2000—2012年京津风沙源治理区植被覆盖时空演变特征

植被是陆地生态系统的主体,分析长时期植被覆盖变化,有助于揭示陆地生态环境的演变规律。研究以京津风沙源重点治理区的MODIS02B产品为数据源,通过数据处理获得2000—2012年的NDVI(Normalized difference vegetation index)时序数据集,采用线性趋势分析、标准差、Hurst指数和相关系数等方法,分析京津风沙源重点治理区植被覆盖的时空变化特征及影响因子。结果显示:(1)近13年来,治理区植被覆盖总体呈上升趋势(R2=0.70),2012年NDVI值达最大值0.324,比2000年增加了135.62%。但增加速率和幅度各异:北部干旱区(Bbghq)增加速度最快,浑善达克区(Hsdkq)次之,农牧交错区(Nmjcq)植被变化相对稳定。(2)工程区地表植被覆盖改善区域的面积明显大于退化区域,其中得到改善且通过显著性检验(P=0.10)的区域约占总面积的94.31%;Hurst指数分析表明,工程区植被变化整体呈中强持续性特征,面积合计约占64.48%;综合分析表明,工程区植被变化以良性发展为主,特别是强持续性的退化区和弱持续性的改善区值得关注,二者合计占35.27%。(3)人类活动是京津风沙地区植被覆盖上升的重要驱动因素;自然因素中,降水是控制工程区植被生长的主要因子,温度的影响相对较弱。

The spatiotemporal changes of vegetation cover in Beijing-Tianjin sandstorm source control region during 2000-2012

Due to grassland degradation and expansion of desertification, the Northern China continuously suffered from several sandstorms in the spring of 2000. In order to control the desertification trend in Beijing and surroundings and improve the ecological condition and environment in Beijing-Tianjin Sandstorm Source Region (BTSSR), the Chinese government launched the Beijing-Tianjin Sandstorm Source Control Program in July 2000. This program has been working for thirteen years. However, it is unknown that how the vegetation and ecosystems were restored and whether or not the program functioned in the BTSSR. It is necessary to assess the effectiveness of this program. The assessment deals not only with the changes of vegetation cover in the ecological vulnerable area, but also with the effectiveness of the policy. The objective of this study was to analyze the spatiotemporal characteristics of vegetation cover changes and the factors that affect the changes in the core control area of the BTSSR using the time series of normalized difference vegetation index (NDVI) derived from a Moderate Resolution Imaging Spectro-Radiometer (MODIS) product MODIS02B for the time period of 2000-2012. Linear trend analysis, standard deviation, Hurst index and correlation analysis were employed. Results showed that (1) During the past thirteen years, although annual NDVI values fluctuated slightly, overall there was an increased trend of vegetation cover in the BTSSR with the coefficient of determination: R² =0.70. The NDVI value increased with 0.01 per year. In 2012, the maximum NDVI value reached to 0.324 and increased by 135.62% compared to that in 2000. However, the change rate and degree varied from place to place. The greatest increase of NDVI took place in the Northern arid area (Bbghq), then in the Hsdk area (Hsdkq), and the vegetation cover was relatively stable in the agro-pastoral ecotone (Nmjcq); (2) The annual NDVI values in the BTSSR varied spatially. It was found that as elevation increased, the average NDVI increased at the beginning, reached its maximum and then decreased. The minimum value 0.257 was mainly distributed in the areas with elevation of 1000 to 1500m. The NDVI values also increased as the increasing slope; (3) In the program region, the area in which vegetation growing showed improved was obviously larger than the vegetation degraded area. There was an area of 94.31% in which the vegetation cover statistically increased significantly at a risk level of 0.1. Hurst index analysis showed that in the sandstorm control area, overall the vegetation showed growing and the vegetation cover continuously increased. The total area of vegetation growing occupied about 64.48%. The results of comprehensive analysis implied that in the program area the vegetation showed a tendency of healthy development. But, it had to be pointed out that there was still an area of 35.3% in which the vegetation was continuously degraded or only slightly improved; (4) Human activities, especially ecological restoration, were the main factor that drove the increase of vegetation cover. Among the natural factors, precipitation was critical because it limited the growth and spatial distribution of vegetation in the BTSSR. The effect of temperature was relatively weak. All the findings greatly enhanced our understanding of how human activities impacted vegetation cover in arid and semi-arid environments and provided scientific guidelines for the management of ecological restoration programs and future directions.