## 4 Discussion  
### Temporal and Spatial Patterns of NPP  
NPP is an important characteristic of vegetation vigor and growth, and is susceptible to environmental impacts [20]. Influenced by zonal and other factors, the NPP of grasslands on the TP exhibited great elevational variability, i.e., increasing with elevation below 3400 m and decreasing with elevation above 3400 m. The difference between the mean values of grassland NPP according to elevation could reach more than 10 times, with the high values occurring in the eastern region. This may be due to a combination of vegetation types, climatic conditions, and human activities. The lower elevations of the TP are dominated by alpine meadows, and their productivity is significantly greater than that of alpine steppes distributed at higher elevations. As elevation increases, anthropogenic activities diminish, and disturbance to the grasslands decreases, resulting in an increase in grassland productivity. When the elevation exceeds the optimum height for vegetation growth, the \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Alpine Grassland** & **Alpine Meadow** & **Alpine Steppe** \\ \hline Coefficient of partial correlation between & \(0.266\) & \(0.182\) & \(0.262\) \\ \(\text{NPP}_{\text{slope}}\) and \(\text{Temp}_{\text{slope}}\) & & \(0.221\) & \(0.118\) & \(0.208\) \\ \(\text{NPP}_{\text{slope}}\) and \(\text{Pre}_{\text{slope}}\) & & \(0.221\) & \(0.118\) & \(0.208\) \\ \(\text{NPP}_{\text{slope}}\) and the DEM & & \(-0.492\) & \(-0.608\) & \(-0.37\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The partial correlation coefficients of \(\text{NPP}_{\text{slope}}\) with \(\text{Temp}_{\text{slope}}\), \(\text{Pre}_{\text{slope}}\), and the DEM. Figure 8: The effects of the DEM, \(\text{Temp}_{\text{slope}}\), and \(\text{Pre}_{\text{slope}}\) on \(\text{NPP}_{\text{slope}}\) obtained using a structural equation model (SEM) (**a**) for all the alpine grassland, (**b**) for the alpine meadow, and (**c**) for the alpine steppe. climatic conditions gradually deteriorate to the point of being unsuitable for vegetation productivity [38]. The eastern TP is relatively low in elevation and located on the windward slopes of the Pacific monsoon, providing better thermal and hydrological conditions for vegetation growth [18; 22], and thus is a high-value area for grassland NPP. Over 90% of the grasslands on the TP experienced a positive trend in NPP over the past 22 years, with a mean value of 1.01 \(\mathrm{gC/m^{2}/yr}\), aligning with results from previous studies [29; 39]. The large slopes were predominantly in the east of the plateau and coincided with the distribution of high values of temperature gains or precipitation gains (Figures S2 and S5). This implied a positive feedback of grassland vegetation vitality on climate change in the TP [40]. In terms of the elevation gradient, the positive trend of NPP ascended below 2500 m and declined above 2500 m with increasing elevation, which was associated mainly with the pattern of changes in temperature and precipitation. At low elevations, the increase of temperature in alpine meadows and the increase of precipitation in alpine steppes was enhanced with elevation (Figures S3 and S6), alleviating the limitations of temperature on meadow growth and water on steppe growth to support higher NPP\({}_{\mathrm{slope}}\) with elevation. Above 2500 m above sea level, the NPP\({}_{\mathrm{slope}}\) declined, although temperature and precipitation variations were still positive. On the one hand, the increase in temperature declined and the positive relationship between precipitation and NPP gradually weakened with elevation (Figure S7). On the other hand, the lower background values of temperature and precipitation above 2500 m weakened the activities of biological enzymes [41; 42], and thus the NPP\({}_{\mathrm{slope}}\) declined with elevation.