## 2 Materials and Methods
### Data Collection and Preprocessing  
#### 2.2.1 Data to Calculate GPP, NPP, and PNPP  
The photosynthetically active radiation (PAR) data used in this study came from the Global Land Surface Feature Parameters (GLASS) data product, developed independently by the team of Professor [PERSON] at Beijing Normal University. The GLASS PAR data product is derived from the inversion of Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Very High Resolution Radiometer (AVHRR) data, wherein the AVHRR data are estimated using an improved lookup table algorithm and MODIS data are estimated using a hybrid algorithm, providing daily data with a resolution of 0.05\({}^{\circ}\)[32; 33; 34].  
The Normalized Difference Vegetation Index (NDVI) is a commonly used vegetation index and is one of the essential parameters reflecting the growth and nutritional information of crops; it is extensively used in estimating forest productivity [35]. MODIS  
Figure 1: Geospatial location and elevation of the study area.  
Normalized Difference Vegetation Index (NDVI, MOD13A1 V6.1) is a continuous index of NDVI derived from NOAA-AVHRR, calculated based on the bidirectional surface reflectance that has undergone atmospheric correction. These reflectances have excluded water, clouds, heavy aerosols, and cloud shadows, according to the dataset which is of high quality and can objectively reflect vegetation dynamics. The NDVI dataset originates from Google Earth Engine ([[http://earthegine.google.com](http://earthegine.google.com)]([http://earthegine.google.com](http://earthegine.google.com)), accessed on 25 July 2023), with a time resolution of 16 days and a spatial resolution of 500 m, built using the Maximum Value Composite (MVC) method to construct monthly NDVI data [36], further eliminating errors caused by clouds, atmosphere, solar angles, and other interfering factors [37].  
The average temperature and dew point temperature data were both sourced from ERA-LAND, a reanalysis dataset providing a consistent view of the evolution of land variables over several decades, at a higher resolution compared to ERA5. The temperature and dew point temperature data provided by ERA are in Kelvin units, which can be converted to Celsius by subtracting 273.15 during specific calculations; the temporal and spatial resolutions are monthly and 0.1\({}^{\circ}\), respectively.  
The precipitation data utilize version 4 of the precipitation data provided by the Climatic Research Unit (CRU), which was cross-verified at the station level to ensure its accuracy; the temporal and spatial resolutions are monthly and 0.5\({}^{\circ}\), respectively. The annual total precipitation was calculated from the accumulation of monthly data. The Digital Elevation Model (DEM) data were acquired from the SRTM DEM dataset provided by the National Qinghai-Tibet Plateau Data Center, with the dataset primarily being the fourth version created by CIAT (International Center for Tropical Agriculture), utilizing a new interpolation algorithm [38] which better fills the data gaps of SRTM 90. DEM refers to the Digital Elevation Model, which is a physical representation of the terrain's elevation, depicted through an array of ordered numerical values. This method has more effectively filled the data gaps of SRTM 90 (accessed on 26 May 2023 from [[https://data.tpdc.ac.cn/zh-hans/data/acb49](https://data.tpdc.ac.cn/zh-hans/data/acb49) ce8-2 bfe-4 abd-97 ff-e6e727110703]([https://data.tpdc.ac.cn/zh-hans/data/acb49](https://data.tpdc.ac.cn/zh-hans/data/acb49) ce8-2 bfe-4 abd-97 ff-e6e727110703)).  
The land cover dataset used was the MODIS Land Cover Type (MCD12Q1) version 6.1 data product, which offers global land cover types at an annual interval. The MCD12Q1 6.1 version data product is derived through the supervised classification of MODIS Terra and Aqua reflectance data, and this study used its International Geosphere-Biosphere Programme (IGBP) classification scheme, with temporal and spatial resolutions of annually and 500 m, respectively.