## 3 Results
### Seasonal Changes in CUE  
Considering the dormancy of vegetation in the SNP in the winter months from December to the following February, we calculated CUE in spring (March to May), summer (June to August) and autumn (September to November), respectively. Figure 5 shows the seasonal variation of ecosystem CUE. The regional average CUE was 0.236, 0.835 and 0.854 in spring, summer and autumn, respectively. The highest and lowest CUEs in spring were observed in the year of 2013 (0.299) and 2012 (0.075), respectively. The lowest value of CUE in spring 2012 may be due to the drought of that year [26]. The maximum summer CUE (0.916) was observed in 2004, whereas the minimum value (0.75) occurred in 2011. CUE in autumn reached the peak (0.974) in 2011, while the lowest value was found in 2014 (0.686).  
In most years, average CUE was the lowest in spring. The average CUE values in summer of 2002, 2009, and 2014 were greater than those in autumn, which was related to successive drought from summer to autumn. It was found that the degree of CUE decrease depends not only on the intensity of the drought, but also the duration of the drought intensity and the time of occurrence [27].  
Spatially, spring CUE in the southwest of the SNP was higher than the east during the 15 years (Figure 6). It ranged from 0 to 0.4, with an average value of 0.24 (Figure 6a). In summer, vegetation grew vigorously, and the carbon sequestration capacity of vegetation increased (Figure 6b). Similar  
Figure 4: Monthly mean CUE of different ecosystems from 2001 to 2015.  
Figure 5: Average CUE variations in spring, summer and autumn from 2001 to 2015.  
spatial distribution pattern was observed in summer with an average CUE of 0.83. With the arrival of autumn, the CUE in most regions also increased and ranged from 0.8 to 1.0 with an average value of 0.88 (Figure 6c). In summer and autumn, the carbon sequestration capacity of natural ecosystems was better, indicating the relatively stronger ecological protection function.  
In terms of spatial distribution, the pixels showing an upward trend in three seasons were mainly found in grasslands in the southwest and deciduous broadleaf forest in the eastern fringe. According to the slope analysis, about two-thirds of the study area showed an upward trend of CUE in spring (Figure 7a). CUE in summer tended to increase in 53.7% of the study area (Figure 7b), while the CUE showed increasing trend in the area of 56.7% in autumn (Figure 7c). This increasing trend suggested that the carbon sequestration capacity of natural ecosystem in the SNP could be improving. More NPP accumulated in natural ecosystem may make their ecological protection function stronger. The change trends of CUE (over 90% pixels) passed the significance level test at \(p<0.05\).  
Figure 6: Spatial distribution of seasonal average CUE of the SNP from 2001 to 2015. (**a**) Spring; (**b**) summer; (**c**) autumn.  
Figure 7: Spatial trend of average CUE in each season of SNP from 2001 to 2015. (**a**) Spring; (**b**) summer; (**c**) autumn.  
The seasonal changes of CUE for different ecosystems CUE were also obviously changing in the SNP (Figure 8). In spring, all types of vegetation had low CUE values (Figure 8a). Relatively good hydrothermal conditions in summer were more favorable to vegetation growth and the CUE of each vegetation type was generally increasing (Figure 8b). In autumn, CUE of MF, DBF and GRA continued to rise, whereas WET CUE declined slightly (Figure 8c). Among different ecosystems, CUE of MF had been the highest (spring: 0.288; summer: 0.902; autumn: 0.928).