Spatiotemporal Evolution of Landscape Ecological Risk and Topographic Gradient Heterogeneity in Huangshan City, China, Based on the intPLUS Model

To characterize the spatiotemporal evolution of land use and landscape ecological risk in mountainous tourism cities, this study uses Huangshan City as a representative case. Land use data from 2002 to 2022 are analyzed, and future land use patterns for 2032–2042 are projected using the integrated patch-generating land use simulation (intPLUS) model. Meanwhile, by coupling landscape ecological risk indices with terrain gradient metrics (T1–T5), we identify the spatiotemporal evolution patterns and quantify how terrain constraints shape risk distribution. The results indicate that: (1) From 2002 to 2042, forest land remains dominant and consistently accounts for more than 85% of the area. Nevertheless, strong pressure from future urbanization is expected. Between 2032 and 2042, built-up land is projected to expand sharply from 163.08 km2 to 255.46 km2, increasing encroachment pressure on the extensive forest matrix. (2) Over the past two decades, landscape ecological risk shows a complex “improve-then-rebound” trajectory. In the future, however, it exhibits distinct phase characteristics: risk is relatively stable and improves from 2022 to 2032, but undergoes a sudden reversal and deterioration from 2032 to 2042. This pattern reflects the intensified risk to ecological security resulting from the concentrated release of construction land quotas during the mid-to-late stage of urbanization. Spatially, risk follows a radiation-like pathway, diffusing outward in concentric patterns, with core towns and tourist scenic areas functioning as the source regions. (3) Terrain gradients impose strong constraints on both land use patterns and ecological risk. Anthropogenic disturbance decreases stepwise as terrain gradient increases. The low-gradient zone (T1) is the primary location for built-up expansion and risk enhancement; the mid-gradient zone (T2) represents a highly sensitive ecotone, marked by pronounced tension between human disturbance and natural recovery. In contrast, the high-gradient zones (T4–T5) preserve exceptionally high forest connectivity due to topographic barriers, thereby serving as a crucial natural ecological barrier. Overall, this study clarifies the spatial conflicts among rapid urbanization, tourism development, and ecological conservation in mountainous urban regions, offering actionable insights for differentiated spatial governance and sustainable development.