Analyzing the spatio-temporal variation and drivers of NDVI in upper reaches of the Yangtze River from 2000 to 2015: a case study of Yibin City

In this paper, we chose Yibin City as a typical case, which is the district of relatively fragile eco-environment in the upper reaches of the Yangtze River, to study the dynamic changes of vegetation growth characterized by Normalized Difference Vegetation Index(NDVI) in past 16 years. Based on MODIS MOD 13 Q1 data, we analyzed the variation characteristics in time, variation tendency, and future sustainable characteristics of NDVI by using the Sen + Manna-Kendall nonparametric test and the Hurst index. The spatial variation characteristics were analyzed by the transition matrix and gravity migration model. Furthermore, we explored the dominant factors, and analyzed distribution characteristics and driving forces of the NDVI with application of Geodetector model. The results showed as follows.(1) During 2000 to 2015, the fractional vegetation cover in Yibin City was in a good growth state, with the tendency of gradually optimized from north to south. The area with average annual NDVI more than 0.6 accounted for 52.06%. With the average annual variation rate of 0.007/a, the annual NDVI increased significantly in fluctuation, which accounted for 70.18%. With the Hurst index, the variation of NDVI was not wide. The potential fluctuation area accounted for 53.65% in the future.(2) During 2000 to 2015, vegetation growth in Yibin City was dominated by positive evolution. The evolution process was consistent with the results of previous studies on the effect of eco-construction projects. And the ecological environment was constantly optimized.(3) The NDVI in Yibin City was determined by a series of surface differences, including altitude, average annual temperature, land-use type, and population density. The explanatory power of these factors exceeded 25%. The combination of natural factors and human activities had a more significant impact on NDVI.