Temporal and Spatial Differences and Its Trends in Vegetation Cover Change over the Loess Plateau
黄土高原地区植被覆盖变化的时空差异及未来趋势

Using 15-day NDVI products of Global Inventory Modeling and Mapping Studies (GIMMS) VGT during the period 1982-2006 and projection pursuit clustering (PPC), standard deviation, linear trend analysis, and the Hurst index, NDVI trends across the Loess Plateau were examined. On the basis of this, general trends in vegetation cover change and its spatial variation in the future were predicted. Results show that 1) over the past 25 years, changes in vegetation cover varied significantly, so it seems to be difficult to draw a definite conclusion about if the coverage of vegetation was improved or degraded. The PPC method can be used to perform evaluation of time series of variables. It was found that vegetation cover over the study region was gradually improved. After 2002, overall, variation in NDVI was found to progressively decrease, which means that vegetation cover improved generally in recent years. It was also found that the PPC method highlights the trend in dependent variables, so it seems to be more suitable for deriving variations of variables. In addition, standard deviation and linear trend analysis were jointly conducted so as to eliminate the inadequacies of the two methods in trend analysis, which can not only reflect the magnitude of changes, but also a change in direction; 2) The Hurst index of NDVI time series in the Loess Plateau over the past 10 years was generally higher, and was significantly different in space. Specifically, it was relatively higher in the east Loess Plateau, but was relatively lower in the west Loess Plateau. It is expected that improvement in vegetation cover will expand to the north Loess Plateau, whereas the degraded areas will likely expand to the south Loess Plateau. 3) The Hurst index of various vegetation types except for the city varied between 0.5 and 1 on average, i.e., a persistent series. Forest land had the strongest persistency; grassland had the weakest persistency. The regional differences resulting from a particular environment and varying management measures of different land use types jointly affect the persistency of vegetation cover. The impact of vegetation cover on climate change often shows some lag effect.