近15年新疆伊犁河谷草地退化时空变化特征

以伊犁河谷为研究区,利用MODIS NDVI数据及像元二分模型,反演草地植被覆盖度,以草地植被覆盖度为评价标准,结合数字高程模型(digital elevation model,DEM)数据及Getis-Ord Gi*冷/热点分析方法,对伊犁河谷2001-2015年草地退化的时空特征进行了分析。结果表明,1)受持续过度放牧及气候条件影响,2001-2015年伊犁河谷草地整体持续退化,15年内退化草地比例达46.18%,但退化以轻度为主;2)空间上退化草地的分布范围逐步向高海拔区域扩展,海拔1 500-3 000 m的中山和中高山区退化草地扩张最明显;3)草地生态保护政策的实施减缓了草地退化速度,草地退化与改善的空间差异逐渐明显,以退化为主的单一变化趋势有所改变;4)利用NDVI反演植被覆盖度对草地退化进行评价的方法存在对高植被覆盖区域草地退化敏感性相对较弱的缺陷。

Spatiotemporal dynamics of grassland degradation in Yili Valley of Xinjiang over the last 15 years

Based on data of fractional coverage extracted from MODIS NDVI data and a dimidiate pixel model,and with the assistance of digital elevation model data and Getis Ord Gi* cold/hot spot analysis method,spatiotemporal variations of grassland degradation during the 2001-2015 period in Yili Valley were investigated.The results showed:1) Affected by persistent overgrazing and variation in climatic conditions,grassland degeneration in Yili Valley exhibited a steady declining trend from 2001 to 2015.The proportion of degraded grassland rose to 46.18％ within 15 years,but was primarily lightly degraded.2) Spatially,degraded grassland expanded to the high-altitude area gradually,where the expansion of the degraded grassland located in the middle and sub-high mountain areas within 1 500-3 000 m above sea level was the most obvious.3) The policies of ecological conservation implemented in the grassland slowed down the rate of grassland degradation.Spatial differences between degraded grassland and improved grassland became distinct gradually,hence the trends in grassland dynamics caused by degradation were altered.4) The method of evaluating grassland degradation by utilizing vegetation coverage inversed from NDVI suffered from weak sensitivity to grassland degradation in high vegetation coverage areas.