Forest greenness stability in response to climate change along forest edge–core gradients

Forest greenness and its interannual variability are key indicators for assessing ecosystem stability and climate sensitivity. Recent studies have mainly focused on the direct greening effects of afforestation and the response of greenness to climate change. However, the extent to which large-scale afforestation in China has reshaped the edge–core gradient pattern of unchanged forests (i.e., forests that remained unchanged from 2001 to 2020), as well as how climate change has influenced greenness and its stability across these horizontal gradients, remains insufficiently explored. This study investigates the relative changes in edge–core gradients of unchanged forests using China’s Annual Tree Cover Dataset (CATCD) and Normalized Vegetation Index (NDVI) data, combined with multiple linear regression and SHapley Additive exPlanations (SHAP) analysis. It also quantifies the contributions of various climatic factors to greenness across different gradient levels and their impacts on the stability of greenness. Our results show that the proportion of unchanged forests located >1 km from the edge increased from 13.12 % in 2001 to 25.80 % in 2020, nearly doubling, indicating improved forest connectivity due to afforestation. NDVI steadily increased with gradient distance, while the coefficient of variation of NDVI (NDVI_CV) trends was significantly negative (p < 0.05), indicating enhanced greenness stability with increasing gradient and over time. Grid-based multiple linear regression analysis revealed that temperature was the dominant factor influencing greenness within 0–2 km, with relative contributions exceeding 35 %, much higher than other factors. SHAP analysis revealed that the variation of solar radiation (SR) and the slope of the coefficient of variation of solar radiation (CVSR_Slope) was the most important factor affecting NDVI_CV variability across all gradients. Our study highlights the shaping effect of afforestation on forest spatial gradients and its indirect impact on greening, emphasizing the key roles of temperature and radiation in driving greenness and stability across forest gradients.