Monitoring Spatiotemporal Forest Fragmentation in Urban Landscapes: An Improved Urban Fringe Mapping Approach Using Time-Series Remote Sensing Data

Urban fringe forests deliver critical ecosystem services yet face irreversible loss and complex degradation under urbanization, while their fragmentation dynamics relative to core forests remain largely unquantified owing to the lack of a spatiotemporally consistent mapping approach. To address this gap, a multi-source remote sensing framework integrating land cover, population, nightlight, and land surface temperature data was developed to delineate urban fringe boundaries and quantify forest dynamics in Zhejiang Province via area-weighted centroids and landscape metrics, where high forest cover and a polycentric urban structure create a highly heterogeneous and dynamic fringe environment, enabling separate quantification of spatiotemporal fragmentation patterns for urban fringe and core forests from 2004 to 2024. The results are as follows: (1) The four dimensions (land, population, economy, and environment) produced spatiotemporally distinct boundaries, with multi-dimensional integration outperforming any single indicator and nighttime light being the best. Meanwhile, the total fringe area grew from 2989.04 to 3990.66 km2 over two decades, with the fastest growth in 2004–2014 and the most rapid boundary shifts in 2014–2019. (2) The fringe forest proportion dropped from 24.72% to 18.37%, with the largest decline in southwestern high-forest cities. Meanwhile, Hangzhou and Ningbo fringe forests increasingly assumed the main ecological carrier role formerly held by core forests, with their centroids moved southwestward most markedly in 2009–2014 and displaying a more consistent direction than core forests. (3) Fragmentation metrics showed a higher patch density and splitting index but lower connectivity in the fringe than in the core, with intensification peaking in Zhoushan and coinciding with intensive edge expansion in 2009–2014, followed by later responses in the core. This study provides a transferable multi-dimensional remote sensing methodology for urban fringe mapping indicating that fringe forests may serve as early-warning signals of urbanization-induced forest degradation, enabling spatially targeted forest management across varied urban contexts.