Ecological-climatic characteristics of the flora of a floodplain landscape in Southeastern Europe

Floodplain ecosystems take on the role of active areas of biodiversity and provide many “ecosystem services”, as evidenced by a number of European scientific references. A biodiversity analysis of river floodplains in six European countries within the temperate zone has shown that the floodplains are habitats with a high-level of structural and functional dynamics. The level of their conservation reflects the floristic diversity of forest territories, which is especially important for subarid areas. Recently, a comparison of bioecological characteristics of flora in floodplain forest areas and treeless territories was conducted on the floodplain landscapes of a subarid region of Europe. The valley-terraced landscape of the Samara River, a tributary of the Dnieper can serve as a reference site of native plant complexes of subarid territory in Eastern Europe. Despite long-term anthropogenic transformation, the landscape has retained a significant phytodiversity level. The flora of the Samara River area includes 887 plant species. Of these, 177 species belonging to the rare and endangered categories. The floodplain landscape is the richest in species and most diverse part of this complex. The flora of the Samara floodplain includes 728 species (including 132 rare ones), of which 631 grow in forest communities, and 487 – in anthropogenically transformed, treeless floodplain areas. As part of the forest flora, the number of tree and shrubby species, scyophytes, hygrophytes, and megatrophs significantly increases compared to treeless sites, and the number of ruderal plant species decreases. The floristic composition of the floodplain forests of the subarid region is much richer and more diverse than the flora of the treeless floodplain areas, and this should encourage measures for their protection and restoration. Afforestation of floodplain territories within the steppe zone of Ukraine should be a priority in comparison with other landscapes. For the protection of the flora studied, a scientific justification for creating the National Park "Samara Bor" was prepared. Under the conditions of anthropogenic and climatic impact, this article is of great global importance for attracting the attention of specialists, authorities and society to the protection and restoration of biodiversity in the most valuable landscapes.


Introduction
Vascular plant flora is one of the most numerous and stable components which represents the state of ecosystems. The territories of European river floodplains are characterized by a high-level of biodiversity. They are represented by habitats with a high level of structural and functional dynamics (Schindler et al., 2016). Some of them can serve as reference sites with the best-preserved floral richness. One of these areas on the territory of the steppe zone of Eastern Europe is the floodplain landscape of the Samara River, a tributary of the Dnieper (Koshelev et al., 2020).
Today, the importance of forest ecosystems (especially in subarid regions) is of inestimable importance. They reduce the effects of global warming and create conditions for biodiversity conservation and recreation (Bussotti et al., 2015;Biurrun et al., 2016;Hossain et al., 2016;Colangelo et al., 2018). The development, sustainability and biodiversity of forest ecosystems, including those within floodplain habitats, depend mainly on climate and hydrological conditions (Mikac et al., 2 018;Stojanović et al., 2016). The only areas of forest growth in the steppe zone are the gullies (ravine forests) and floodplains (floodplain forests). The lack of atmospheric moisture is compensated by floodwaters. Tree species respond positively to floodwaters, even when floodplains are at their maximum (Heklau et al., 2019). The floodplain landscape is characterized by a richness of biotopes (forest, meadow, wetland), which create a variety of climatopical conditions not typical for steppe landscapes, especially in recent decades of global warming (Lavergne et al., 2010;Ortmann-Ajkai et al., 2018).
One of the significant woodlands of the steppe zone within Eastern Europe is the Samara Forest, which serves as the "southern outpost" of forest distribution in the steppe zone of Ukraine (Bel'gard, 1950). It is loca-ted downstream of the Samara River, a tributary of the Dnieper. On the territory of the steppe zone of Ukraine, this woodland is the only one of the remaining large floodplain forests of the middle river valleys. On the territory of the floodplain landscape of the Samara River, forest vegetation is represented by oak forests. The forests alternate with meadows, swamps, and water bodies (Brygadyrenko, 2015(Brygadyrenko, , 2016. The diverse landscapes and rich flora of the Samara River area have attracted the attention of several well-known botanists. One of the species from the territory of of the Samara River area was described by Carl Linnaeus: Veronica incana L. (Elenevskij, 1978), the type stored in London. Detailed flora and vegetation surveys in this territory (including the floodplain landscape) were initiated at the end of the XIX century by Akinfiev (1908), the famous Ukrainian botanist. In the following years, the study of flora and vegetation was carried out by followers of the geobotanical scientific school of A. L. Belgard (Alekseyev et al., 1986, Tarasov et al., 1988Tarasov, 2012;Baranovsky, 2016).
The flora of the Samara River area has historically been composed of different geographical elements and is distinguished by a large set of species and their ecomorphs (Belgard, 1950). The elements include a large number of rare plant species and a minimal number of adventive ones (Chervona knyha, 2010;Tarasov, 2012;Baranovsky, 2016). They are widely represented in all landscape elements of the Samara River area and especially in the floodplain. A variety of ecological conditions was developed in the forest area of the floodplain, which provides greater flora richness and diversity compared to transformed territories (Schindler et al., 2016).
In this regard, the task of our work was to compare the climatopes and ecofloristic diversity of the floodplain forest areas of the Samara River and anthropogenically transformed territories where the forest was destroyed. This is well illustrated by the A. L. Belgard Ecomorph System, which was the first vascular plant ecomorph system (Baranovsky et al., 2018). Finding out the specific values of phytodiversity in floodplain forest areas will help initiate measures for their protection and restoration.

Materials and methods
The survey subject was the microclimatic features and floodplain vascular flora of the Samara River in its downstream area (between Kocherechki village and Volnoye village  (Fig. 1). One of the climate features in the Samara River area, which is located within the steppe Atlantic-continental climate type, is the high temperatures of the warm period with reduced precipitation and significant air dryness. The average annual air temperature is 8.1 °C, and the average annual precipitation is 545 mm (Gorb & Duk, 2006). Average annual amplitude of air temperature is 26.8 °C. The average temperature and humidity (Table 1) of this region (Hubynykha weather forecast) show the most intense temperature rise in the period from March to April (8.8 °C) and the most intense drop in temperature from September to October (6.9 °C).
The scientific basis for the survey of climatopes in native forest biogeocenoses was the doctrine of the forests' influence on environmental change (the doctrine of forest pertinence) of Vysotsky (1950). Climate research was conducted according to widely accepted methods. A set of standard methods of hydrometeorological observations was carried out at the test sites, and the gradient climate stand proposed by Gritsan (2000) was used, which allows meteorological elements to be measured at any level from the soil surface and tree crowns (Grytsan et al., 2005) The hydrological features of the of the middle river floodplains in the steppe zone are characterized by short-term flooding (Ramenskiy, 1971). The territory of the Samara River floodplain is flooded during high water for a period of about one week (Belgard, 1950). This determines mainly sufficient moisture for oak forests under short-term flooding conditions.  The vascular plant flora survey was conducted from 1998 to 2019. To compile a complete flora list, materials from the herbarium collections of Dnipropetrovsk University and the National Herbarium of the Institute of Botany, National Academy of Sciences of Ukraine were used. 1300 herbarium samples were collected. The flora of different floodplain biotopes and different types of water bodies was surveyed (Roshchyna & Baranovski, 2019). The definition of vascular plant species was carried out according to the following identification manuals and "Flora" (Flora of the USSR, 1935USSR, -1963Prokudin & Dobrochaeva, 1987). Plant species names were given according to the nomenclature publication adopted in Ukraine (Mosyakin & Fedorochuk, 1999).
Bioecomorphic flora analysis was carried out according to the ecomorph system of Belgard (1950). For ecomorphic characterization of species, we used existing publications (Belgard, 1950;Baranovski, 2000;Tarasov, 2012), consultations with leading experts of the Institute of Botany of National Academy of Sciences of Ukraine, as well as our presonal data (Baranovsky et al., , 2018.

Results
The greatest differences in meteorological elements were observed between the floodplain and steppe landscapes in summer during the period of the greatest development of crown leaves. The weakened turbulent air exchange reduces the average daily temperature by 0.5-2.0 °C and increases the average daily relative humidity by 10-16%. The maximum temperature of the soil surface is reduced by 7-15 °C in comparison with the values of steppe landscape. With variable cloud cover, amplitude of temperature fluctuations of the soil surface in forest areas on average is up to 7.0 °C, and in conditions of the steppe landscape the amplitude is more than 20 °C.
Forested areas have inherent climate features in a number of different biotopes: riverbeds, lakes, swamps, forest margins, and glades. Surveys of climatopes have revealed that light intensity and energy supply in the field of PAR (photosynthetically active radiation) increases in the following sequence of biotopes: oak forest → forest margin → woodland water body. Light intensity under the canopy of oak forest during the light period of the day is reduced by an average of 95.2% in comparison with nonwooded areas. Significant reflectivity of the surface of a water body provides additional solar radiation in the FAW range of the lower part of the crowns and undergrowth. Amplitude of daily temperature fluctuations in the forest glade biotope is 1.4 times greater than in the water body and forest biotopes.
All these factors impact significantly on the ecosystem functioning. They provide the basis of the floral diversity of floodplain biogeocenoses. Floral diversity of the biogeocenoses is incomparably richer than the rest of the territory. The flora of vascular plants of the Samara floodplain is very diverse both in systematic and ecological terms. A wide range of different ecomorphs are represented here ( Table 2).

Discussion
The analysis of floral richness shows that the floodplain forests of the Samara contain 628 species of vascular plants from 728 species of the entire floodplain territory and 887 species of the entire flora of the Samara River area. This confirms the position that the floodplains of the valley landscapes of the temperate zone of Europe (Schindler et al., 1916;Filippová & Pohanka, 2019) have the highest level of biodiversity.
A significant number of ruderal species (16.2%), of which 92.4% are adventive, evidence anthropogenic transformation of the Samara floodplain flora. Of the floodplain landscape flora, 628 species of vascular plants grow in forest communities, and 486 species grow in transformed, treeless floodplain territories (Table 3).
Of the total flora composition in the floodplain landscape of the Samara River area, within the spectrum of biomorphs and climamorphs there are almost twice as many wood and shrubby species as in treeless areas, and the remaining categories increased by approximately one third. Among helimorphs, there were several times more sciophytes in the forests, among the hygromorphs and there were a third more hygrophytes and mesophytes, and among trophomorphs there were twice as many megatrophs.
Ruderalization of the flora is more typical for the treeless floodplain areas (103 species) compared to the wooded floodplain (90 species).

Conclusion
Despite the long-term anthropogenic transformation, the valley-terrace landscape territory of the Samara River area, as one of the reference native complexes of the subarid territory of Eastern Europe, has preserved a significant level of phytodiversity. In this regard, it is planned to develop a National Park "Samara Bor" within this area.
The floodplain landscape is the richest in species and most diverse part of this complex. Its flora includes 728 plant species (including 132 rare ones), of which 631 grow in the forest communities, and 487 in anthropogenically transformed, treeless floodplain areas.
In the forest flora, the number of tree and shrubby species, scyophytes, hygrophytes, and megatrophs significantly increases, and the number of ruderal plant species decreases.
The floristic composition of the floodplain forests of the middle river of the subarid region is much richer and more diverse than the flora of the treeless floodplain areas, and this should encourage measures for their protection and restoration. Afforestation of floodplain territories within the steppe zone of Ukraine should be a priority in comparison with other landscapes.