Strong increase in coverage but accelerated fragmentation in China's urban forests under rapid urbanization

Urban forest fragmentation (UFF) undermines the integrity, stability, and ecological functioning of urban ecosystems. However, under rapid urbanization, its spatiotemporal dynamics and driving mechanisms at the national scale remain insufficiently understood. Based on a constructed UFF index, this study quantifies the evolution of UFF across 291 Chinese cities from 2000 to 2022. Quadratic polynomial fitting was used to identify critical thresholds of key drivers across various climate zones and city levels, and the Geodetector model was employed to further investigate these driving factors. Results indicate that the average urban forest coverage in China was 20.32 %, with higher urban forest coverage observed in small cities and subtropical monsoon climate zones, and a general upward trend over the study period. The national average UFF index was approximately 0.59, with the highest fragmentation occurring in small cities and subtropical monsoon regions. UFF has shown continuous intensification in both mega and small cities, particularly in temperate monsoon climate zones. A distinct threshold effect was observed in the drivers of UFF: below a certain level, increases in driving factors did not reduce fragmentation and may even have exacerbated it; beyond the threshold, fragmentation decreased significantly. Furthermore, the primary drivers of UFF varied by climate zone and city level. Notably, the interaction between natural and anthropogenic factors demonstrated stronger explanatory power than individual factors. These findings enhance our understanding of UFF under rapid urbanization and provide valuable insights for sustainable urban forest management.