[Fractional vegetation cover and topographic effects in Pisha sandstone area of Northwest China in 2000-2018].

Based on MODIS-EVI data and binary pixel model, we calculated fractional vegetation coverage (FVC) in Pisha sandstone area of Northwest China from 2000 to 2018. The topographic effects on fractional vegetation coverage was analyzed from elevation, slope, and aspect using digital elevation model (DEM) data. From 2000 to 2018, FVC in Pisha sandstone area of Northwest was relatively low, which increased with a rate of 2.43·a. The positive anomaly of FVC existed in 12 years, with higher positive anomaly FVC mainly in 2018 and 2013. The negative anomaly of FVC existed in seven years, with the lower negative anomaly of FVC mainly in 2000 and 2001. The spatial distribution of FVC in Pisha sandstone area decreased from southeast to northwest, which differed significantly among different types, with that in bare Pisha sandstone being the lowest, followed by sand-covered Pisha sandstone, and soil-covered Pisha sandstone area being the highest. The FVC in this area had an average increasing rate of 0.0031, suggesting an improved vegetation growth status. Analysis of elevation-aspect effect showed that aspect had a great influence on FVC at the elevation no more than 1000 m and more than 1500 m in bare Pisha sandstone and sand-covered Pisha sandstone areas, respectively. The aspect of soil-covered Pisha sandstone had a small influence on FVC, its influence was relatively large at 1200-1300 m. When the slope of Pisha sandstone area was no more than 15°, the slope-aspect effect of FVC was not obvious. When the slope was more than 25°, FVC of shady and semi-shady slope in bare and sand-covered Pisha sandstone areas was larger than that of sunny and semi-sunny slopes. For soil-covered Pisha sandstone, when the slope was more than 15°, FVC of semi-shady and semi-sunny slopes was larger than that of shady and sunny slopes.