## 5 Discussion  
As the direct energy source for vegetation photosynthesis, solar radiation directly determines the light use efficiency of vegetation. Therefore, \(\mathcal{E}_{\mathrm{max}}\) should have corresponding spatiotemporal dynamics to the background of spatiotemporal solar radiation variation. The spatiotemporal dynamics of \(\mathcal{E}_{\mathrm{max}}\) were the result of the long-term adaptation of vegetation to variations in solar radiation (especially seasonal variations), and vegetation usually has different \(\mathcal{E}_{\mathrm{max}}\) values under different radiation conditions. The seasonal dynamics of \(\mathit{PAR}\)-\(\mathcal{E}_{\mathrm{max}}\) presented in this paper show a \"U\"-shaped trend, with a large value in spring and autumn and a small value in summer. During vegetation green-up, vegetation has a large \(\mathcal{E}_{\mathrm{max}}\) to make full use of the limited PAR and thus promote vegetation growth and development. However, during the peak growing season, solar radiation is more abundant, and vegetation photosynthesis tends to be saturated (high solar radiation will even reduce photosynthesis) and has a relatively small \(\mathcal{E}_{\mathrm{max}}\). There were similar explanations in the studies of [PERSON] and [PERSON] [32] and [PERSON] and [PERSON] [34]. The study of Propastin and Ibrom [41] in a tropical rainforest found that the LUE model would overestimate vegetation GPP under high radiation conditions if the saturation effect of vegetation photosynthesis on solar radiation is not considered. In addition, some studies showed that \(\mathcal{E}_{\mathrm{max}}\) was different in clear and cloudy skies [37], as well as in sunlit and shaded leaves [31], which partly explains the influence of PAR on \(\mathcal{E}_{\mathrm{max}}\). Some cloudiness indices that regulate \(\mathcal{E}_{\mathrm{max}}\) used in CFlux [26], CI-LUE [27], and CI-EF [28] can also be considered a sort of radiation-regulated \(\mathcal{E}_{\mathrm{max}}\) because the cloudiness index was calculated based on PAR [42]. In this paper, a cubic polynomial function was used to calculate \(\mathit{PAR}\)-\(\mathcal{E}_{\mathrm{max}}\), which is simple to calculate and easy to fit. The fitted curve can effectively characterize the relationship between PAR and GPP in the actual range of PAR variation, and the shape of the fitted curve is consistent with existing studies [32; 33; 34; 36]. The \(\mathcal{E}_{\mathrm{max}}\) can be estimated based on different data and methods, and there are differences in their physical meanings [43]. Some studies estimated \(\mathcal{E}_{\mathrm{max}}\) based on the flux data observed during the peak growing season [44; 45; 46], which obtained the specific \(\mathcal{E}_{\mathrm{max}}\) under radiation saturation. In terms of the full growing season, \(\mathcal{E}_{\mathrm{max}}\) of the peak growing season is only a special case of its seasonal dynamic changes. For example, the specific constant \(\mathcal{E}_{\mathrm{max}}\) used in the EC-LUE model was comparable to the seasonal minimum of \(\mathit{PAR}\)-\(\mathcal{E}_{\mathrm{max}}\) (Figure 3). In the comparison of \(\mathcal{E}_{\mathrm{max}}\) defined by different studies, extra attention needs to be paid to their essential meanings. For example, the study of [PERSON] and [PERSON] [43] indicated that the daily \(\mathcal{E}_{\mathrm{max}}\) exhibits less variation across biome types and seasons, which contradicts the spatiotemporal dynamics of \(\mathit{PAR}\)-\(\mathcal{E}_{\mathrm{max}}\). It is important to note that the \(\mathcal{E}_{\mathrm{max}}\) defined by [PERSON] and [PERSON] [43] contains the FPAR, while the FPAR in \(\mathit{PAR}\)-\(\mathcal{E}_{\mathrm{max}}\) is assumed to be 1. In terms of seasonal trends, \(\mathrm{PAR}\)-\(\mathcal{E}_{\mathrm{max}}\) was consistent with the reference LUE in the study of [PERSON] and [PERSON] [43]. Dynamic \(\mathcal{E}_{\mathrm{max}}\)-based PAR-LUE performed better in GPP estimation than that of constant \(\mathcal{E}_{\mathrm{max}}\)-based MODIS GPP and EC-LUE.