Spatiotemporal dynamics and climatic-anthropogenic drivers of kernel NDVI across China’s subtropical forests

Subtropical forests play a crucial role in global carbon cycling and climate regulation, yet their vegetation dynamics remain highly sensitive to climate variability and anthropogenic pressures. These ecosystems, characterized by diverse forest types, face complex challenges in responding to changing climatic-anthropogenic conditions. Understanding the spatiotemporal dynamics of vegetation greenness and its drivers is essential for predicting forest responses and informing conservation strategies. We analyzed the kernel Normalized Difference Vegetation Index (kNDVI) across subtropical forests in China from 2001 to 2017 to investigate vegetation dynamics, their relationship with carbon uptake, and their responses to climatic, anthropogenic, and geographic factors across spatial scales. Our results showed a predominant increase in kNDVI across all six forest types in 91.96% of the forested areas, particularly in the central and southern regions dominated by natural forests. A strong and significant positive correlation between kNDVI and gross primary productivity was observed in 73.99% of the forested areas, validating the important role of vegetation greenness in ecosystem photosynthetic activity in subtropical regions. Partial correlation analysis at the pixel scale showed that while temperature and precipitation generally promoted greenness, surface solar radiation and water balance exhibited widespread negative correlations. This finding suggests that subtropical vegetation is limited not only by resources but also by excessive surpluses, where cloud cover (diffuse radiation) and moderate moisture are more favorable than direct heat stress and hydrological saturation. Piecewise structural equation modeling at the regional scale revealed that population density surpassed climatic factors to become the strongest negative driver, highlighting a critical human footprint that constrains vegetation recovery. Furthermore, we identified divergent physiological strategies, with Deciduous Broadleaf Forests showing a unique vulnerability to warming and excessive moisture compared to evergreen species. This study highlights the combined influence of anthropogenic pressure and climatic constraints. While affirming the success of conservation policies in promoting forest recovery, our findings underscore the critical need for integrated management strategies that address divergent physiological vulnerabilities and target degraded areas under a changing climate.