## 4 Discussion
### The Temporal Variation Characteristic of SIF
The influence of the canopy structure on the seasonal dynamics of SIF has been seen to be greater than that of the chlorophyll content. However, other research studies have found that there are similar trends in coniferous forests, with little seasonal variation in canopy structures. The canopy structures of coniferous forests have been seen to be basically stable throughout the year, and the chlorophyll content and temperature were seen to be the main causes of seasonal changes [75]. The decrease in SIF in the winter has been observed as due to the seasonal physiological changes mediated by photochemical quenching and non-photochemical quenching [3]. Under sunny conditions, the diurnal variation of the SIF has shown a \"unimodal\" pattern, which was similar to the diurnal dynamics of PAR [76; 77]. As we know, plant photosynthesis is prone to midday inhibition during sunny days. In fact, due to the pressure of high Ta or high VPD, chlorophyll fluorescence at the leaf-scale has occurred as a phenomenon similar to photosynthetic midday depression. In their research on temperate forests, [PERSON] et al. found that there was midday inhibition of fluorescence after they ignored the canopy scattering and reabsorption processes [19]. In this study, top-canopy leaves may have occurred midday inhibition under direct sunlight and high VPD conditions, while inner-canopy leaves were not stressed due to the canopy shading or microclimate. The tower-based system collected fluorescence throughout the canopy, so the leaves inside the canopy were full of this inhibition, which may have been one reason for the absence of midday inhibition. Another possibility is that Chinese cork oak is a drought-resistant species and, thus, the environmental conditions did not reach the threshold for inhibition during the observation period. By observing the diurnal variation trend, we found that the SIF changed asymmetrically in the morning and afternoon, even under the condition of symmetrical PAR. Other research studies on crops have concluded that the asymmetry was caused by the obvious row structure of farmland, and further research has found that the diurnal variation of maize-canopy SIF was caused by the variation of incident solar radiation and solar observation geometry [78] The zenith angle can change the ratio of sun and shade leaves, thus changing the observed canopy structure [79]. The \(\mathrm{f_{esc}}\) is affected by the LAI and the leaf inclination angle distribution, and the growth differences, water stress and wind direction lead to uneven distribution and variation in the canopy geometry [19]. Therefore, in this study, the \(\mathrm{f_{esc}}\) in the afternoon was higher than that in the morning (Figure 4), which may have been caused by changes in the canopy structure, zenial angle and leaf movement, and the asymmetry of APAR also confirmed this possibility. The diurnal inflection points of SIF on the sunny days gradually advanced with the advance in growth; however, this was not found on cloudy and rainy days. Clouds will reduce the total solar radiation and increase the diffuse reflection, possibly greatly reducing the effect of the zenith angle on the \(\mathrm{f_{esc}}\). The \(\mathrm{f_{esc}}\) changed slightly on cloudy and rainy days, which may be the reason why the relationship between SIF and GPP on cloudy days was closer than it was on sunny days.