## 3 Results and Discussions
### Annual Variation of Vegetation Productivity in the Altay Mountains  
From 2000 to 2017, the change in vegetation GPP in the Altay Mountains fluctuated greatly, showing an increasing trend (Figure 4).  
It can be seen from Figure 4 that the vegetation GPP of the Altay Mountains presents an overall increasing trend, but the interannual fluctuation is still large. The annual average value peaked in 2007, 2011 and 2016, and their values were 428.49 g c/m\({}^{2}\), 428.18 g c/m\({}^{2}\) and 446.61 g c/m\({}^{2}\). In 2003, 2008, 2009 and 2014, the values were relatively low, with 385.18 g c/m\({}^{2}\), 384.90 g c/m\({}^{2}\), 384.49 g c/m\({}^{2}\) and 393.10 g c/m\({}^{2}\), respectively.  
In this study, the significance of variation trend of the vegetation GPP in the period of 2000-2017 was also analyzed, and the results are shown in Figure 5.  
It can be seen from Figure 5 that in 73.16% of the study area, the vegetation GPP has an increasing trend; the significantly increased area accounts for 7.72%, which is mainly distributed in the middle and eastern parts of the mountain. However, the vegetation GPP  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**April**} & \multicolumn{2}{c}{**May**} & \multicolumn{2}{c}{**June**} & \multicolumn{2}{c}{**July**} & \multicolumn{2}{c}{**August**} & \multicolumn{2}{c}{**September**} \\ \hline
**GPP/g** & **Proportion/\%** & **GPP/g** & **Proportion/\%** & **GPP/g** & **Proportion/\%** & **GPP/g** & **Proportion/\%** & **GPP/g** & **GPP/g** & **Proportion/\%** & **GPP/g** & **Proportion/\%** \\ \(\textbf{c}\)\(\textbf{c}\)\(\textbf{m}^{-2}\) & &  
shows a decreasing trend in 26.84% of the study area, and is mainly distributed at the western part of the mountain; the significantly decreased area accounts for 0.98%.  
The results of the annual variation rate of the vegetation GPP during the study period are presented in Figure 6.  
It can be seen from Figure 6 that the variation rates of the vegetation GPP in the study area are significantly different. The area with a GPP variation rate of \(-\)2.0-0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\), 0-1.0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) and 1.0-2.0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) accounts for 21.23%, 27.46% and 23.43%. The areas with a GPP variation rate of \(-\)2.0-0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) are mainly distributed in the middle-height area of the western part of the Alday Mountains; the areas with a GPP variation rate of 0\(\sim\)1.0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) are mainly distributed in the western and eastern parts of the mountain; and the areas with a GPP variation rate of 1.0-2.0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) are distributed discretely in the whole part of the mountain. The areas with a GPP variation rate of 2.0\(\sim\)4.0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) account for 18.76%, and are mainly distributed at the lower-mountain area of the middle and east parts of the Alday Mountains. The areas with a GPP variation rate of \(<\)\(-\)2.0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) account for 6.03%, and are mainly distributed in the middle-latitude area of the western section. The area with a GPP variation rate of \(>\)4.0 g c\(\cdot\)m\({}^{-2}\).a\({}^{-1}\) accounts only for 3.09%, which is scattered in the low-latitude areas in the middle and east sections.