Urban Expansion and the Evolution of Morphological Polycentricity in Shandong Province, China, 2000–2020: Evidence from Long-Term Nighttime Light Data

Rapid urban expansion reshapes urban morphology and the spatial organization of regional city systems. However, provincial-scale studies rarely integrate expansion dynamics, morphological change, and the evolution of morphological polycentricity, while stage-specific shifts in associated factors remain insufficiently understood. This study examines 16 prefecture-level cities in Shandong, China, from 2000 to 2020 to quantify urban expansion and morphological change, assess morphological polycentricity and the city-size hierarchy, and evaluate stage-specific factors associated with newly added built-up land. Built-up land was extracted from the Prolonged Artificial nighttime-light Dataset of China (PANDA-China) using a statistics-constrained bisection threshold method and evaluated against independent land-cover products, followed by complementary spatial, morphological, rank–size, and GeoDetector analyses. The results show that the built-up area increased from 1912.10 to 5646.04 km2 in Shandong, while the annualized expansion rate declined from 7.09%/yr to 4.14%/yr despite persistently high absolute expansion after 2010. The dominant northeast–southwest axis remained stable, while urban forms became less compact and more complex. Secondary built-up nodes strengthened, whereas Jinan and Qingdao retained their dominant positions, indicating strengthening morphological polycentricity within a persistent dual-core structure. GeoDetector revealed a temporal shift from greater GDP prominence in 2000–2010 toward stronger population and road associations in 2010–2020. Initial built-up density remained the strongest explanatory factor, and factor interactions consistently exceeded individual explanatory power. These findings clarify the stage-dependent evolution of urban expansion and support differentiated land management and coordinated regional development.