Abstract: The "Grain for Green Project" initiated by the governments since 1999 were the dominant contributors to the vegetation restoration in the agro-pastoral transitional zone of northern China. Climate change and human activities are responsible for the improvement and degradation to a certain degree. In order to monitor the vegetation variations and clarify the causes of rehabilitation in Shaanxi-Gansu-Ningxia Region, this paper, based on the MODIS-NDVI and climate data during the period 2000-2009, analyzes the main characteristics, temporal-spatial distribution and reasons of vegetation restoration, using methods of linear regression, the Hurst index, standard deviation and other methods. Results are shown as follows. (1) From 2000 to 2009, the NDVI of the study area was improved progressively, with a linear tendency being 0.032/10a, faster than the growth of the Three-North Shelter Forest Program (0.007/10a) from 1982 to 2006. (2) The vegetation restoration is characterized by two fast-growing periods, with an "S-shaped" increasing curve. (3) The largest proportion of the contribution to vegetation restoration was observed in the significantly improved area, followed by the slightly improved area and the significantly degraded area, the last of which is distributed sporadically over southern Ningxia as well as Huanxian, Zhengyuan and eastern Dingbian of Gansu. (4) Climate change and human activities are the two driving forces in vegetation restoration; moreover anthropogenic factors such as "Grain for Green Project" were the main causes leading to an increasing trend of NDVI on local scale. However, its influencing mechanism remains to be further investigated. (5) The Hurst index of NDVI time series shows that the vegetation restoration was sustainable. It is expected that improvement in vegetation cover will expand to the most parts of the region.

Key words: the vegetation cover, "Grain-for-Green Project", vegetation spatial-temporal patterns, climate change, Shaanxi-Gansu-Ningxia Region