Assessment of land degradation using time series trends analysis of vegetation indictors in Beijing-Tianjin dust and sandstorm source region

Assessment of land condition is a basic prerequisite for finding the degradation of a territory under climatic and human pressures leading to desertification. The temporal change in vegetation productivity is a key indicator of land degradation. In this paper, taking the Beijing-Tianjin dust and sandstorm source region (BTDSSR) as a case, the annual-maximum normalized difference vegetation index (NDVI_max) and net primary production (NPP) dynamic trends during 2001-2010 were analyzed, with the Mann-Kendall test and the Correlation Analysis method. The results showed that the two vegetation indicators (NDVI_max and NPP) had shown a downward trend with the two methods in the past 10 years and the land was in the degradation. For the analysis of the two vegetation indicators (NDVI_max and NPP), it indicated a decreasing trend in 66.60% and 70.26% of the study area according to the Mann-Kendall test and in 59.53% and 64.35% of the study area according to the correlation analysis method. However, the change trends were not significant, the significant trends at the 95% confidence level accounted for only a small proportion. Analysis of NDVI_max and NPP series showed a significant decreasing trend in 6.79% and 12.3% with the Mann-Kendall test. The NDVI_max and NPP change trends showed obvious positive link with the precipitation in the study area.