The variability of growing season of different vegetation types in Xilingol League

With the help of the time-series data of NOAA/AVHRR NDVI at 1-kilometer resolution from 1989 to 2008, and the vegetation chart of Xilingol League, this paper abstracted the NDVI time-series curves of eight types of vegetation after filtering the time-series images with the Savitzky-Golay method. After filtering the NDVI time-series curves with the S-G method, the paper estimated the annual beginning, end date and length of growing season of various vegetation using the improved dynamic threshold method, and performed the linear fitting. Finally, the accuracy of estimated phenophases and length of growing season of temperate needlegrass arid steppe was verified according to surveyed groud observations. The results indicated that there was significant difference among all vegetation types in beginning of growing season. Six vegetation types showed advanced trend of change in various degree, while temperate grass and forb holophytic meadow and temperate leaflet deciduous woodland showed delayed trend about 4 and 1 d·20 a-1, respectively. The end of growing season of all kinds of vegetation showed delayed trend in similar degree and the length of growing season of all kinds of vegetation extended. The end of growing season has greater impact on the length of growing season. Both surveyed and estimated values appeared similar trend of change which were 9 and 7 d·20 a-1 in advance, while end of growing season appeared converse. The RMSE between surveyed and estimated values was 16.59 d for the bginning of growing and 14.40 dfor wilting date, respectively.