2000—2018年西北砒砂岩区植被覆盖度与地形效应

利用2000—2018年的MODIS-EVI数据,基于像元二分模型计算植被覆盖度,利用数字高程模型(DEM)数据从高程、坡度和坡向等方面分析西北砒砂岩区植被覆盖度的地形效应。结果表明: 2000—2018年西北砒砂岩区植被覆盖度(FVC)相对较低,平均按2.43·a^-1的速率递增,FVC正距平年主要有12年,其中以2018年和2013年相对最高,负距平年主要有7年,其中以2001年和2000年相对最低。砒砂岩区FVC空间分布由东南向西北逐渐降低,但不同类型砒砂岩区FVC存在较大差异,其中裸露砒砂岩区FVC相对最低,覆沙砒砂岩区次之,覆土砒砂岩区相对最高;2000—2018年砒砂岩区FVC的平均变化率为0.0031,植被生长状况趋于改善。高程-坡向效应表明,裸露砒砂岩和覆沙砒砂岩区坡向分别在高程≤1000 m和>1500 m处对FVC的影响较大,覆土砒砂岩区坡向对FVC的影响较小,但在高程1200～1300 m处影响较大。坡度-坡向效应表明,砒砂岩在坡度≤15°时,FVC的坡度坡向效应不明显,坡度＞25°时裸露砒砂岩和覆沙砒砂岩的阴坡和半阴坡FVC大于阳坡和半阳坡;覆土砒砂岩坡度＞15°时,半阴坡和半阳坡的FVC大于阴坡和阳坡。

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^-1. 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.