Evolution of the Central Yunnan Urban Agglomeration over three decades: administrative hierarchy dependence and topographic constraints

Context The Central Yunnan Urban Agglomeration (CYUA), a representative plateau-mountainous system, has experienced rapid urbanization. However, the spatiotemporal patterns and multi-scale drivers of its expansion under such topographic constraints remain insufficiently understood. Objectives This study analyzes the spatiotemporal patterns and hierarchical characteristics of built-up land expansion from 1990 to 2023 and quantifies the multi-scale drivers and topographic constraints shaping urbanization. Methods Annual built-up land datasets were generated using Landsat imagery and a Random Forest model. Multi-level units were constructed based on administrative hierarchy. Spatiotemporal differentiation was examined using kernel density estimation, expansion intensity, and differentiation indices. Random Forest was employed to quantify the relative contributions of natural, socio-economic drivers across scales. Results (1) Pronounced expansion occurred post-2010, with sequential acceleration from provincial capital to town levels. (2) Spatially, expansion followed a compound pattern of extensional growth supplemented by leapfrogging. Kunming, the provincial capital, expanded southward (2000–2015), then northeastward (2016–2023). Built-up land increasingly encroached into higher elevations (≥ 2000 m) and steeper slopes, with high-altitude expansion comprising 21.55% in 2020–2023. (3) Road service capacity, population density, GDP, and topography were key drivers, exhibiting hierarchical dominance: roads at the agglomeration scale, population/GDP at prefecture/county levels, and topography as a critical constraint of urban area growth at the township level. Conclusion Urbanization in CYUA followed a sequential ‘prefecture-county-town’ pattern with extensional and leapfrogging spatial growth, including an uphill phase. It is driven by broad-scale socio-administrative factors and locally constrained by topography, revealing a clear multi-scale hierarchical mechanism.