基于SPOT NDVI的华北北部地表植被覆盖变化趋势

Indicating variation of surface vegetation cover using SPOT NDVI in the northern part of North China

为分析华北北部地表植被覆盖变化趋势,探寻合理土地利用方式,基于1999年1月至2006年12月的SPOT-VEGETATION逐旬NDVI数据,采用国际通用的MVC(最大合成法)获得月NDVI值,用均值法求出年均NDVI数值。在此基础上,用一元线性回归斜率定量描述地表覆盖动态变化,以Hurst指数表示其时间依存性,利用GIS工具表征其空间格局并进行空间统计分析。研究结果表明:近8年来华北北部地表植被覆盖整体得到改善的区域比植被覆盖退化的区域面积大,得到改善的区域约占总面积的66.04%,基本不变的区域约占14.39%,退化区域约占19.57%。各种土地利用类型Hurst指数平均值均为0.5<H<1,即为可持续性序列,林地、耕地NDVI值有持续性增加的趋势,部分草地NDVI值有持续性减小的趋势,须采取相应措施以防草地退化。

To investigate dynamics of surface vegetation, and optimize regional land use structure and pattern, monthly and annual NDVI were computed respectively by using maximum value composites and average method with 10-day NDVI of SPOT-VEGETATION from Jan. 1999 to Dec. 2006, and then variation of surface vegetation cover and its temporal dependence were indicated by the slope of one-variable linear regression and Hurst exponent. The results show that the greening area is larger than the degraded area in the northern part of North China as an indicator of positive or negative slope of NDVI series. The greening area, stable area, and degraded area account for 66.04%, 14.39% and 19.57% of the total area respectively. The dynamics of surface vegetation cover shows significant regional differentiation. For example, vegetation cover would show the decrease trends in transitional zones among Hebei Province, Shanxi Province and Inner Mongolia Autonomous Region, while persistent increase of vegetation cover appears in the Taihang Mountain, Yanshan Mountain and Luliang Mountain. The fact that Hurst exponents of the NDVI time series greater than 0.5 suggests NDVI series are generally persistent and a rising tendency in the past will cause possible increase in the future. So, the variation of surface vegetation cover in cropland, woodland, and built-up area would show the increasing tendency, while grassland would appear a possible decrease. Moreover, dynamics of surface vegetation exhibits obvious difference in different land cover types. The stability of coniferous forest ecosystem is larger than the deciduous broadleaf forest and shrub vegetation, while the stability of grassland vegetation is lower than the forestland system. Influencing factors of vegetation cover such as temperature, precipitation and evaporation differ in different elevations, and so the stability of ecosystem variation presents alti- tude differences. To increase the sustainability of regional agro-forestry ecosystems, more natural factors should be considered above 1000 m, while much more attentions have to be paid to the anthropic factors below 1000 m in landuse planning according to the research results.