Spatio-temporal Dynamic Changes Of Grassland Vegetation Cover And Phenology In Gannan Prefecture

Vegetation cover is an important indicator for reflecting grassland vegetation growth condition, and vegetation phenology is a sensitive indicator of ecological response to climate change. Vegetation cover and phenology play an important role in evaluation and analyzation of vegetation restoration.Using Terra/MODIS product of NDVI (Normalized Difference Vegetation Index) and EVI (Enhanced Vegetation Index) from May to October during2001-2011, combined with ground observed data and meteorological data, the inversion models of grassland vegetation cover were established, their precisions were compared, and the characteristics of grassland vegetation cover were analyzed in growing seasons during2001-2011in Gannan prefecture. Using10-day synthetic vegetation index, the inversion models of grassland vegetation phenology were established, by use of the maximum slope method the green-up, dormancy and growing season lengths (GSL), as well as grassland vegetation cover and EVI dynamic changes in different phenological phases were analyzed, and the influence factors of meteorological factors in phenology was also discussed. Results suggested that:(1)The logarithmic function(y=33.6581n(x)+112.65) of MODIS-EVI is the best model to simulate grassland vegetation cover in Gannan pastoral area, its precision reaches93.31%.(2)MODIS-EVI has more advantages in high vegetation cover region, which can finely reflect the spatial and temporal difference of vegetation cover, and reduce the effect of environmental factors to maximum extent. In recent11years, grassland vegetation cover in Gannan prefecture has an increasing trend. The grassland vegetation cover was78.43%in2005, which reached a maximum value, and the vegetation cover of2001,2003and2008were obviously lower than those in other years. The vegetation cover in the midwest and southwest areas is better than that in the eastern area.(3)Grassland vegetation was dominated by high vegetation cover and higher vegetation cover levels, the proportion of other grassland vegetation cover levels was small (6.98%) and stable. Overall, grassland in Gannan prefecture for the eleven years was characterized by transforming from high vegetation cover into higher vegetation cover.(4)The occurrence time of grassland vegetation green-up is151±9d in Gannan prefecture; the occurrence time of grassland vegetation dormancy is267±17d; the growing season lengths(GSL) is115±11d(about4month).(5) During2001-2011, the range of EVI in green-up is0.23-0.41with a rising trend. In dormancy, the range of EVI is0.31-0.4and there was a downward trend. The maximum value of EVI is0.55-0.63.(6)With increasing temperature, green-up was advanced, dormancy and growing season lengths haves prolonged. Meteorological factors have influence on the phenology of Gannan prefecture. The effect of temperature was greater than precipitation. The impact mechanism is complexity in different grassland types, we need to do further study.