Spatiotemporal characteristics of the expansion of an urban agglomeration and its effect on the eco-environment: Case study on the northern slope of the Tianshan Mountains

Urban agglomerations, which are highly integrated urban groups, form during the advanced stages of industrialization and urban development. The urban agglomeration on the northern slope of the Tianshan Mountains is one of the 19 urban agglomerations promoted by China’s 13th Five-Year Plan and a key area of urbanization in Xinjiang. Based on remote sensing data, we analyzed the expansion of construction land in the urban agglomeration on the northern slope of the Tianshan Mountains and its effect on ecological value in 1980–2015. The results show that the expansion of construction land significantly affected ecological value in the study area over the past 35 years. Among land use types, grassland was the dominant contributor to changes in ecological value; its contribution rate reached 66.27% with an interannual variation of only 5%. The contribution rate of urban and rural residential land along with industrial and mining lands was very low (0.82%), while the interannual variation was large (86%). The area of construction land in the urban agglomeration expanded from 1,067 to 2,586 km2, and the conversions of different land use types had different effects on ecological value. Among land use types, the conversions of low-coverage grassland and dry land made the largest contributions to changes in ecological value. During the study period, a total of 1,685 km2 of ecological land, 69.91% of which belonged to cultivated land and grassland, was converted into construction land. A total area of 1,082 km2 of dry land and low-coverage grassland was converted into construction land, accounting for 64.21% of the total ecological land that was converted into construction land. In 2000–2015, the area of urban construction land increased significantly along with the proportion of unused land, including gobi (172 km2), bare land (65 km2), and salt-affected land (65 km2), being converted into construction land. In contrast, the area of construction land that was converted into ecological land was less than 166 km2. The areas with the largest expansions of construction land were concentrated in Urumqi, Karamay, and Changji, where the areas of construction land increased by 399, 186, and 126 km2, respectively. The areas with the most rapid expansions in construction land were concentrated in Kuytun, Toksun, and Usu. The environmental effects of construction land expansion and land use change were more positive in the late stage of urban agglomeration (2000–2015) compared to in the early stage (1980–2000). Kuytun City, Shihezi City, and Wujiaqu City showed large changes in ecological value caused by the expansion of construction land, whereas the changes in ecological value in the 14 other cities were relatively small, and the expansion of construction land slowed in 2000–2015 compared to in 1980–2000. In the future, further development of the urban agglomeration on the northern slope of the Tianshan Mountains should fully consider the sensitivity and carrying capacity of the vulnerable ecological environment in this area. Population growth and industrial development in the urban agglomeration should be controlled based on the available water resources and ecological capacity to reduce the pressure on the environment and ensure sustainable development.