**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]. Tang 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]. Tang et al. [3] found the largest CUE for WET on a global scale. Khalifa 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 grea<sup>t</sup> changes in CUE [33]. Campioli 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.


**Table 4.** Comparison of estimated CUE at different time scales in different researches.

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 grea<sup>t</sup> 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. This study found that CUE of GRA in summer was higher than that of DBF, possibly because GRA had less investment in plant tissue respiration than that of broad-leaved forest, as reported by Law et al. [36]. Forest types showed high CUE in autumn, because trees with higher carbon storage might be more beneficial to the growth in the next year. After analyzing the abnormal values of different vegetation in different years (Figure 8), this study found that in the spring of 2012, CUE of all types of vegetation decreased to the lowest level, which would be related to different degrees of spring drought occurring in the western part of the SNP region [26]. With lower average temperature in the autumn of 2002, CUE decreased in the SNP as the temperature decreased, along with the CUE value. In the autumn of 2014, the CUE of vegetation decreased significantly, which was associated with moderate drought in the south-central Northeast China [37].

Phenological records can not only directly reveal the changes of natural seasons, but also illustrate the response and adaptation of ecosystem process and results to global environmental changes. Few previous studies have discussed the relationship between phenology and CUE. The phenological metrics that we extracted were similar to the study of Huang et al. [2]. Most of the existing literatures have focused on the relationship between NPP, GPP and phenology. Earlier SOS may extend the growing season longer and lead to an increase in GPP [38]. Similarly, the delay in EOS may also prolong the growth season, causing increases in GPP and NPP [18]; therefore, the CUE value of the vegetation will increase. Vegetation requires relatively less energy to maintain living tissues in lower temperature conditions, resulting in less respiration costs and higher CUE [39]. On the other hand, vegetation growth is generally constrained by the short growing season. Rising temperature could extend the growing season length and significantly increase GPP [12]. The sensitivity of CUE to temperature under lower-temperature conditions is lower because the temperature sensitivities of GPP and autotrophic respiration are of comparable size. In warm regions, especially in the tropics where the growing season is long, by contrast, the respiration consumption of vegetation are higher, leading to a lower CUE [1].

CUE is sensitive to environmental conditions and climate change [40]. Previous studies found that net productivity would increase linearly with higher average annual precipitation and temperature in cold and dry ecosystem [1]. As a function of GPP, NPP and respiration, CUE of vegetation (for instance, forest) may be affected by temperature and precipitation [41]. One reported study suggested that CUE exhibited a decreasing trend with the increase of precipitation when precipitation was less than 2300 mm year–1. CUE showed an increasing trend along temperature when it was between −10 ◦C and 20 ◦C, as well as an increasing trend with rising temperature [1]. In this study, CUE showed an increasing trend from May to July and from August to October, respectively, possibly because the hydrothermal condition was more suitable during those two time periods. Increased precipitation may lead to a higher NPP/GPP ratio [6]. The variations of temperature a ffect both the photosynthesis and Ra rates, resulting in the changes of vegetation CUE [42]. The ratio of NPP to GPP might increase as the annual temperature increased between −10 and 20 ◦C [1], which was partially explained by the findings of this study.

In addition, in recent decades, to improve the local ecological environment and enhance the ecological protection barrier function, the Chinese governmen<sup>t</sup> and local citizens have taken multiple measures and implemented actions for ecological and environmental protection [2]. Ecological and environmental restoration projects such as the "Three-North Shelterbelt Project" and the "Grain for Green Project" have achieved some positive e ffects [43,44]. We used the same method to calculate the CUE of farmland. By comparison, we found that the average CUE of the natural ecosystem in SNP showed a similar variation as that of the internal farmland from 2001 to 2015 (Figure 15). The CUE of farmland and natural ecosystem increased simultaneously. The respiration consumption of vegetation decreased. This also showed that the ecological protection function of natural ecosystem may have been strengthened during the past 15 years.

**Figure 15.** Average CUE variations of natural ecosystems and farmland in the SNP from 2001 to 2015.
