## 4 Conclusions  
The MODIS-GPP product is a global GPP product using remote sensing data, and is developed according to the light-use efficiency principle. The MODIS data can effectively reflect the landscape information on a regional scale. Combined with meteorological data and field observation data, the variation trends of vegetation GPP in relation to meteorological factors can be accurately evaluated by using the MODIS-GPP model.  
The spatial distribution pattern of annual vegetation GPP and growing season GPP of the Altay Mountains was almost consistent. In the whole study area, the proportion of the areas with a vegetation GPP of 400-600 g c/m\({}^{2}\) was the highest, and was mainly distributed in the middle- and high-altitude areas of the middle and west parts of the Altay Mountains with the best plant growth conditions.  
Due to the differences in meteorological factors such as temperature and precipitation in the Altay Mountains, the GPP in different stages of the vegetation growth season (April to September) had a great difference, reaching the highest value in July; the area with a GPP of 100-150 g c/m\({}^{2}\) had the highest proportion, with 36.15%. In April, there was still a large area of snow cover in the high-altitude area of the Altay Mountains, and the vegetation growth conditions were poor. Among the areas with vegetation coverage, the proportion of area with a GPP of 1-5 g c/m\({}^{2}\) was the highest, with 36.73%, and was mainly distributed in the areas below the snow line in the middle-altitude area.  
The vegetation GPP of the Altay Mountains had an overall increasing trend in the study period, but there was a large gap in GPP values in different years--the peak value appeared in 2007, 2011 and 2016, and the lower values appeared in 2003, 2008 and 2009, respectively. In 73.16% of the study area, the vegetation GPP showed an increasing trend, and 7.72% of the area was in the middle and east parts of the mountain, this increasing trend was more significant. However, the decreasing trend appeared in 26.84% of the study area, and was mainly distributed in the west part of the mountain.  
Temperature and precipitation were the most important meteorological factors affecting vegetation GPP in the Altay Mountains. In 71.61% of the area of the Altay Mountains, the vegetation GPP was positively correlated with precipitation, while the vegetation GPP in 64.85% of the study area was positively correlated with the annual average temperature.  
Our study has quantitatively analyzed the long-term dynamics of vegetation GPP and its responses to meteorological factors in the Altay Mountains. The results of our study would be helpful in carrying out further research on the assessment of the degradation of the forest ecosystem, evaluation of the ecosystem function, identification of the key restoration areas, as well as mitigating the impact of various meteorological disasters on mountain forest ecosystems.  
Conceptualization, methodology, validation, formal analysis, investigation, A.A. and H.X.; Software visualization, A.A., P.Z., X.Z. and R.Y. All authors have read and agreed to the published version of the manuscript.  
This study was jointly supported by the Natural Science Foundation of Xinjiang Uyghur Autonomous Region <<Study on the Cooperative Configuration of Oasis Wind break and Sand fixation System in Extremely Arid Areas>> and <<Key technologies for natural forest protection and restoration in 2022 (ZX-2022040)>>.  
Data Availability Statement: Data could be provided on reasonable request from the first author.  
**Acknowledgments:** The authors are grateful to the staff of the Department of Forestry of Altay Prefecture for providing the necessary facilities during the study. We are also thankful to anonymous reviewers for their valuable suggestions and comments on the overall improvement of the manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.