Desertification of the Amu Darya River Delta and Vegetation Dynamics in the Conditions of the Aral Sea Crisis

This paper discusses the dynamics of ecosystems and their components in connection with the development of the Aral Sea environmental crisis. The theoretical basis of this study was the idea that the vegetation dynamics under the conditions of deltaic landscape desertification represents an anthropogenically-induced natural hologenetic process involving the replacement of vegetation typical for hydromorphic floodplain and reed-bed biotopes with vegetation of semihydromorphic meadow and solonchak biotopes resulting in the formation of zonal vegetation typical for automorphic biotopes. These endo–ecogenetic successions are determined by the directed reduction in moisture supply in biotopes and accompanying salinization and evolution of soils. The changes occur in both successional and catastrophic ways. In the northern, undeveloped part of the Amu Darya River delta, the reduction of the sea water surface and a sharp drop in water reserves resulted by the 1990s in the formation of environmental conditions typical for desert landscapes: the climate parameters and their regime became close to desert ones, while the groundwater level fell to a depth of 5–10 m, thus, making groundwaters inaccessible to plant roots. Reconstruction of the reservoir system and flooding of former marine bays contribute to the formation of hydromorphic conditions on local sites. Geobotanical studies commenced in the Amu Darya River delta in 1979 and involved route surveys and surveys of topo–ecological profiles passing through the main deltaic relief elements (levees, their slopes, and interchannel depressions) were repeated in the monitoring mode in 1985, 1993, and 1999. Route surveys performed in 2017 showed that the current vegetation dynamics stage involves the formation of desert plant communities. Black saxaul ( Haloxylon aphyllum (Minkw.) Iljin) first discovered in the Muynak district in 1993 is actively spreading in the most part of the undeveloped delta that has turned into a wasteland after the extinction of common reed ( Phragmites australis (Cav.) Trin. ex Steud.) communities in the 1970s–1980s. The desert species Krasheninnikovia ceratoides (L.) Gueldenst. that has invaded degrading tugai and sparse arborescent saltwort ( Salsola dendroides Pall.) monocoenoses on takyr solonchak soils around the same years formed extensive thickets north of the city of Kungrad. Observations on topo–ecological profiles made it possible to examine individual changes and stages in more detai