Vegetation cover change and its driving factors over northwest China

In this paper the spatio-temporal variation of vegetation cover in northwest China and its driving factors were analyzed with the average method,trend line and correlation analysis during 1982 to 2006 using GIMMS/NDVI data.The annual average NDVI has been increased with a rate of 0.5 %/10 a in northwest China and there is an obvious difference in the space.The trend line slopes of NDVI were higher than 0.000 5 in the Tianshan and Altay Mountains of Xinjiang,the Qilian Mountains of Gansu and the eastern part of Qinghai,and it indicated the vegetation cover was significant increased in these areas.The trend line slopes of NDVI were lower than-0.000 5 in the southern region of Qinghai,Shaanxi and Ningxia border region,parts of Gansu and Tarim Basin,Turpan,Tarim River,Tuoli in Xinjiang,and it indicated the vegetation cover was declined in these areas.The NDVI of woodland,grassland and cultivated land had an ascending tendency during the study period.Studies show that the vegetation cover change was caused by both natural and human activity in northwest China.The natural vegetation(forests,etc.) change was influenced by climate change,while human activity was the main reason to change the artificial vegetation(cultivated land,etc.).The vegetation cover change for different elevation,slope,and aspect was quite different.When the elevation was exceeded 4 000 m,the NDVI increasing trend was very low;the ecological construction project increased the NDVI at the slope of less than 25 °;the NDVI on sunny slope was changed stronger than that on shady slope.The temperature rose significantly in past 25 years in northwestern China by an average rate of 0.67 ℃/10 a,and precipitation increased by an average rate of 8.15 mm/10 a after 1986.It was positive correlation between vegetation cover and temperature and precipitation annual changes.Temperature rising increases evaporation and promotes drying of the soil,which is not conducive to vegetation growth,and the irrigation in the irrigated agricultural areas will reduce the correlation between agricultural vegetation NDVI and precipitation.The improvement of agricultural production level and ecological construction project are the very important reasons to increase the NDVI in northwest China,and the ecological effect of large scale ecological construction project has been presented.