| Ulanqab City is located in transitional area between pastoral and agricultural of northern China,and the change of vegetation cover is directly related to the ecological security of Beijing-Tianjin and even North China.In order to reasonably evaluate the effect of ecological construction and monitor the dynamic change characteristics of vegetation cover,in this paper,the spatiotemporal change of the Normalized Difference Vegetation Index(NDVI)and its affecting factors in Ulanchab City in recent 16 years were analyzed.This study was based on the monthly syntheticproduct data of MODIS China 500 m NDVI from 2000 to 2015,combined with the precipitation and temperature data in the same period,Land use data for 2000 and 2015,DEM data.The trend line analysis,differential method,complex linear regression analysis,residual method and Land use transfer Matrix were used.The results showed that:(1)The vegetation NDVI in Ulanqab City varied in a range of 0.354-0.555 during the period of 2000-2015,and the overall trend fluctuated and its spatial distribution was obvious.was high in the southeast but low in the northwest.(2)The vegetation types in Ulanqab City were in a trend of improve-ment ?degeneration ? improvement during the periods of 2000-2004,2005-2009 and 2010-2015,and the degenerated vegetation was regenerated slowly.(3)From northwest to Southeast,the change of MRD of vegetation in Ulanqab city is less than the average,equal to the average,higher than the average,accounted for 40.88%,13.25% and 45.87% of the study area respectively;three types of SDRD stable area,sub stable area and unstable area are staggered in the whole city,accounted for 29.40%,63.23%and 7.37% respectively;The representative points of vegetation time stability and sub stability accounted for 2.67% and 9.56% of the total area of the city respectively.(4)Vegetation growth in Ulanqab City was affected by both precipitation and temperature.There was a significantly positive correlation between the NDVI and the annual precipitation(r=0.730,P=0.001),which passed the significance test at 0.01 level,but a negative one between the NDVI and the temperature(r=-0.351,P=0.182),and the correlation between the two climatic factors was not significant.Without considering the influence of human activities,the contribution of precipitation and temperature to vegetation NDVI was gradually increased from 0.213 to 0.805 from the northwest to the southeast.(5)The average NDVI of vegetation in the study area increases with the elevation and the slope,but the slope direction has little influence on NDVI,and the average NDVI of each slope direction is small.(6)The land use types of Ulanqab city generally showed a change pattern of reduced cultivated land and unused land area,increased grassland,forest land,residential land and water area,from 2000 to 2015;the change of NDVI mean value caused by the mutual conversion of various land use types was greater than 0,among which the mutual conversion between cultivated land and other land use types contributed the most to the change of NDVI mean value.(7)The areas where human activities played a role in reducing vegetation index and increasing it accounted for 55.92% and 44.08% of the total study area respectively. |
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