Land Use Change and Landscape Ecological Risk in the Hangzhou Bay Urban Agglomeration: Spatiotemporal Dynamics and the Nonlinearity and Heterogeneity of Associated Factors

Rapid urbanization is driving land-use and land-cover change (LUCC) and reshaping landscape ecological risk (LER) in coastal bay urban agglomerations, where intensive development affects interconnected terrestrial and aquatic systems. We examined the spatiotemporal evolution of LUCC and LER across the Hangzhou Bay Urban Agglomeration (HBUA) from 2000 to 2020. We integrated eXtreme Gradient Boosting (XGBoost), SHapley Additive exPlanations (SHAP) and Geographically and Temporally Weighted Regression (GTWR) to characterize nonlinear responses, interactions and spatiotemporal heterogeneity in the statistical associations between potential associated factors and LER. The results showed that (1) built-up land expanded continuously, primarily at the expense of cropland. This expansion shifted the regional landscape from one dominated by forests and cropland to a more intensively developed, mixed structure. (2) Although the lowest and lower risk classes remained dominant, areas of moderate and higher risk expanded, particularly across bayfront plains, urban expansion frontiers, and in parts of the Zhoushan Islands. In contrast, cells at the lower end of the risk classification occurred primarily in mountainous terrain where landscape continuity remained comparatively high. LER also remained significantly spatially clustered. (3) Among the natural factors, elevation (DEM), slope (SLP), and the normalized difference vegetation index (NDVI) contributed most strongly to LER prediction and exhibited pronounced nonlinear responses and interactions. The associations between LER and river network density (RND), population density (POP), and gross domestic product per capita (GDP) varied across space and time. The local positive association between POP and LER weakened, whereas that between GDP and LER strengthened. These results offer evidence to support differentiated risk control, dynamic monitoring, and coordinated governance in coastal bay urban agglomerations.