Spatiotemporal dynamics of grassland net primary productivity and its driving mechanisms in the Yellow River Basin

The Yellow River Basin（YRB） is a sensitive area for ecosystems. The grassland ecosystem in the YRB has changed significantly under climate change and human activities. Combing with multi-source remote sensing images and meteorological site data, we estimated the grassland net primary productivity（NPP） in the Yellow River Basin from 2001 to 2018 using the improved CASA model in this paper. After quantitatively evaluating the accuracy of simulated NPP data, we analyzed the spatiotemporal dynamics of grassland NPP and its responses to climate changes and human activities in the YRB. Results showed that 1) the improved CASA-based model could simulate the grassland NPP in the Yellow River Basin with high precision. 2) Grassland NPP in the YRB showed an increasing trend from 2001 to 2018, with a sudden change in 2013. Climate and human activities contributed to the increase of grassland NPP from 2001 to 2013. After 2013, the effect of human activities inhibiting the increase of grassland NPP increased significantly, and the area of the inhibited area increased by 34.89% over the previous period. Overall, the influence intensity of human activities on grassland NPP was lower than that of climate factors. 3) Change of cropland and built-up land area was the important human activity that drove the dynamics of actual NPP. The increase of cropland and built-up land closely related to the decrease of grassland area and the reduction of increase rate of actual NPP. With the development of the economy, the "Grain for Green" program should be persistently implemented, and the supervision of grassland ecological protection should be strengthened.