What drives the uphill expansion of cultivated land in hilly regions? Nonlinear responses and threshold effects from the Jiangnan hilly region, China

Amid the severe challenges to global food security and sustainable development, changes in cultivated land resources have become a key issue in global development research. However, in the context of rapid urbanization, high-quality cultivated land in hilly regions continues to be lost, forcing agricultural activities onto marginal lands. This has led to the phenomenon of uphill expansion of cultivated land, which affects both agricultural sustainability and ecological security. This research characterizes the spatiotemporal evolution of cultivated land expansion in the Jiangnan hilly region of China, revealing a shift from low-gradient areas to increasing encroachment into medium- and high-gradient regions. The results show that while cultivated land continues to expand into higher slopes, the process slows significantly beyond a 10° slope due to topographic limitations. To explore the underlying driving mechanisms, a Geographically Weighted Random Forest model is employed to examine the spatial heterogeneity of these driving factors. The findings indicate that net income per farmer is the primary driver, reflecting the dominant role of farmers' economic decisions. Additionally, built-up land expansion area exerts a spatial compression effect, driving cultivated land into steeper areas. Ultimately, the uphill expansion of cultivated land is a nonlinear response to socio-economic pressures exceeding the region's ecological carrying capacity. This study provides policy implications for hilly regions, suggesting the integration of slope-based ecological constraints within the “land occupation–compensation balance” framework to promote sustainable land use.