Comparison of the multi-year average AGSI and NDVI at four sites. The NDVI curves were acquired using (a, c, e, g) MODIS NDVI from 2000 to 2017 and (b, d, f, h) SPOT NDVI from 2000 to 2012, which have a temporal resolution of 16 days (n = 23) and 10 days (n = 36), respectively. AGSI values were the maximum values in the corresponding 16-day or 10-day period of NDVI data. The r and P indicate the Pearson’s correlation coefficient and significance level, respectively.