2) NDVI
The normalized vegetation index (NDVI) is a ratio parameter derived from the reflectance in the near-infrared and infrared bands of remote sensing images, which can be used to determine vegetation coverage, biomass, and the leaf area index (Geerken et al., 2005; Dorigo et al., 2007). The NDVI data used in this study were obtained from NASA’s Land Processes Distributed Active Archive Center (LPDAAC). We selected MOD13C2 (Collection 6) with a spatial resolution of 0.05° × 0.05° sampled monthly between 2000 and 2015. The wavelengths of the near-infrared and red bands were 0.84–0.88 and 0.62–0.67 µm, respectively.
3) Actual net primary productivity (NPP-a)
The actual net primary productivity (NPP-A) data for the period between 2000 and 2014 were obtained from the MOD17A3 product (Collection 5.5), which is provided by the NTSG (Numerical Terradynamic Simulation Group) from the University of Montana. We converted the NPP-A units to g·C·m−2·yr−1. The blank (white) areas in Figure 2 indicate an absence of NPP-A data.