Abstract:

Objective: This research was aimed to investigate the spatiotemporal distribution and coverage changes of forest and grass vegetation in the Yellow River Basin during 2009 and 2018, and to forecast future development trend of the forest and grass coverage in order to provide a basis for scientific protection and precise restoration of forest and grass vegetation, and for the promotion of positive ecosystem succession in the Yellow River Basin. Method: Based on the SPOT/VEGETATION NDVI time series data from 2009 to 2018, the spatial distribution and temporal changes of the vegetation coverage of forest and grass in the Yellow River Basin were studied by using annual mean method, Theil-Sen median trend analysis and Mann-Kendall test. The sustainability characteristics and future development trend of vegetation coverage were analyzed using Hurst index method. Result: In the total area of forestland and grassland in the Yellow River Basin, the area with a high vegetation coverage (NDVI≥0.7) accounted for 42.34%, distributed in aggregation in the Qilian Mountains, the east of Three-Rivers' Origin and Zoige grassland in the upper reaches of the basin, the northern foot of Qinling Mountains in the middle reaches, the south of Huhuanyong line in the Loess Plateau, and the surrounding area of Jinan City in the lower reaches; The area with a medium vegetation coverage area (0.3 < NDVI < 0.7) accounted for 44.92%, mainly distributed in the west of Three-Rrivers' Origin in the upper reaches of the basin and the junction of Ningxia, Inner Mongolia, and Shaanxi in the middle reaches. The area with a low vegetation coverage (NDVI < 0.3) accounted for 12.74%, mainly distributed in the Mu Us sandy land, the Loess Plateau in central Gansu Province, and the southern foot of Yinshan Mountains in the upper reaches of the basin. From 2009 to 2018, the annual mean NDVI of vegetation in forestland and grassland of the Yellow River Basin showed an overall increasing trend, with a growth rate of 0.007 4 a^-1. In the past 10 years, the area with improved vegetation coverage accounted for 81.62% of the total area of forest and grass, of which the area of significant improvement accounted for 33.36%. The area with vegetation degradation accounted for 13.86%, which was basically the same as the area with a low vegetation coverage. In the future, 61.89% of the vegetation coverage will develop in a positive direction, 33.60% in a negative direction, and 4.51% in a stable and uncertain direction. Conclusion: The vegetation coverage in forestland and grassland in the Yellow River Basin is high in the southeast and northwest mountain areas, and low in the north. From 2009 to 2018, the improved area is much larger than the degraded area, mainly concentrated in the Qilian mountain forest area and Zoige grassland in the upper reaches, Liupanshan forest area, Ziwuling forest area, Huanglongshan forest area, Luliang Mountain forest area and the northern foot of Qinling Mountain in the middle reaches.The degraded areas are mainly distributed in the Mu Us sandy land and the Loess Plateau in central Gansu Province with low vegetation coverage, where is the priority area for harness and restoration. The future development of vegetation coverage in the Yellow River Basin tends to be good in general. It is predicted that the positive development area is much larger than the negative development area. In the negative development area, although the vegetation coverage in Three-Rivers' Origin and Mu Us sandy land has slightly improved, the sustainability is poorer, and there is a possibility of degradation in future, therefore it should be given great attention in the ecological conservation and restoration of the Yellow River Basin.

Key words: Yellow River Basin, NDVI, vegetation coverage, spatiotemporal variation, forestland and grassland