## 4 Discussion  
Vegetation index is the simplest and most effective measure of vegetation cover and growth. At present, hundreds of vegetation indices have been developed ([[https://www.indexdatabase.de/db/i.php](https://www.indexdatabase.de/db/i.php)]([https://www.indexdatabase.de/db/i.php](https://www.indexdatabase.de/db/i.php))). The combination of vegetation index and ground-measured data has been widely used in grassland production estimation research [22]. Due to differences in topography and elevation between different study areas, the vegetation index is not universally applicable. In this study, four indices--NDVI, EVI, MSAVI, and PsnNet--were selected for grassland research. Multivariate regression analysis was conducted to consider the influence of precipitation factors and establish grassland production estimation models. Compared with other models, the overall estimation accuracy  
Figure 4: Interannual variation in grassland production along the China–Mongolia Railway in Mongolia from 2006–2015. of the models based on MSAVI and NDVI was better than that of other models, with average accuracies of 73% and 74.67%, respectively. The average estimation accuracy based on PsnNet and EVI was 71% and 65%, respectively. The optimal model was an exponential model based on MSAVI with a significance level of 0, a coefficient of determination of 0.72, and an accuracy of 78%. MSAVI has the feature of being suitable for different vegetation coverage, and can eliminate or reduce the noise of soil background [23]. It is scientific and reasonable to apply it to the grassland production estimation in this study area. The results of this study are consistent with the changing trend of vegetation coverage in Mongolia or the Mongolian Plateau, as demonstrated by other research. For example, [PERSON] et al. [24] studied the spatial distribution pattern of grassland plants in Mongolia, and found that the grassland plant species along Ulaanbaatar to Tov and Dundgovic decreased gradually; and [PERSON] et al. [25] studied the spatial distribution of vegetation cover on the Mongolian Plateau from 2000 to 2012. Their findings are consistent with the distribution of grassland production found in this study. [PERSON] et al. [26] found that the vegetation coverage of the Hentiy Mountain region and the eastern part of Hangay Mountain in the Mongolian Plateau increased significantly from 2000 to 2013, while [PERSON] et al. [27] found that the vegetation coverage of Hangay Mountain in Mongolia and its adjacent areas that extend southeast to the Dornogovi region increased significantly from 2001 to 2010. This is consistent with the change in trend of grassland production in corresponding areas during the same period found in this study. From 2006 to 2015, grassland production along the Mongolian portion of the China-Mongolia Railway increased slightly, from 2889.99 kg/ha in 2006 to 4036.78 kg/ha in 2015. The regional characteristics of negative growth of grassland production were apparent and mainly distributed in the southeast of the Tov Province, southwest of the Sukhbaatar Province and south of the Dornogovi Province. Grassland production in the Gobi area in the southern portion of the study area and the transitional areas from desert steppe to steppe and dry steppe decreased significantly. The expansion of grassland degradation transitional areas reflected that the risk of grassland degradation still exists. The areas with decreased grassland production along the railway were mainly in the Sainshand-Bor Undur Railway, the Choir-Ulaanbaatar Railway, and the Zonghara-Sukhbaatar Railway. Nowadays, research indicates that climate change, grazing activities, and rat infestations are the main causes of grassland degradation [28; 29]. The study area is located in arid and semi-arid areas, with uneven distribution of precipitation and significant zonality. These factors, combined with fragile ecological conditions, make the area sensitive to climate change.