Temporal and spatial variation of vegetation cover of the water-wind erosion crisscross region in the Loess Plateau | 黄土高原水蚀风蚀交错带植被覆盖时空演变分析

صينى. 分析近25年来黄土高原水蚀风蚀交错带植被覆盖的时空演变特征，为黄土高原生态极其脆弱区的环境发展方向及宏观决策提供依据。利用1982-2006年GIMMS（Global Inventory Modeling and Mapping Studies）-归一化植被指数（Normalized Difference Vegetation Index，NDVI）8 km/15 d分辨率数据，统计检验年、季节及月等不同时间尺度上NDVI的演变特征，在ArcGIS软件中表达10年尺度间NDVI均值的空间分布差异，并用面积累积频率曲线表达研究区NDVI值的整体分布及演变状况；同时用时间相关及空间对应法分析研究区NDVI与降水量的时空联系。近25年来，黄土高原地区水蚀风蚀交错带植被NDVI总体呈显著（P0.05）上升趋势，秋季及9-10月NDVI分别呈极显著（P0.01）和显著（P0.05）上升趋势。在10年尺度上，研究区植被NDVI空间分布差异较大，持续性增加面积仅为22.3%，集中分布在陕北吴旗、志丹所在北洛河上游，安塞、延安所在延河地区以及西宁西南部。年降水量与年NDVI均值有较好的正相关关系，月尺度上则存在着明显的滞后效应；在10年尺度上，西宁西南部、吴旗-延安等NDVI持续性增加地区与降水量增加的空间分布比较吻合，而内蒙古东胜附近NDVI改善但降水量呈减少趋势，本区属于鄂尔多斯草原禁牧、退耕还林还草生态恢复区，说明生态环境建设产生了较好的生态环境效益。黄土高原水蚀风蚀交错带25年来NDVI整体呈显著改善趋势，但空间差异较大；其改善程度的空间差异性既有降水的作用，也有人为活动的影响。

إنجليزي. The time-space evolution characteristics of vegetation cover during 25 years of the water-wind erosion crisscross region in the Loess Plateau was analyzed to provide scientific basis for the environmental development and macroscopic decision of ecological fragile region in the Loess Plateau. Utilizing 8 km/15 d resolution GIMMS (Global Inventory Modeling and Mapping Studies)-NDVI (Normalized Difference Vegetation Index) date from 1982 to 2006, NDVI revolution characteristics on year, month and seasonal time scales were analyzed by statistical test, and spatial distribution of average NDVI on 10 years time scale was studied based on ArcGIS. Area accumulation frequency curves was used to manifest the NDVI integral distribution and evolution condition of the study area. The approach of time correlation and space corresponding analysis was adopted to estimate the relationship between NDVI change and precipitation. In recent 25 years, the water-wind erosion crisscross region showed a significant (P0.05) increase trend in NDVI. NDVI also showed very significant (P0.01) increase trend in autumn and significant (P0.05) increase trend in 9-10 months. On 10 years time scale, NDVI had great difference in spatial distribution. Continuous increasing area was only 22.3%, and centralized in northern Shaanxi of Wuqi/Zhidan in upstream of the Beiluohe river, Yanhe river area where Ansai/Yanan is located, and southwest of Xining. There was a positive correlation between annual precipitation and annual NDVI, and NDVI had lag reaction to precipitation on monthes time scale. The region of NDVI continuous increase was corresponding to the spatial distribution of precipitation increase trend except the region nearby Dongsheng in Inner Mongolia where the NDVI increased while precipitation decreased which belongs to the ecological restoration zone of banning grazing and conversion of cropland to forest and grassland of the Ordos grassland. The phenomenon testifies that the ecological environment construction has brought better ecological environment benefit in this region. NDVI indicated significant improvement in recent 25 years in the water-wind erosion crisscross region, but had great difference in spatial distribution. Spatial distribution difference of improvement degree was due to both precipitation and human activity influence.