Composition and Structure of Tugai Communities in the Indication of Ecological Conditions in the Lower Amu Dar’ya

Abstract

Tugai ecosystems of desert river valleys are one of the most affected and endangered landscape types. The largest areas of tugai vegetation in Uzbekistan are preserved in the lower reaches of the Amu Dar’ya River. The main massifs of poplar forests are there. To assess their current status, 284 relevés, representing tugai plant communities in the lower reaches of the Amu Dar’ya, have been analyzed. For each one, the position on ecological gradients was determined by species indicator values. It is shown that the scatter of relevés is the greatest for moisture gradient: from 17 to 97 grades, and it is noticeably smaller for pasture digression (2.5–6.5) and soil salinity (16.1–19.5). To reveal the relationships between the species composition and environmental factors, CCA ordination was performed. Groups of relevés, representing the native tugai vegetation and three ecological series connected with salinity, soil moisture, and anthropogenic pressure, are identified. According to them, four generalized plant communities, differing in species composition, floristic diversity, and structure, are distinguished. Indicator plants are identified on the basis of a formalized assessment of species constancy and importance values. The changes in species composition and structure of communities (life form spectra) are shown. It has been established that vegetation halophytization is the most active process following progressive desertification. This process is reflected in the replacement of poplar forests by shrub communities dominated by Halostachys belangeriana and Tamarix hispida. As a result of intensive anthropogenic impact, the vegetation is transformed and the importance value of ruderal annual plants becomes higher.