Spatio-Temporal Patterns of NDVI and Its Influencing Factors Based on the ESTARFM in the Loess Plateau of China

The Loess Plateau is ecologically vulnerable. Vegetation is the key factor in ecological improvement. The study of the distribution patterns of vegetation and its impact factors has important guiding meaning for ecological construction in the region. The existing single sensor cannot provide long-term and high-resolution data. We established data of NDVI with a great spatial resolution by fusing the GIMMS NDVI and the MODIS NDVI based on the ESTARFM. Furthermore, we analyzed the variation in NDVI under different topographies and its response to climatic factors and human activities in the Loess Plateau. The results manifested that: (1) The fused NDVI by the ESTARFM had a high correlation with the MODIS NDVI and can be used in subsequent studies. (2) The multi-year average NDVI of this region ranged from 0.027 to 0.973, which is specifically low in the northwest and high southeast. The NDVI manifested an upward trend in the last 31 years. Its growth rate was 0.0036/a (p < 0.01). Spatially, the area with an upward trend of NDVI accounted for 89.48% of the plateau. (3) For topography, the larger area with the extremely significant upward of NDVI was found at elevations of 500-1500 m, with slopes of 6-15 degrees. The larger area with the extremely significant downward trend of NDVI was found at an elevation of higher than 3000 m, with a slope of greater than 35 degrees. (4) The response of the NDVI to the climatic factors manifested a significant spatial heterogeneity. The temperature had a more significant impact on NDVI than precipitation. (5) Human activities contributed more to NDVI than climatic factors (65.22% for human activities and 34.78% for climatic factors). Among them, the area with a high contribution of human activities to NDVI increase was consistent with the area where the GGP was implemented. The distribution of areas with high contribution of human activities to NDVI decrease was in line with that of the provincial capital cities. The results served as the theoretical foundation for assessing the efficacy of environmental stewardship and for optimizing ecological restoration measures.