Results of Our Long-Term Observations on Stationary Transects in the Volga-Akhtuba Floodplain

Only two large regions with natural vegetation had remained in the Volga River valley by the beginning of the 1970s, namely, the Volga-Akhtuba floodplain and the Volga River delta. The largest part of the remaining valley had been converted to a cascade of artificial reservoirs. The conservation of meadow, marsh and forest plant communities in the Lower Volga, in the semi-desert and desert zones, is due to regular special water releases to the downstream of the Volgograd hydroelectric complex. These artificial water discharges to the Lower Volga valley in the spring-summer period took the place of natural floods. However, in the conditions of regulated water flow, the water rise height during high water diminished, and the duration of flooding of inundated ecotops shortened. In the northern part of the flood-lands, on the dam site of the Volgograd Hydroelectric Power Station, the river bed went deep along a 100 km section. They were dams which became hindering water inflow to the Volga-Akhtuba floodplain during spring and summer floods. They surround agricultural lands with artificial irrigative systems and settlements in the flood-lands. Roads are built on such dams. The recreational use of the Lower Volga valley has increased in the last decades. Before the building of the largest Volga hydroelectric complexes (the Kama, Gorky, Kuibyshev, Volgograd and Saratov ones), in 1954-1955, the Caspian expedition of Moscow State University laid five geobotanical transects in the Volga-Akhtuba floodplain. Subsequently, four of these transects were surveyed in 1982 and in 2008-2013. It was established that xerophytization and ruderalization of the vegetation occurred on all transects. These phenomena were not similar at different sections of the floodplain. They were most pronounced near the Volgograd Hydroelectric Power Station and in places where the valley is narrowing. The main causes of these xerophytization and ruderalization of vegetation are the reduction in the height and duration of floods after the beginning of water flow regulation, the high local grazing, and recreation pressure. Among the invasive species, the most important role in changes of the vegetation cover of the Volga-Akhtuba floodplain is played by Fraxinus pennsylvanica, Conyza canadensis, Bidens frondosa, and Xanthium strumarium s.l. Thus, not all phenomena related to the dynamics of the flora and vegetation of the Volga-Akhtuba floodplain were caused by the restructuring of the Volga hydrological regime after the beginning of its water flow regulation. The invasion of alien plants, the increased recreational load, and changes in the economic use of vegetation have played some role in this process.

1. Atlas ruslovoi morfodinamiki Nizhnei Volgi (Volgograd - Astrakhan') [Atlas the Lower Volga channel mophodynamics (Volgogrd - Astrakhan). V. N. Korotaev, D. B. Babich, R. S. Cha-lov, eds.]. Moscow, Izdatel'stvo Moskovskogo universiteta, 2009. 232 p. (in Russian).

2. Bolgov M. V., Demin A. P. Water-Management and environmental problems of the Lower Volga and ways to their solution. Water Resources, 2018, vol. 45, no. 2, pp. 297-305 (in Russian). DOI: 10.7868/S0321059618020116

3. Golub V. B. Using vegetation releves as a sample collection for classification of plant communities. Vegetation of Russia, 2011, no.17-18, pp. 70-83 (in Russian).

4. Golub V. B., Barmin A. N., Iolin M. M., Starichkova K. A., Sorokin A. N., Sharova I. S., Nikolaychuk L. F. Estimate of vegetation dynamics along the transect in the southern part of the Volga-Akhtuba floodplain near Hosheutovo village. Proceedings of the Samara Scientific Centre ofthe Russian Academy ofSciences, 2011, vol. 13, no. 5, pp. 107-113 (in Russian).

5. Golub V. B., Bondareva V. V., Sorokin A. N., Nikolaychuk L. F. Reed (Phragmites australis agg.) dominated plant communities in the Lower Volga valley. Vegetation of Russia, 2015, no. 26, pp. 26-37 (in Russian).

6. Gorelits O. V., Zemlyanov I. V. Modern mechanism of flooding the territory of the Volga-Akhtuba floodplain during the flood period (within the Volgograd region). The Scientific Potential of the Regions in the Service of Modernization, 2013, no. 2 (5), pp. 9-18 (in Russian).

7. Gorelits O. V., Zemlyanov I. V., Sinenko L. G. Estimation of the morphometric characteristics of the channel when planning activities for water supply in the Lower Volga. In: Management of water-resource systems in extreme conditions: Collection of reports of the International Conference. Moscow, GIDROOKS Publ., 2008, pp. 306-307 (in Russian).

8. Zajtsev A. A., Ivanov V. V., Korotaev V. N., Labutina I. A., Luk'yanova S. A., Tszunsyan' Li, Rimskij-Korsakov N. A., Rychagov G. I., Svitoch A. A., Sidorchuk A. Yu., Sychev V. A., Chernov A.V. Nizhniaia Volga: geomorfologiia, paleogeografiia i ruslovaia morfodinamika [Lower Volga: Geomorphology, Paleogeography and Channel Morphodynamics]. Moscow, GEOS Publ., 2002. 224 p. (in Russian).

9. Iolin M. M., Sorokin A. N., Starichkova K. A., Barmin A. N., Nikolaychuk L. F., Golub V. B. Vegetation dynamics along a transect in the Volga-Akhtuba floodplain near Kapustin Yar village. Povolzhskiy J. of Ecology, 2011, no. 4, pp. 431-442 (in Russian).

10. Kapitonova O. A. Synopsis of macrophyte flora of the Vyatka-Kama Cis-Urals. Phytodiversity of Eastern Europe, 2015, vol. 9, no. 4, pp. 4-85 (in Russian).

11. Ramensky L. G. Uchet i opisanie rastitel'nosti (na osnove proektivnogo metoda) [Accounting and Description of Vegetation (Based on the Projective Method)]. Moscow, VASKhNIL Publ., 1937. 100 p. (in Russian).

12. Ramensky L.G., Tsatsenkin L. G., Chizhikov O. N., Аntipin N. А. Ekologicheskaia otsenka kormovykh ugodii po rastitel'nomu pokrovu [Ecological assessment of fodder land by vegetation cover]. Moscow, Sel'khozgiz Publ., 1956. 472 p. (in Russian).

13. Sorokin A. N., Barmin A. N., Iolin M. M., Starichkova K. A., Nikolaychuk L. F., Golub V. B. Indication of environment change on transect in the Volga-Akhtuba floodplain near Kapustin Yar village by using Ramenskiy indicator values and DCA-ordination. Bulletin of the V. N. Tatishchev Volga University, Ser. Ecology, 2010, iss. 10, pp. 74-80 (in Russian).

14. Starichkova K. A., Barmin A. N., Iolin M. M., Sharova I. S., Sorokin A. N., Nikolaychuk L. F. Estimate of vegetation dynamics along the transect in the northern part of the Volga-Akhtuba floodplain. Aridnye ekosistemy, 2009, vol. 15, no. 4, pp. 36-48 (in Russian).

15. Tarasov M. N., Beschetnova E. I. Hydrochemistry of the Lower Volga in the regulation of runoff (1935-1980). Gidrokhimicheskie materialy, 1987, vol. 101. 120 p. (in Russian).

16. Tsatsenkin I. A. Vegetation and natural forage resources of the Volga-Akhtuba floodplain and delta Volga. In: Priroda i sel'skoe khoziaistvo Volgo-Akhtubinskoi doliny i del'ty r. Volgi [Nature and Agriculture of the Volga-Akhtuba Valley and the Delta Volga]. Moscow, Izdatel'stvo Moskovskogo universiteta, 1962, pp. 118-192 (in Russian).

17. Bondareva V. V., Golub V. B. Dynamics of vegetation in the Volga-Akhtuba floodplain (by the example of a transect near the settlement of Bolkhuny). Biology Bulletin, 2016, vol. 43, no. 10, pp. 1370-1376. DOI: 10.1134/S1062359016100022

18. Chytry M., Tichy L., Holt J., Botta-Dukat Z. Determination of diagnostic species with statistical fidelity measures. J. of Vegetation Science, 2002, vol. 13, pp. 79-90. DOI: 10.1111/j.1654-1103.2002.tb02025.x

19. Tichy L. JUICE, software for vegetation classification. J. of Vegetation Science, 2002, vol. 13, pp. 451-453. DOI: doi.org/10.1111/j.1654-1103.2002.tb02069.x

20. Tichy L., Chytry M., Bruelheide H. Statistical determination of diagnostic species for site groups of unequal size. J. of Vegetation Science, 2006, vol. 17, pp. 809-818. DOI: 10.1111/j.1654-1103.2006.tb02504.x

21. Flora Europaea on CD-ROM. T. G. Tutin, V. H. Heywood, N. A. Burges, D. H. Valentine, S. M. Walters, D. A. Webb, eds. Cambridge, Cambridge University Press, 2001, vol. 1-5.

22. Zeleny D., Tichy L. Linking JUICE and R: New developments in visualization of unconstrained ordination analysis. 18th Workshop of European Vegetation Survey in Rome. Roma, La Sapienza Univerzita Publ. House, 2009, pp. 123.