Urban core–edge gradients in vertical and horizontal forest structure reveal nonlinear and threshold responses across Chinese cities

Urban forests are inherently three-dimensional, yet whether their vertical and horizontal structural components respond synchronously to urbanization remains poorly understood. Here, using GEDI LiDAR observations from 31 provincial capital cities across China, we simultaneously quantified foliage height diversity (FHD) and canopy cover (CC) along morphologically adaptive urban core–edge gradients. Urban forests exhibited two distinct modes of three-dimensional structural organization. In 71% of cities, FHD and CC followed coordinated trajectories despite differences in trajectory shape, indicating synchronized structural development across urban gradients. In the remaining 29% of cities, the two structural dimensions became decoupled, exhibiting contrasting spatial trajectories under comparable urbanization gradients. Machine-learning analyses further showed that coordinated systems were consistently dominated by population density, whereas precipitation, land surface temperature and soil properties regulated structural variation through type-specific nonlinear interactions. By contrast, decoupled systems were influenced by largely the same climatic and anthropogenic drivers, yet FHD and CC exhibited fundamentally different response functions despite similar driver importance. These findings provide a mechanistic explanation for why similar urbanization pressures produce contrasting three-dimensional forest structures across cities.