## 4 Discussion  
Previous studies on CUE using remote sensing methods mainly focus on changes at annual scale. In this paper, CUE at the seasonal and monthly scales were investigated. Thus, the change trends of CUE and the climate factors affecting CUE in different growth stages could be explained. CUE was considered to be a constant value regardless of ecosystem types or species [28; 29]. However, this assumption at a global scale might be controversial, because it ignores the influence of environmental factors [30; 31]. [PERSON] et al. [3] estimated global average CUE using site data, which varied widely between 0.201 and 0.822. In this study, the estimated monthly CUE from satellite observations ranged from 0.021 to 0.999 in the SNP. The results suggested that CUE among ecosystems could not be a constant. The assumption of a constant CUE of 0.5 might lead to biased estimates for carbon cycling modelling across temporal-spatial scales. We compared the CUE calculated by the same model of different ecosystems at the annual scale from other reported studies (Table 4). The order of annual CUE of different ecosystems in SNP was as follows: GRA (0.567) \(>\) WET (0.542) \(>\) MF (0.480) \(>\) DBF (0.479) [19]. [PERSON] et al. [3] found the largest CUE for WET on a global scale. [PERSON] et al. [32] estimated the CUE of different vegetation in sub Saharan area and found that the annual average CUE deceased in the following sequence: WET \(>\) GRA \(>\) MF \(>\) DBF. However, in our study, the order of average CUE of the growing season in the SNP was: MF \(>\) WET \(>\) GRA \(>\) DBF. This difference may be due to the different time scales and regions with of the studies. Previous studies indicated that plant CUE might demonstrate a significant seasonal variation. In the short term, such as over one year, the dynamic patterns of carbohydrate storage and plant carbon allocation may lead to great changes in CUE [33]. [PERSON] et al. [7], using biometric methods and vortex correlation techniques, evaluated temporal and spatial variation of CUE in Fagus sylvatica forest and found that CUE in spring was the highest. Artificially grown apples have higher CUE in summer, which may be consistent with the higher accumulation of biomass and the lower respiratory consumption [34]. In contrast, as SNP is at mid-high latitudes, vegetation in the SNP may reduce the consumption of respiration and increase the carbon sequestration capacity in autumn, leading to the highest CUE. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Month** & **Positive Pixels (\%)** & **Negative Pixels (\%)** \\ \hline May & 67,293 (91.60\%) & 6170 (8.40\%) \\ June & 58,717 (79.93\%) & 14,746 (20.07\%) \\ July & 49,180 (66.95\%) & 24,283 (33.05\%) \\ August & 61,691 (83.98\%) & 11,772 (16.02\%) \\ September & 69,366 (94.42\%) & 4097 (5.58\%) \\ October & 44,115 (60.05\%) & 29,348 (39.95\%) \\ November & 20,398 (27.77\%) & 53,065 (72.23\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The number of pixels and their proportions of correlation coefficients between monthly precipitation and CUE. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Month** & **Positive Pixels (\%)** & **Negative Pixels (\%)** \\ \hline May & 25,181 (34.28\%) & 48,282 (65.72\%) \\ June & 48,525 (66.05\%) & 24,938 (33.95\%) \\ July & 43,585 (59.33\%) & 29,878 (40.67\%) \\ August & 39,588 (53.89\%) & 33,875 (46.11\%) \\ September & 7667 (10.44\%) & 65,796 (89.56\%) \\ October & 38,603 (52.55\%) & 34,860 (47.45\%) \\ November & 72,079 (98.11\%) & 1385 (1.89\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The number of pixels and their proportions of the correlation coefficients between monthly temperature and CUE. CUE is regarded as a dynamic parameter, and differs among species of the same biome [35]. In this study, we found that the CUE of MF ecosystems in the SNP had great potential for carbon sequestration in different seasons. GPP and NPP of GRA were very small in spring, resulting in the lowest CUE. In summer and autumn, the CUE of GRA gradually increased.