zones of Eastern Europe" in Mucina et al. (2016) is fully consistent with the steppe depression vegetation. Thus, we synonymize the previously described alliances of the mesic vegetation of the steppe depressions, Carici praecocis-Elytrigion pseudocaesiae, Poo angustifoliae-Ferulion orientale, and Lythro virgati-Elytrigion pseudocaesiae, as was done in a previous publication (Shapoval 2006), and consider them within the Althaeion officinalis alliance.
The wettest associations of depression bottoms (clusters 6-8) showed some inconsistency in their interpretation by expert systems -on the one hand, the EVC expert system assigned most of their plots to the Molinio-Arrhenatheretea class, and on the other hand the EUNIS-ESy expert system interpreted most of their plots as C (Surface waters) and Qb (Wetlands) groups. But this inconsistency is quite understandable given the ephemeral and complex nature of these habitats and irregularity of flooding. In view of this, we propose that the nature of these communities best fits the class Isoёto-Nanojuncetea, defined as "Pioneer ephemeral dwarf-cyperaceous vegetation in periodically freshwater flooded habitats of Eurasia" in Mucina et al. (2016). We include these units (two associations and one additional subassociation) to an alliance of steppe depression vegetation, which is currently accepted in the EVC -Myosuro-Beckmannion eruciformis -within the order Nanocyperetalia. The floristic composition of these communities is quite unique and differs significantly from other alliances of this order, such as the Verbenion supinae alliance, which includes pioneer ephemeral communities in the nemoral zone in habitats flooded with fresh water without signs of salinity or sweetening. Moreover, the fluctuating nature of ephemeral communities of pody hardly makes it possible to consider them as pioneer.
Cluster 9, according to the list of diagnostic species and the analysis using expert systems, can be assigned to the class Festuco-Puccinellietea. This is the only community that has a pronounced halophytic character, which distinguishes it from all other analyzed units. This difference, both floristic and ecological, might explain the erroneous attribution of the steppe depression vegetation in general to the halophytic type. This unit should probably be attrib-uted to the order Scorzonero-Juncetalia gerardi. However, the transitional nature of the communities as well as the source of the chloride salinity does not currently allow them to be attributed to any of the existing alliances.
The obtained results once again showed that the vegetation of steppe depressions (pody) is indeed rather complex, but not «mosaic», because it was not possible to isolate phytocenoses of annual (ephemeral) plants characteristic for the class Isoёto-Nanojuncetea, and separate them spatially or in time from grassland or wetland communities of perennial plants. Even in the plots of small size in small depressions and bottom depressions with the longest duration of flooding, both ephemeral annual and perennial species were present. Of course, the increase in the plot size slightly changed the proportions of individual and total cover, but in no way affected the homogeneity and integrity of the studied plant communities. It can be assumed that with sufficiently long floods and increasing depth of a water body, some mesophytic or xeromesophytic plants, which are common in dry, non-flooded depressions, would disappear from the communities. Then we would probably get localized occurrences of ephemeral annual vegetation, confined to drying puddles. But irregular and short-term flooding of depressions (every 7-10 yrs, sometimes 20 yrs, lasting only 2-3 months), as well as the shallowness of temporary standing water (about 30-40 cm deep at the peak of the flood and then becoming shallow, 5-10cm) do not adversely affect perennial mesophytic species. It is worth noting that the closed bottoms of the depressions in the natural intact state is a perfectly flat surface, so the edaphic conditions, moisture regime and other abiotic parameters are almost identical throughout a flooded bottom. Thus, when the depressions are flooded and then begin to dry in the same season, peculiar combinations of ephemeral annual aquatic plants and perennial grassland and wetland plants are observed. These plants grow in different layers, but within the same phytocenosis. Such an original complex of hydrophytic vegetation ("ephemeretum") is indivisible either territorially or chronologically.
When interpreting the obtained units, we tried to compare them with the units described in the very first work on the pody vegetation (Solomakha et al. 2005). However, we did not succeed, since the diagnostic species of those associations were in most cases not concentrated in one cluster but distributed among different units in the dataset. We believe that the reason for this is that these units were identified using insufficiently representative data. With the increase in the number of vegetation plots from 34 to 367 (Shapoval 2006), and in the present work to 641, the blocks of diagnostic species have been dissolved. Therefore, we can say that, although they are somewhat similar to our associations, we cannot synonymize them. For example, we can assume that the association Achilleo micranthoides-Poetum angustifoliae is close to Herniario glabrae-Poetum angustifoliae; however, from the three species that are listed as diagnostic for Achilleo micranthoidis-Poetum angustifoliae, Achillea micranthoides has a fairly high fidelity in our clusters 3 and 4, Poa angustifolia in clusters 1 and 2, and Potentilla argentea in clusters 2 and 4, which may indicate their diagnostic significance for syntaxa of a higher rank than the association.
Our testing of two expert systems showed that they can be used as an additional tool for interpreting the results of vegetation classification, especially for assigning associations to syntaxa of a higher hierarchical rank. However, for such complex communities, and, accordingly, complex habitat types, the use of expert systems has limitations, since their nature is such that communities can contain species of different ecological groups, different vegetation classes, and, accordingly, different discriminant or functional species groups, which often overlap. These features prevent the correct interpretation of the relevés by an expert system.

Nomenclatural notes
Taking into account that all previously described units of the steppe depression vegetation are invalid, because the nomenclature type was not indicated using expressis verbis the Latin words 'typus' or 'holotypus' (ICPN Art. 5,par.3), we validly describe the syntaxa of the steppe depression vegetation which we accepted, according to the analysis presented in this paper. When validating the previously described syntaxa, we have kept all their nomenclature types, which are also presented in this article in the Suppl. material 1, but we have slightly modified the lists of diagnostic species of these syntaxa, in accordance with the taxonomic nomenclature used in this paper and the results of calculating their fidelity on the basis of the phi coefficient (Chytrý et al. 2002). Shapoval ex Shapoval et Kuzemko suball. nov

Conservation values
We have noted 21 taxa in the depression communities that have a protected status, including nine species protected at the regional level in the Kherson oblast (Andriyenko and Peregrym 2012, Anon. 2013), six species from the Red Book of Ukraine (Didukh 2009), two species from the European Red List (Bilz et al. 2011), one species from the IUCN list (Anon. 2020) and three species having several protection statuses (Table 3). Elytrigia repens subsp. pseudocaesia, which is protected at the regional level and is found in all syntaxa of the pody vegetation, has the greatest frequency in the dataset. Among the species of national and international protection status, Allium regelianum has the highest frequency and is present in six units. In cluster 4, it has a constancy of 65.6%. This cluster, which we interpret as the association Herniario glabrae-Poetum angustifoliaе, is characterized by the largest number of red listed species, 13 in all (see Table 3). It should also be not-   ed that the species protected at the national level, Damasonium alisma, has a clear coenotic confinement to cluster 8 (Elatinо-Butometum umbellatі damasonietosum alismae), in which its constancy reaches 97.7%. Given the floristic, coenotic and habitat specificity of steppe depressions, as well as the absence of such units in the existing EUNIS hierarchy, and accordingly to Resolution 4 of the Bern Convention (Anon 1997), which makes it impossible to protect this habitat type in the Emerald Network of Ukraine, we have prepared proposals to include them into Resolution 4 (Kuzemko et al. 2017). In 2018, our proposals were adopted by the Steering Committee of the Bern Convention, and the depressions (pody) of the steppe zone of Ukraine were included as a complex type X36 to Resolution 4, accordingly, to the EUNIS classification (https://eunis.eea.europa.eu/habitats/8009), which requires a comprehensive study and protection of this habitat type. In 2019, we prepared proposals for the inclusion of seven new sites to the Emerald Network of Ukraine specifically for the preservation of the X36 habitat type. All these sites were officially recognized at the end of 2019 and included in the existing Emerald network. Taking into account their international conservation status, as well as the high proportion of red listed taxa, which was also confirmed by our research, the next step should be to develop effective management plans for the protection and maintenance of these communities and habitat types. The most important task is the maintenance of the optimal moisture regime, as well as the limits on land issues related to the current land reform in Ukraine; namely that it be impossible to plow them further.

Conclusions
Our analysis allowed us to propose an updated syntaxonomic system of mesic and wet grassland vegetation of the steppe depressions, which reflects their ecological and territorial differentiation, to restore a syntaxonomic status of a number of syntaxa that were considered doubtful, and to find a proper place of the steppe depression vegetation in the syntaxonomic system of the European vegetation (Mucina et al. 2016). Our study confirmed the existence of at least eight associations of the pody vegetation. We tried to correct nomenclatural aspects according to the current addition of the ICPN, and we have validated all syntaxa of the steppe depression vegetation of Ukraine, the existence of which has been proven by a comprehensive analysis using currently accepted methods of phytosociological research. The results of our study will contribute to further inventory of the steppe depression vegetation, organization of proper management and effective protection, which will preserve these unique habitats and provide a system of phytocenotic monitoring of their current state, structure, functional organization and dynamic trends.

Data availability
The data used in the paper are available as Supplementary material in *.xlsx format and in *.csv format.

Author contributions
V.S. formulated the idea of the paper, prepared the dataset for the analysis (85% of the relevés are his own), reviewed the literature, wrote a description of the obtained vegetation units and interpreted them at the level of associations, subassociations and alliances. A.K. planned the research, made all analyzes and interpreted the obtained units at the level of orders and classes. The authors jointly prepared the manuscript.