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 detail. On the right bank of the Akdar’ya River that feeds the Mezhdurechenskoe Reservoir, on the Porlytau topo–ecological profile located 3 km southwest of the upland of the same name, over the course of the 40-year observation period, the river washed away a section of the near-channel floodplain and levee 500 m wide and was incised 9 m into the ground following a drop in the erosion base level (i.e., water level in the eastern part of the Greater Sea) by 26 m. Plant communities successions develop slowly. They follow the path of successional replacement of an arboreal Рoplar tugai (Populus ariana + Populus pruinosa + Elaeagnus angustifolia–Mixteherbosa with a shrubby tugai with Tamarix ramosissima Ledeb. ending in a nearby section of the interchannel depression with a catastrophic change: the death of the Halostachys belangeriana (Moq.) Botsch. community. This community was formed in 1985 as a result of soil salinization in the area previously occupied by reeds. In 2017, a slow surface desalination process that began in 1993 due to the illuviation of salts from upper (0–10 cm) horizons to lower ones continued in the soil at all survey points on the profile. For the first time, the desert shrub species Krasheninnikovia ceratoides (L.) Gueldenst. was recorded on the tugai fringe in 2017; over time, this species will probably colonize this entire site characterized by the profile. In the northeastern part of the delta, on the Kunyadarya topo–ecological profile that begins on the right bank of the dry channel of the same name and descends in the southeastern direction to the coastal plain, a degrading Populus diversifolia-Halimodendron halodendron tugai on the levee is being replaced by a tamarisk tugai, which is indicated by the species composition of the community: Populus diversifolia-Tamarix laxa + Halimodendron halodendron + T. ramosissima–Atriplex tatarica. In 2017, black saxaul was first recorded in a community formed by tall well-developed tamarisks and succulent saltworts located further along the profile on the levee slope. The profile enters a coastal solonchak plain with a Halostachys belangeriana–Climacoptera aralensis community. The salinity profiles of soils under all plant communities have a similar feature: the salt content reaches its maximum in the upper part of the profile (0–10 cm); then it sharply decreases in the 10–20-cm horizon and does not change down the profile to a depth of 50 cm. Salinity profiles of soils on two sites occupied by the tugai have the maximum similarity. The uppermost (0–5 cm) horizon is less saline or somewhat washed out from salts compared to the deeper (5–10 cm) horizon featuring the maximum salt content. In the Halostachys belangeriana community, the salt content reaches its maximum (8.28%) in the surface soil horizon (0–5 cm), then it sharply decreases to 2% in the 10–20-cm horizon, and further decreases down the profile to almost 1%. The salinity profile under tamarisk communities differs from other profiles: overall, it is slightly saline and has two maxima: mild surface salinity (0.5% at a depth of 0–5 cm) and medium salinity (0.8%) at a depth of 20–30 cm. Salinity profiles of soils on the Kunyadarya topo–ecological profile indicate progressive salinization of soils with a clearly pronounced surface-accumulative profile under the impact of pulsating secondary hydromorphism in this area provoked by water flows from the Dzhiltyrbas Bay towards the sea that occur on a periodic basis. In 2017, a newly-formed young woody tugai on the bank of an irrigation canal crossing the degrading Yerkin tugai was described. It differs from the degrading tugai forests described earlier in its full-fledged structure (it has three storeys) and in richness of its plant species composition (12 species). Two liana species are present, and renewal of predominant tree species occurs. The soil under the young tugai is alluvial–meadow–tugai (hydromorphic); it is slightly saline (0.36%) only in the near-surface horizon (0–5 cm). Comparison of data collected on topo–ecological profiles with data collected in the course of previous observations made it possible for the first time to conclude that the changes of plant associations in woody tugai communities in the undeveloped part of the delta under the desertification conditions occurs mainly in a successional way; while herbaceous and shrub communities, especially halophilic ones and their variants, are characterized by predominantly catastrophic changes. After their death, long-lasting wastelands are formed on lands previously occupied by such communities where favorable conditions for the introduction of species with different ecology are created. In local hydromorphic conditions with floodplain regimes, the formation and existence of tugai plant communities is possible. The practical significance of the results is that they characterize the diversity of environmental conditions and processes currently occurring in the vegetation and landscapes in the undeveloped part of the Amu Darya River delta and can be applied in resource use and biodiversity conservation practices.