The Impact of Location of Neophyte Plants in the Volga River Delta on the Moisture Gradient According to the Results of the Geobotanical Database Analysis

An experiment was carried out to assess the possibility of using a large geobotanical database to characterise neophyte plants and the invasibility of habitats of the Volga River delta with different moisture. Combinations of habitats of various moisture are a characteristic feature of the Volga River delta. Water and desert habitats are often separated by only 15-20 meters of height (See Fig. 2). The degree of habitat moisture in the Volga River delta is one of the leading environmental factors that affects the composition of plant communities. The database of releves of the Lower Volga valley was created using the computer software package TURBOVEG. For research, only releves made in the Volga River delta were extracted from this database. Sample plots located in crop fields, ruderal communities, and fallows were excluded. A total of 7985 sample plots with natural vegetation were analysed. Only vascular plants were considered, the names of which are indicated by "Flora Europaea". The abundance of plants in the database is indicated by the size of their cover. Plant species with a cover of more than 50% were conventionally referred to as dominants. The moisture of habitats was characterised by Ramensky's indicator values with 120 gradations of moisture divided into 6 types of habitats: 1-30 - desert and semi-desert moisture, 31-46 - dry steppe, 47-63 - meadow-steppe and dry meadow, 64-88 - wet meadow and raw meadow, 89-103 - marsh-meadow and marsh, 104-120 - habitats of riverine and aquatic vegetation. The ecological amplitude of plants on the moisture gradient was established by the totality of releves in which they occurred. Of the adventitious species, we consider only a group of neophytes, i.e., species that appeared in the Volga River delta since the 17th century. Information about their homeland, life form, and method of drifting into the Volga River delta was included in the database from the monograph by Laktionov. We distinguish two groups of neophytes according to the mode of introduction: xenophytes and ergasiophytes. Xenophytes are species that are unintentionally or accidentally introduced to a new territory from the original area_as a result of economic activity. Ergasiophytes are species deliberately cultivated in a given region, "left" from culture and settling on their own. In total, 22 neophytes were identified in the Volga River delta among the releves stored in our database. The families Asteraceae and Amaranthaceae are the richest in neophytes, each of them containing 4 species. The greatest abundance and number of neophytes are in the range of moisture levels 31-46 on Ramensky's indicator values (dry steppe habitats) (See Tab. 1). The representation of neophytes in this moisture interval is three times higher than in other habitats. There is a reason to assume that this is not an accidental phenomenon. The vegetation of steppe humidification is quite sparse, but the soil here is not as dry as in desert and semi-desert habitats. Therefore, it is possible that steppe moistening habitats are the most favourable for introducing alien plants into them. However, in the Volga River delta, the ecological amplitudes of species in relation to their moisture are most often crossed in the interval of 64-68 of Ramensky's indicator values - wet-meadow and raw-meadow moisture habitats (See Fig. 3). It could be assumed that the number of neophyte plants should have been the largest in this interval of moisture. However, it was shown above that the number and abundance of neophytes are greater in drier habitats, namely in the range 31-46 of Ramensky's indicator values (See Tab. 1). This suggests that moisture is not the only filter that will determine the invasibility of certain habitats. Attention is drawn to neophytes growing in different moisture conditions, which have become dominant plant communities in the Volga River delta, and have an impact on the functioning of its ecosystems. Four of them are ergasiophytes (Amorpha fruti-cosa, Fraxinus pennsylvanica, Elaeagnus angustifolia, Zizania latifolia), one is xeno-phyte (Bidens frondosa). They named a number of syntaxa: Salvinio natantis-Zizanie-tum latifoliae Krutskikh et al. 2013, Rubo caesii-Amorphetum fruticosae Golub et E.G. Kuzmina 2004, Bidenti frondosae-Salicetum triandrae Golub et E.G. Kuzmina 2004, Bidenti frondosae-Salicion triandrae Golub et E.G. Kuzmina 2004, Elaeagne-tum angustifoliae Golub et E.G. Kuzmina, 2004, Artemisio austriacae-Elaeagnetum angustifoliae Golub et E.G. Kuzmina 2004, and Plantagini majoris-Elaeagnetum angustifoliae Golub et E.G. Kuzmina 2004. By origin, North American species predominate among the neophytes (See Fig. 4). Dry habitats are saturated with neophytes of more diverse origin. Among the neophytes, perennial grasses are completely absent and invasive plant are represented only by annual plants, trees and shrubs up to 64 on Ramensky's indicator values with the proportion of woody plants increasing to 104, respectively. Adventive trees and shrubs are disappearing into wetland and aquatic habitats. Only native species of trees and shrubs can be found in such habitats, namely Salix alba and S. triandra. The proportion of annuals is maximum in the driest habitats, gradually decreasing with the increasing moisture content of ecotopes (See Fig. 5). The occurrence of xenophytes is highest in the driest habitats, decreasing in wetter ones. Their role increases in the habitats of riverine and aquatic vegetation. The change in representation of ergasiophytes on the axis of moisture has the opposite trend (See Fig. 6). Our work confirmed the prospects of using geobotanical databases for analysing the adventisation of vegetation cover. The article contains 6 Figures, 2 Tables and 47 References. The Authors declare no conflict of interest.

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