## 3 Results  
### Diurnal Patterns in GPP and SIF760  
The diurnal courses of GPP, SIF\({}_{760}\), and PAR during our campaign are shown in Figure 3. Since the canopy spectra were collected at four different positions, four SIF\({}_{760}\) values together with one GPP value were obtained at a time. As shown in Figure 3, both SIF\({}_{760}\) and GPP exhibit similar patterns to that of PAR with lower values in the early morning and late afternoon and higher values in the middle of the day. These values tend to exhibit a single peak curve that follows the variation of PAR. The distinct decrease in GPP at midday that is commonly found in C3 species, known as the \"midday depression\", was not observed in our experiment, which is consistent with the studies by [PERSON] et al. [82] and [PERSON] et al. [25] For C3 species, higher temperatures can decrease the affinity of enzymes for CO\({}_{2}\) and cause the carboxylation enzyme, Rubisco, to be less active, which leads to the midday reductions in carbon assimilation. In addition, the higher evaporative demands at midday reduces the stomatal conductance, which also affects the assimilation rate. In contrast, for C4 species, CO\({}_{2}\) is delivered to Rubisco, which is localized in the bundle sheath chloroplasts, by a metabolic pump that concentrates CO\({}_{2}\)[19]. This mechanism can inhibit photorespiration and promote carbon assimilation. Consequently, C4 species exhibit higher photosynthesis capacities at high temperatures where photorespiration is stimulated. In addition, C4 species generally have lower stomatal conductance than C3 species [19]. Lower conductance coupled with higher photosynthetic capacity in C4 species results in higher water use efficiencies in comparison to C3 species. The similar pattern between SIF\({}_{760}\) and PAR indicates the capacity of SIF in protecting plant tissues from light energy that may be excessive for photosynthesis and could otherwise damage tissues. Although some research has demonstrated that SIF will increase with PAR at low light then decrease when NPQ increases and further increase when NPQ reaches saturation [83], we did not find this kind of variation clearly in our experiment. Actually, only one set of data on 17 July showed slight decreases at approximately 11:30 and 14:30, which might correspond to the aforementioned decreasing trend. However, we also noticed that the SIF\({}_{760}\) values of the four canopy positions had relatively higher diversity on 17 July (as well as 10 July) than on the other days. It is not clear whether it can to be attributed to the NPQ increment or observation bias. Thus, further studies are needed, and both data quality and quantity should be considered before drawing such a conclusion. For our study, we found that SIF\({}_{760}\) showed a pattern with a single peak that followed the variation of PAR as concluded by [PERSON] et al. [25]. We also analyzed the relationships between APAR and both \(\text{SIF}_{760}\) and GPP. Both showed strong linear correlations, and an R\({}^{2}\) of 0.713 (Figure 4a) and 0.591 (Figure 4b) was achieved between APAR and \(\text{SIF}_{760}\) and APAR and GPP, respectively. The averaged \(\text{SIF}_{760}\) values were also generated to reduce some uncertainties introduced during our spectral acquisition. As our measurements were conducted at four different canopy positions, we assumed their mean values could adequately represent the situations of our study site. By analyzing the relationship between APAR and the averaged \(\text{SIF}_{760}\), the R\({}^{2}\) increased to 0.798 (Figure 4c). As illustrated in Figure 4, the slope of the \(\text{SIF}_{760}\)-APAR relationship is approximately the same \(\textit{LUE}_{\text{F}}\), and the slope of the GPP-APAR relationship can be considered as \(\textit{LUE}_{\text{P}}\).