Spatial-temporal variation of growing-season NDVI and its responses to climate change over the source region of the Yellow River

The spatial-temporal distribution of vegetation variation in the source region of the Yellow River was investigated by using the MODIS/NDVI data from 2000-2011.Combining the vegetation maps at a scale of 1:1 000 000 and meteorological data,we discussed the relationship between growing-season NDVI and climatic variables.The results were shown as follows: 1) NDVI increased at a rate of 2.75%/10a in the source region of the Yellow River since 2000.Inter-annual variations in NDVI of alpine grassland,alpine meadow,alpine shrub were +2.84%/10a,+2.65%/10a and +2.77%/10a respectively.2) NDVI increased and decreased area covered about 29.39% and 0.98% of the whole study area.The increased area was mainly distributed in upper reaches of Kariqu River and Maqu River,Zhaqu watershed,south of Buqing Mountains,north of Zhaling Lake and the surrounding areas of Eling Lake.NDVI decreased area was mainly distributed in the southeastern mountains of Yueguzonglie River and northern mountains of Kariqu River.Controlled by thermal and hydrological conditions: ①Amelioration area was more in the sunny slope than in the northern slope.②Amelioration area firstly increased and then decreased while the elevation rose up.③Amelioration area rapidly decreased while the slope increased.3) There was a significantly positive correlation between NDVI and climatic factors(temperature and precipitation).The results also revealed that a correlation between NDVI and precipitation in alpine grassland and alpine meadow zone was stronger than temperature,and NDVI had a closer correlation with temperature than with precipitation for the alpine shrub.The main reasons for the positive change of vegetation in the source region of the Yellow River were the heat and wet tendency of climate.