## 4 Discussion  
### Individual Tree fAPAR/fIPAR and Influencing Factors  
Most previous fAPAR/fIPAR studies were focused on continuous vegetation such as crops and forests [11]. To our best knowledge, this is the first study focused on the urban individual trees. Individual trees have a more complex apparent structure and external environment including illumination and background. This study shows all the above factors would influence the fAPAR/fIPAR. As to the background influence on fAPAR/fIPAR, a previous study shows it is stronger for a clear day (small SKYL) than for a cloudy day (large SKYL) [11]. Similarly, the current study shows that the difference between grass background fAPAR/fIPAR and concrete background fAPAR /fIPAR is smaller with the increase of SKYL (Figure 6). However, this effect can only be clearly observed for radiar solar angle (SZA = 0\({}^{\circ}\)) while it is not obvious for oblique solar angle (SZA = 45\({}^{\circ}\)). Considering the influence of illumination, both fAPAR and fIPAR tend to be more stable with the increase of the diffuse ratio (Figure 3), which is consistent with the results of a previous study which indicated that total diffusion shall reduce sample error in open forests [54]. In addition, this current study shows the fAPAR/fIPAR decrease with the diffuse ratio for both sunlit leaves and shaded leaves for the urban trees (Figure 3 and Table 2). This finding is different from the measurements in rice [28] which indicated that fAPAR and fIPAR under white sky conditions (SKYL = 1) are higher than under black sky (SKYL = 0). The differences might be caused by the condition of the leaves and the multi-scattering of the trunk and branches. Previous study shows that low chlorophyll content leads to a higher fAPAR under direct sunlight conditions and conversely higher chlorophyll content leads to a higher fAPAR under diffuse conditions [12]. This current study finds that the oblique solar illuminations can increase both fAPAR and fIPAR, especially for the shaded leaves (Figure 7). That is probably because the oblique illumination can increase the length of the path to penetrate the canopy which shall increase the chance of the PAR to be intercepted and absorbed. As to the tree structure influence, the distribution of trunk and branches, which are usually characterized by strong reflectance, can decrease fAPAR. At the same time, the multi-scattering caused by the trunk and branch will increase fIPAR. The current study also explores the chlorophyll content influence on the fAPAR and fIPAR (Figure 6). The senescent leaves (C\({}_{\text{ab}}\) = 10) with small chlorophyll content demonstrate lower fAPAR and higher fIPAR. The complexity of the structure shall also determine the sunlit and shaded leaves distribution, which makes the illumination influence even more complex. Thanks to the high resolution of the studied scenes, it was possible to show that the distribution  
Figure 8: Relative differences between fAPAR and fIPAR of single trees with grass and concrete backgrounds at different illumination conditions. of sunlit and shaded leaves within the crown of the tree has a significant impact on both PAR indices. In the investigated scenarios with free-standing trees, sunlit leaves intercept and absorb more PAR than shaded leaves (Figure 3). Consequently, even though sunlit leaves have lower light use efficiency, they may produce more GPP with larger fPAR [55]. Furthermore, the effect of background reflectance is more pronounced in the case of shaded leaves. This is perhaps less significant in scenarios with dense, continuous canopies, but might contribute to accurate estimation of PAR availability in the built environment, where sections of the crown might be predictably shaded by the surrounding structures at different times of the day.