Exploring the Critical Thresholds of Environmental Factors on Net Primary Productivity in the Yellow River Basin

Net primary productivity (NPP) is an important indicator for assessing ecosystem productivity and carbon cycling. The Yellow River Basin (YRB), as an important ecological conservation zone and economic region in China, is highly sensitive to climate change, land use change, and ecological restoration. Understanding the spatiotemporal variation in NPP and its relationships with environmental factors is therefore important for regional ecological management. In this study, MODIS NPP data, ERA5-Land environmental variables, land use data, machine learning algorithms, and SHAP-based model interpretation were used to analyze the spatiotemporal patterns of NPP and the nonlinear responses of NPP to environmental factors in the YRB from 2001 to 2020. The results showed the following: (1) NPP exhibited a spatial pattern of higher values in the south and lower values in the north. The annual average NPP showed a fluctuating upward trend, and most pixels showed varying degrees of increase during the study period. (2) Moisture-related variables contributed more strongly to model-predicted NPP variations in the entire basin than thermal variables. (3) For different ecosystem types, surface solar radiation downwards (SSRD) made the largest contribution to model-predicted NPP variations in cropland and forest ecosystems and showed a negative relationship with NPP, whereas evapotranspiration (E) contributed most strongly to model-predicted NPP in grassland ecosystems and showed a positive relationship with NPP. (4) Most environmental factors showed nonlinear associations with model-predicted NPP, and SHAP-derived response thresholds differed among ecosystem types. These thresholds should be interpreted as model-based nonlinear response points rather than confirmed ecological tipping points or ecological regime shifts. This study provides a reference for understanding the heterogeneous responses of vegetation productivity to environmental factors in the YRB.