## 3 Results
### Changing Trend in NPP and Driving Factors of the Tibetan Plateau  
The NPP simulated by the CLM5.0 model is shown in Figure 5. The multi-year average of NPP simulated by the CLM model for the period 1979-2018 is 256 (g C\(\cdot\)m\({}^{2}\cdot\)yr\({}^{-1}\)). The NPP shows an increasing trend over 40 years with an average value of 2.38 (g C\(\cdot\)m\({}^{2}\cdot\)yr\({}^{-1}\)). NPP is affected by different meteorological factors; thus, the NPP in the Tibetan Plateau showed large variations over four decades. Generally, the NPP trend that considers all meteorological factors has increased in most areas of the Tibetan Plateau (Figure 6a). Precipitation led to an increasing trend of the NPP in the central and west-central regions, while the decreasing trend occurred in the central-eastern and southeast regions (Figure 6b). Radiation change led to an increasing trend in the central region (Figure 6c). Temperature change led to an increasing trend in the central and east-central regions of the TP, while the decreasing trend in the western and southern regions of the TP (Figure 6d). The interannual change of NPP under consideration of different meteorological factors is shown in Figure 7. As shown in Figure 7, NPP exhibited a noteworthy increase under the influence of precipitation, with a rate of 2.4 (g C\(\cdot\)m\({}^{2}\cdot\)yr\({}^{-1}\)) (\(p<0.05\)). During the period 1979-2018, the interannual variations of NPP under precipitation matched closely with the control experiment, which indicated that precipitation was the main contributor to the interannual variation of NPP. The precipitation accounts for approximately 91% of the NPP variation (Figure 7). Under the influence of temperature, NPP changed at a rate of 0.14 (g C\(\cdot\)m\({}^{2}\cdot\)yr\({}^{-1}\)), indicating a slight upward trend, accounting for 5.3% of the interannual variation of NPP. Conversely, the radiation factor led to a decrease in NPP by \(-0.1\) (g C\(\cdot\)m\({}^{2}\cdot\)yr\({}^{-1}\)), suggesting a weak declining trend and accounting for 3.8% of the NPP  
Figure 4: Spatial patterns of NPP simulated by (**a**) CASA model and (**b**) CLM 5.0 from 2000 to 2018. Figure 3: Comparison of the measured NPP and the NPP simulated by CLM 5.0.  
change. However, it is noteworthy that NPP was not significantly affected by temperature and radiation factors. Figure 5: Trends in the simulated NPP by CLM 5.0 from 1979 to 2018. Figure 6: Spatial trends in NPP in the Tibetan Plateau region for the period 1979–2018 under different environmental factors, white areas indicate invalid values. (**a**) All factors. (**b**) Precipitation. (**c**) Radiation. (**d**) Temperature. Interannual Variation of NPP in Grassmanns with Different Coverage and Response to Meteorological Factors  
The grasslands are the main land use type in the Tibetan Plateau region. The difference in NPP changes across different grasslands in the TP and their driving meteorological factors were analyzed. The alpine grassland in the TP was divided into high-coverage grasslands, medium-coverage grasslands, and low-coverage grasslands. Although grasslands of all three types are widely distributed over the plateau, their distribution patterns vary significantly (Figure 8a). There is a relatively small area of high-coverage grasslands on the plateau, primarily found in the eastern and southern parts of the plateau, while the plateau's central portion is dominated by grasslands with medium cover. grasslands with low cover have the greatest distribution range, and they are widely distributed throughout the TP. As shown in Figure 8b, the high-coverage grasslands, the medium-coverage grasslands and the low-coverage grasslands have an average annual NPP of 306, 259 and 243 (g C\(\cdot\)m\({}^{2}\cdot\)yr\({}^{-1}\)), respectively. The NPP in low-coverage grasslands, medium-coverage grasslands, and high-coverage grasslands increases significantly by 2.61, 2.39 and 1.97 (g C\(\cdot\)m\({}^{2}\cdot\)yr\({}^{-1}\)), respectively.