Results of long-term observations on stationary transects in the Volga-Akhtuba floodplain.
By the beginning of the 1970s, only two large regions with natural vegetation, namely the Volga-Akhtuba floodplain and the Volga delta, had remained in the Volga River valley. The largest part of the remaining valley had been converted into a cascade of reservoirs. The conservation of meadow, marsh, and forest plant communities in the lower reaches of the Volga River located in the semi-desert and desert zones is due to regular water releases of the Volgograd hydroelectric power station downstream. These artificial water discharges into the Lower Volga valley in the spring-summer period have taken the place of natural floods. However, in the conditions of regulated water flow, the rise in the water level decreased during floods and the duration of flooding of the floodplain ecotopes shortened. The river channel deepened along a 100-km river section in the near-dam part of the Volgograd hydroelectric power station in the northern part of the floodplain. These dams hinder water flow to the Akhtuba-Volga floodplain during spring and summer floods. They surround agricultural lands with artificial irrigation systems and settlements in the floodplain. The recreational use of the Lower Volga valley has increased in the past few decades. Before the construction of the largest Volga hydroelectric complexes (the Kama, Gorky, Kuibyshev, Volgograd, and Saratov ones) in 1954-1955, five geobotanical transects were laid out in the course of the Caspian expedition of Moscow State University. Subsequently, four of the transects were surveyed in 1982 and 2008 -2013. It was established that xerophytization and ruderalization of vegetation occurred on all transects. These phenomena were not similar in different sections of the floodplain. They were most pronounced near the Volgograd Hydroelectric Power Station and in the places of narrowing of the valley. The reduction in the height and duration of floods after the regulation of the water flow, high local grazing, and recreation pressure is the main reason for xerophytization and ruderalization of vegetation. Among the invasive species, Fraxinus pennsylvanica, Conyza canadensis, Bidens frondosa, and Xanthium strumarium s.l. play the most important role in changes in the vegetative cover of the Volga-Akhtuba floodplain. Thus, not all phenomena related to the dynamics of the vegetation and flora in the Volga-Akhtuba floodplain were caused by restructuring of the Volga hydrological regime after regulation of its water flow. The invasion of alien plants, increased recreation load, and changes in the economic use of the vegetation cover in the territory have played a particular role in this process.