Some Data on the Adventive Component of Flora in Communities of Aquatic and Coastal-Aquatic Vegetation in Saline Ecotopes of Southeastern Europe

In the article, the author analyzes the features of the invasions of alien plant species in the European Southeast into communities of two classes of aquatic and coastal aquatic vegetation-Crypsidetea aculeatae Vicherek 1973 and Phragmito-Magno-Caricetea Klika in Klika et Novák 1941. In the saline habitats of this mega-region in the above communities the alien species turned out to be represented in very limited quantities [2 species-Atriplex tatarica L. (Chenopodiaceae Vent.) and Echinochloa crus-galli (L.) P. Beauv. (Poaceae Barnhart)], in terms of syntaxonomy (only in 4 lower units of the “association-community” rank), and geographically (only in the Lower Volga region, the lower reaches of the Volga River-Astrakhan oblast). The total share of alien species in the cenoflora of the lower syntaxa of the classes under consideration is insignificant and varies from 2 to 5.5%. Echinochloa crus-galli is most successfully adapted to local conditions. It invades communities of both of these classes, which are found on soils of the entire range of salinity (from weak to strong). But at the same time, the level of soil salinity, obviously, is a certain limiting factor for it. Since this species is more massively present in communities growing on weakly and moderately saline soils than in highly saline soils.


Introduction
The scientific community for more than a decade has claimed the problem of biological invasions to be one of the most important on our planet [1-4, etc.]. Water bodies (including large rivers), along with the fact that they can be natural barriers, contribute to the active penetration of alien plant species into the territory. Vegetation in contact with water bodies or located on the banks, in the first place, can be affected by adventive flora.
A number of publications have already been devoted to the invasions of alien plant species into saline soils of the South-East of Europe [5][6][7][8]. The purpose of this study is to assess how relevant it is here for aquatic and coastal aquatic vegetation, namely for the communities of classes Crypsidetea aculeatae Vicherek 1973 and Phragmito-Magno-Caricetea Klika in Klika et Novák 1941 found in saline ecotopes. The first class consists of pioneer communities with a predominance of annuals in periodically inundated habitats with a very variable moisture regime and salinity of the upper soil layer; the second class are the communities of swampy meadows and herbaceous bogs, as well as rooted plants rising above water along the shores and coastal zone of water bodies [9].

Materials and Methods
By the European South-East, the author understands the previously adopted concept of mega-region, its geographical and vegetative component [10, 11]: 1) territory: Russia -Republic of Kalmykia, Astrakhan, Volgograd, Saratov, Samara and Orenburg (only its southwestern part) oblasts, Kazakhstan -European parts of West Kazakhstan and Atyrau regions; 2) vegetation of saline ecotopes of this mega-region (its classification is ecological-floristic).
We found the lower syntaxa of the classes Crypsidetea aculeatae Vicherek 1973 and Phragmito-Magno-Caricetea Klika in Klika et Novák 1941 described in saline ecotopes of the European South-East, where alien species were recorded. Then we assessed the role of invasive species in their formation (based on the indicators of abundance, constancy, and proportion in coenoflora); we studied the geography of the introduction of species in the class range as well as ecological conditions of their habitats. We included in the analysis the lower syntaxa of the classes of the "association-community" rank. But only in ass. Phalaroido-Scirpetum Golub et Mirkin 1986 we considered one subass. Ph.-S. bolboschoenetosum Golub et Mirkin 1986, since only it is found on saline soils.
The article uses abbreviations: ass. -association, WSI -Western steppe ilmen (area to the west of the main central system of the Volga River branches in its delta); C -constancy, classcl., subass.subassociation.

Results and Discussion
In coenoses of the classes Crypsidetea aculeatae and Phragmito-Magno-Caricetea in the European South-East, we recorded only one alien species from 2 families -Atriplex tatarica L. (Chenopodiaceae Vent.) and Echinochloa crus-galli (L.) P. Beauv. (Poaceae Barnhart). We found the first species only in the first class, and the second one -in both. Both of them are annuals, archaeophytes, xenophytes, epecophytes; Atriplex tatarica is of Iranian-Turanian origin, and Echinochloa crus-galli is of South Asian origin [17][18][19].
Alien plant species are found in coenoses of 4 lowest units of the "association-community" rank: Both species are seldom found in communities of each class (C = I). Atriplex tatarica is present there with a low (1 point) abundance, while Echinochloa crus-galli is generally more abundant: with a variable abundance index (1-3), the cereal is more abundant in cl. Phragmito-Magno-Caricetea (table  1).
Floristic composition of almost all of the lower syntaxa of both classes found in the European South-East is rather poor: the total number of species is about 10-20, the average is 5-10 [10, 11]. This, in general, is accurate for those classes where we find alien species (table 2). The total share of alien species in the coenoflora of lower syntaxa is small -no more than 5.5% (table 2), but in specific coenoses where invasive species have been present for some time, it can increase to 10-20% -due to their species depletion in comparison with the lowest unit.  The invasions of both alien species occur in an insignificant part of the range of each class in Southeastern Europe [10, 11, 21-24]: in Crypsidetea aculeatae -only in the extreme west, in Phragmito-Magno-Caricetea -in the extreme southwest of the range. Geographically, it is limited to the Lower Volga region [10, 11, 20, 25] and since it is noted exclusively in Astrakhan oblast, in order to avoid unnecessary repetitions further in the text, characterizingthe geography of the introduction of species, we indicate only its administrative regions (omitting the name of the oblast).
The information for both classes is detailed below. Cl. Crypsidetea aculeatae Vicherek 1973: both alien plant species (table 1) are recorded in its communities as rare (C = I) and low-abundant (abundance = 1 point). They are found in WSI (Ikryaninsky district) and the Volga delta (Volodarsky district). The total share of alien species in the coenoflora of associations of the class is small -about 4-5% (table 2). However, in some coenoses where there are invasive species, it can be 10-20% due to the greater species depletion. Ass. Alismato-Salicornietum Golub 1985 -Atriplex tatarica is registered only in one of its coenosis in WSI (Ikryaninsky district, low-lying meadow on the right bank of the Podstepok River) [20]. The share of alien species in the coenoflora of the association is 3.7% (table 2), but in the floristically highly depleted (5 species) coenosis, where A. tatarica is found, the share increases to 20%. However, A. tatarica, obviously, is unable to significantly transform the coenosis, due to the strong competitive dominant Aeluropus littoralis s. l. (incl. A. littoralis ssp. pungens) and codominant Crypsis schoenoides (species abundance is 5 and 2 points, respectively), which mainly form the herbage.
Ass. Argusio-Phragmitetum Golub et Mirkin 1986 is distributed in highly saline (salt content -1.2-1.8%) alluvial meadow soils in plant complexes of the Volga delta. Echinochloa crus-galli is found in one of its coenosis (Volodarsky district, leveled area of the island) [25]. The share of an alien component in the coenoflora of the association is 5.0% (table 2), but in a specific coenosis with an invasive species it is 10%. However, since the abundance of this cereal is rather small, one cannot expect its strong influence on the coenosis due to the more massive presence of quite numerous dominants and codominants (Scirpus maritimus ssp. Maritimus, Phragmites australis, etc.).
Cl. Phragmito-Magno-Caricetea Klika in Klika et Novák 1941: only Echinochloa crus-galli is registered in its communities. It is also found rarely here (C = I), as in Crypsidetea aculeatae, but more massively (abundance = 2-3 points) (table 1). It is found in WSI (Narimanov district) and the Volga delta (Volodarsky district). The share of alien species in the coenoflora of the lower units of this class, as in the previous one, is small -2.0-5.5% (table 2) (table 2). But in its only coenosis (Volodarsky district, depression), where Echinochloa crusgalli is found not very massively (abundance = 2 points), this indicator increases to 5.5%.
Typha laxmanii-community [20]: its coenoses occupy weakly and moderately saline (0.5-0.9%) soils, occurring on the periphery of slightly saline and fresh water bodies. Echinochloa crus-galli is found in one of them in WSI (Narimanov district, southern coast of fresh ilmen) [10, 11, 20]. The share of the alien component in the whole coenoflora of the Typha laxmanii-community is 5.5% (table  2); but in the coenosis with Echinochloa crus-galli it is 11.1%. In addition to the dominant Typha laxmanii (abundance = 4 points), most of the other species in this coenosis are random. Therefore, we can assume that the invasive cereal significantly influences its formation as a rather massive codominant (abundance = 3 points).

Conclusion
The author notes a rather weak invasion of alien species into the communities of aquatic and coastal aquatic vegetation found in saline ecotopes in Southeastern Europe Echinochloa crus-galli invades coenoses of this vegetation type most successfully. Unlike Atriplex tatarica, it penetrates into the communities of both classes, is present in them more massively and has a wider geography of invasiveness. This cereal crop can invade communities of lower syntaxa, which are found throughout the entire range of soil salinity. It is found in them equally rarely (C = I), but is more abundant (2-3 points) in syntaxa with soils of the category of weakly and moderately saline

Funding
This work was carried out within the framework of the Program of Fundamental Research of the State Academies of Sciences in 2013-2020 (projects nos. AAAA-A17-117112040040-3 and AAAA-A17-117112040039-7).