Background  The implementation of forestry ecological engineering such as the Three-North Shelterbelt System and Returning Farmland to Forests has an important and positive effect on the construction of forest and grass vegetation on the Loess Plateau. In order to explore the distribution pattern of forest and grass vegetation and its main influencing mechanism during the implementation of semi-arid loess area, Wuqi county, the first county of conducting China's Returning Farmland to Forests in northern Shaanxi province, has implemented Returning Farmland to Forests for over 20 years, and was selected as the typical representative area of this study.

Methods  Based on the Landsat series data, the image binary model was used to extract vegetation coverage data of Wuqi county in 1997, 2005, 2010 and 2016. Then these data were processed by correlation analysis and principal component analysis. The relationship between vegetation coverage and spatio-temporal distribution characteristics and its relationship with climate and forest vegetation location factors were studied.

Results  1) From 1997 to 2016, the vegetation coverage of Wuqi county was generally on the rise, and the vegetation coverage from the southeast to the northwest gradually increased. 2) The vegetation coverage structure was significantly improved. The land area with low vegetation coverage was mainly transferred out, and the land area with medium and high vegetation coverage was mainly transferred in. 3) In the long-term sequence, the influence of temperature on vegetation coverage was greater than that of precipitation, and both had hysteresis. Precipitation in winter and spring had an important impact on vegetation coverage. 4) The forest and grass vegetation was mainly distributed in the range of 500-2 500 m from the residential area, 0-800 m from the main road and 0-400 m from the main water system, and had the greatest influence on the distribution position of the forest vegetation in the middle level vegetation cover, and no influence in the low level.

Conclusions  Under the background of the implementation of the project of Returning Farmland to Forests, the living environment of vegetation such as temperature and precipitation has gradually become an major factor affecting the coverage of local vegetation.