## Materials and Methods  
### Study Area and Data Sources  
The ecotone between agriculture and animal husbandry is bound by a 400 mm precipitation line, but the precipitation is largely random and sudden, resulting in annual differences in the location of the 400 mm precipitation line. Therefore, on the basis of the precipitation data collected since 1951 by 2474 meteorological stations in Mainland China, this study identified the 400 mm annual precipitation line in each year of the study period and took the fluctuation area of the 400 mm annual precipitation line in each year as the study area. Professional meteorological data interpolation software, ANUSPLIN, was used to interpolate the annual precipitation data from weather stations from 1951 to 2015. The interpolated raster of annual precipitation data was used to derive the annual 400 mm precipitation lines for each year. The research area was obtained from the fluctuation range of the 400 mm precipitation line (Figure 1). The study area extends to 303,104 km\({}^{2}\) and transects 17 provinces, including Heilongjiang, Jilin, Liaoning, Inner Mongolia, Shanxi, Shaanxi, Ningxia, Beijing, Hebei, Henan, Shandong, Qinghai, Gansu, Xinjiang, and Tibet. These provinces span the eastern monsoon, northwest arid, and semi-arid regions and the Tibetan Plateau. The unique geographical location and poor climate conditions of the study area lead to an ecologically sensitive environment that is extremely vulnerable to climate change and anthropogenic activities. The occurrence of frequent droughts, the erosion of soils, desertification, and the loss of biodiversity have been more severe in recent years [31]. NPP data with the temporal resolution of monthly from 2000 to 2015 were extracted from the Moderate-Resolution Imaging Spectroradiometer (MODIS) MOD17A3 dataset and based on the Biome Bio Geochemical Cycles Model (BIOME-BGC), the improved Carnegie Ames Stanford Approach (CASA), and the corrected influence of clouds and aerosols from MODIS LAI-FPAR [16]. This dataset was published by the Numerical Terra Dynamic Simulation Group, University of Montana ([[http://www.ntsg.umt.edu/project/mod17](http://www.ntsg.umt.edu/project/mod17)]([http://www.ntsg.umt.edu/project/mod17](http://www.ntsg.umt.edu/project/mod17))). To meet the needs of this study, we used the statistical analysis tool in ArcGIS software (Environmental Systems Research Institute, Redlands, USA) to convert the temporal resolution of NPP data from monthly to seasons and annual. The Land Surface Temperature (LST) dataset with the temporal resolution of annual were provided by the International Scientific & Technical Data Mirror Site, Computer Network Information Center, Chinese Academy of Sciences ([[http://www.gscloud.cn](http://www.gscloud.cn)]([http://www.gscloud.cn](http://www.gscloud.cn))). The MODIS MOD16A2/ET data with the temporal resolution of annual were provided by the National Aeronautics and Space Administration (NASA) and the Numerical Terra Dynamic Simulation Group ([[http://www.ntsg.umt.edu/project/modis/](http://www.ntsg.umt.edu/project/modis/)]([http://www.ntsg.umt.edu/project/modis/](http://www.ntsg.umt.edu/project/modis/))). [PERSON] et al. [32]used the MODIS daily average meteorological reanalysis data and 8-day remote sensing vegetation attributes as inputs to improve the ET algorithm. Their improved algorithm was then used to calculate the net radiation of vegetation and soil by comprehensively considering information such as vegetation coverage, albedo, air temperature, air pressure, and relative humidity. Lastly, the Penman-Monteith model [33] was used to calculate the grayscale remote sensing image dataset of global land surface evapotranspiration. The meteorological monitoring data of all meteorological stations in China from 1951 to 2015 were obtained from the Meteorological Data Sharing Network ([[http://www.nmic.cn/](http://www.nmic.cn/)]([http://www.nmic.cn/](http://www.nmic.cn/))). From 1951 to 2015, the temporal resolution of the meteorological stations' precipitation data is monthly.