Understanding the ecological impacts of vertical urban growth in mountainous regions

Approximately 35 % of Chinese cities are situated in mountainous areas, where rapid urban expansion on slopes has led to the degradation of urban ecosystem health, marked by the loss of natural landscapes, diminished resilience, and reduced ecosystem services. However, the impact of vertical urban growth on ecosystem health, especially in terms of temporal dynamics, distribution, and underlying mechanisms, remains poorly understood. Chongqing, a typical mountainous metropolis, was selected to investigate these impacts over the past two decades. By integrating the slope spectrum (SS), climbing index (CI) of built-up land, and ecosystem health index (EHI), we explored how urban expansion has affected the EHI. Additionally, we developed a method for identifying the expansion advantage slope range (EASR), which can track dynamic slope-climbing urban expansion. Our findings revealed that the built-up land expansion in Chongqing was primarily concentrated in the central areas of the western region. From 2000 to 2010, this expansion occurred primarily in urban built-up areas and shifted to other built-up areas between 2010 and 2020. Although the Climbing Index was −0.11, the EASR results indicated an increasingly significant slope-climbing trend, with slopes of built-up land increasing from [1.17, 8.25] to [1.42, 8.48]. Notably, when expansion occurs on slopes exceeding 12°, the decline in the EHI becomes significantly more pronounced, with the impact coefficient rising from 0.201 to 0.447. This study reveals the fundamental relationship between slope-climbing urban expansion and ecosystems, providing valuable insights for urban planning and sustainable development in mountainous regions.