Combining gradual and abrupt analysis to detect variation of vegetation greenness on the loess areas of China

The annual peak growth and trend shift of vegetation are critical in characterizing the carbon sequestration capacity of ecosystems. As the well-known area with the fastest vegetation growth in the world, the Loess Plateau (LP) lands find an enhanced greening trend in the annual and growing-season. However, the spatiotemporal dynamics of vegetation peak growth and breakpoints characteristics on time series still needs to be explored. Here, we performed tendency analysis to characterize recent variations in annual peak vegetation growth through a satellite-derived vegetation index (NDVI, Maximum Normalized Difference Vegetation Index) and then applied breakpoint analysis to capture abrupt points on the time series. The results demonstrated that the vegetation peak trend had been significantly increasing, with a growth rate at 0.68 × 10·a during 2001–2018, and most pixels (70.81%) have a positive linear greening trend over the entire LP. In addition, about 83% of the breakpoint type on the monthly NDVI time series is a monotonic increase at the pixel level, and most pixels (57%) have detected breakpoints after 2010. Our results also showed that the growth rate accelerates in the northwest and decelerates in the southeast after the breakpoint. This study indicates that combining abrupt analysis with gradual analysis can describe vegetation dynamics more effectively and comprehensively. The findings highlighted the importance of breakpoint analysis for monitor timing and shift using time series satellite data at a regional scale, which may help stakeholders to make reasonable and effective ecosystem management policies.