Characteristics of grassland change in the upper Yellow River over the past 40 years based on multi-source data

Grasslands provide key support for the development of animal husbandry in the upper reaches of the Yellow River, profoundly impacting its water conservation capacity. Exploring the spatio-temporal variation and characteristics of abrupt changes in grassland vegetation over the past 40 years is vital for the management and protection of grassland ecosystems in the upper reaches of the Yellow River. This study first extracted the grassland area in the upper reaches of the Yellow River from 13 common land-use and land-cover (LULC) datasets. Using the most accurate LULC datasets and normalized difference vegetation index (NDVI) dataset; trend analysis, cumulative anomaly analysis, Mann-Kendall, and BFAST (breaks for additive seasonal and trend) methods were used to study the trend and mutation characteristics of grassland area and NDVI value in the upper Yellow River. Results showed that: 1) Different LULC datasets show considerable differences in the upper Reaches of the Yellow River, and CLUD-A (China’s annual land use/land cover datasets) datasets can more accurately reflect the change characteristics of grassland areas in the region. 2) The total grassland area in the upper reaches of the Yellow River showed a non-significant (P > 0.05) decreasing trend during the past 40 years; at the spatial grid scale, 93.6% of the grassland area tended to be stable, and a few degraded areas were mainly distributed in the west and east. An abrupt change in grassland area occurred in 5.4% of the regions, and more than 70% of the mutated areas kept decreasing after mutation; The year 2001 indicated an abrupt change in grassland area. 3) NDVI analysis showed that 50.1% of the grassland distribution areas indicated a significant (P < 0.05) increase in NDVI, mainly in the northeast, southeast, and southwest regions of the upper Yellow River. 10.9% of the NDVI was mutated in the grassland distribution area, and more than 75% of the mutated areas continued to increase post-mutation. The year 2005 was the time node for NDVI mutation. In summary, under the comprehensive influence of human activities and climate change, the trend of grassland area and NDVI in the western and eastern parts of the upper reaches of the Yellow River has changed substantially over the past 40 years, and 2001－2005 was the key period of abrupt change. Overall, the grassland area in the upper reaches of the Yellow River showed a non-significant decreasing trend, and the state of grassland growth developed positively.