UDC 581.5

Novikova N.M., Kuzmina Zh.V., Mamutov N.K.DESERTIFICATION OF THE AMU DARYA DELTA AND VEGETATION DYNAMICS DURING ARAL CRISIS // Arid Ecosystems. 2023. Vol. 29. № 4 (97). P. 4-20. | PDF

The study is devoted to dynamics of ecosystems and their components in connection with the development of the Aral ecological crisis. The theoretical basis of the work was the idea that vegetation dynamics under desertification of the delta landscape is caused by anthropogenically influenced natural hologenetic process of vegetation change from hydromorphic floodplain and floodplain biotopes to semi-hydromorphic meadow and saltmarsh biotopes, which ultimately leads to the formation of zonal vegetation of automorphic biotopes. These are endo-ecogenetic shifts caused by a directed reduction in the moisture availability of biotopes and the accompanying salinization and evolution of soils. Shifts occur both successively and catastrophically. In the northern unused part of the Amu Darya Delta, due to the reduction of the water surface of the sea and a sharp reduction in water availability by the 1990s, environmental conditions inherent to desert landscapes have developed: the values and regime of climatic parameters approached the desert, groundwater was buried  to the depth of 5-10 m and became inaccessible to plant roots. The reconstruction of the system of reservoirs and the filling of former sea bays contribute to the formation of hydromorphic conditions in local areas.

Geobotanical studies initiated in 1979 in the Amu Darya Delta using  route observations and  topo-ecological profiles crossing the main elements of the delta relief (riverine ramparts, their slopes and inter-river depressions) were repeated in monitoring mode in 1985, 1993 and 1999. Route observations conducted in 2017 showed that the modern period of vegetation dynamics is the formation of desert-type plant communities. On most of the territory of the undeveloped delta, which turned into a wasteland after the death of reed (Phragmites australis (Cav.) Trin. ex Steud.) communities (1970-1980), there is an active settlement of the black saxaul (Haloxylon aphyllum (Minkw.) Iljin), encountered for the first time in the Muynak district in 1993. The desert species teresken gray (Krasheninnikovia ceratoides (L.) Gueldenst.), which settled around the same years in the degrading tugai and sparse monocenoses of the tree-like salsola (Salsola dendroides Pall.) on the salt marshes, formed extensive thickets to the north of Kungrad.

Observations on topo-ecological helped to describe trends of particular shifts and stages more detailed. On the right bank of the Akdarya channel, feeding the Mezhdurechensk reservoir, on the topo-ecological profile of Porlytau, 3 km southwest of the hill of the same name, for almost 40 years of observations, the river washed away a section of the riverbed floodplain and the riverbed shaft 500 m wide and eroded 9 m of banks – the level of the eastern part of the Big Sea – fell by 26 m. Vegetation changes are slowly developed. The succession is as follows: from woody sucker-poplar taut (Populus ariana+Populus pruinosa+Elaeagnus angustifolia-Mixteherbosa) through shrubby with tamarix multi-branched (Tamarix ramosissima Ledeb.) and finally to catastrophic change in the nearby site of the interrustal depression – the death of the karabarak (Halostachys belangeriana (Moq.) Botsch.) community. This plant community was formed in 1985 as a result of salinization of the soil on the site of dead reeds. In 2017 the process of slow surface soil desalination which began in 1993 continued in all points of the profile by the irradiation of salts from the upper horizons (0-10 cm) to the lower ones. On the outskirts of Tugai in 2017, for the first time, the presence of a desert species of teresken gray was noted, which possibly will settle throughout this area.

In the north-east of the delta, on the topo-ecological profile of the Kunyadarya, starting on the right bank of the dry channel of the same name and descending in a south-easterly direction to the coastal plain, the degrading Chingil-turang tugai on the riverbed shaft is replaced by tamarix, as evidenced by the species composition of the described community Populus diversifolia-Tamarix laxa+Halimodendron halodendron+ T. ramosissima-Atriplex tatarica. In the community of tall, well-developed tamarix and salt plants located further along the profile on the slope of the riverbed shaft, a black saxaul was first detected in 2017. Then the profile goes to the coastal salt flat Halostachys belangeriana-Climacoptera aralensis community. The salt profiles of soils under all plant communities have similarities : the maximum salt content falls on the upper part of the profile 0-10 cm, it sharply decreases in the horizon 10-20 cm and further to a depth of 50 cm remains unchanged. The salt profiles of the soils under the two sections of the tugai have the greatest similarity. The uppermost horizon of 0-5 cm is less saline or somewhat washed from salts than the deeper horizon of 5-10 cm with the maximum salt content. In the community of Karabarak, the maximum salt content (8.28%) is in the surface horizon of the soil 0-5 cm and sharply decreases to the horizon of 10-20 cm to 2% and decreases to almost 1%. The salt profile under the tamarix communities differs from the rest in that it is all slightly saline and has two maxima – surface weak salinity (0-5 cm – 0.5%) and average salinity (0.8%) at a depth of 20-30 cm. The salt profiles of soils on the topo-ecological profile of Kunyadarya indicate progressive salinization of soils with a pronounced surface-accumulative profile under the influence of pulsating secondary hydromorphism of this site, provoked by the periodically appearing flow of water from the Gulf of Dzhiltyrbas towards the sea.

In 2017 a newly formed young tree tugai was described in a small area on the bank of the irrigation canal crossing the degraded tugai Yerkin. It differs from the degraded ones described earlier : its full–fledged structure has 3 levels, and the richness of the species composition of plants consists of 12 species. Two types of lianas, undergrowth and renewal of woody dominant species of the community are noted. The soil under the young tugai is alluvial-meadow-tugai (hydromorphic), slightly saline (0.36%) only in the near-surface horizon of 0-5 cm.

Comparison of the data obtained on topo-ecological profiles with the data of previous observations, allowed us to assume that the change of plant communities in the conditions of desertification of the unused part of the delta in the communities of woody tugai is mainly successional. At the same time grass and shrub communities, especially their halophilic variants, are changing catastrophically. After they disappear  non-overgrown wastelands are formed for a long time. Conditions for the introduction of species with opposite ecology are prepared.

In local hydromorphic conditions with the floodplain water regime the formation and existence of tugai-type plant communities is possible. As a result the diversity of environmental conditions were characterize and modern processes occurring in the vegetation and landscapes of the unused part of the Amu Darya Delta are detected. These results are useful for organizing and conducting of natural resources use and biodiversity conservation.

Keywords: Uzbekistan, the Republic of Karakalpakstan, used and unused lands of the Amu Darya delta, desertification, hydromorphic, semi-hydromorphic, automorphic stages, topo-ecological profiles, observations, hologenesis, endo-ecogenetic successions, catastrophic and successional changes of vegetation, salt profile of soils, salinization and desalination of soils.

Acknowledgements. Auhors are grateful to M.V. Konyushkova, PhD in Agricultural Sciences, for her assistance in organizing and conducting field work in 2017, processing and analyzing soil data.

Funding. The expedition work was carried out with the financial support of the Regional Representative Office of the International Center for Agricultural Research in Arid Regions – ICARDA, Tashkent (project manager K. Toderich), the analysis of materials and the writing of the article was carried out within the framework of the topic no. FMWZ-2022-0002 of the state task of the Institute of Water Problems of the Russian Academy of Sciences «Research of geoecological processes in hydrological systems of land, formation of surface and groundwater quality, problems of water resources management and water use in the conditions of climate change and anthropogenic impacts».

DOI: 10.24412/1993-3916-2023-4-4-20

EDN: BXJHWA