## 5 Conclusions  
The fraction of absorbed photosynthetically active radiation (fAPAR) is an important parameter in analysing the gross primary productivity of plants at the canopy level and is often equated with the fraction of intercepted PAR, fIPAR. In the case of individual trees found in urban scenes, factors such as the illumination conditions, distributions of sunlit and shaded leaves, and background have a pronounced impact on estimations of fAPAR and fIPAR. In this study, the effect of the above factors on the fAPAR and fIPAR of individual trees were investigated. fAPAR and fIPAR were estimated separately for sunlit and shaded leaves. The sunlit and shaded leaves were automatically classified using an artificial neural network-based model. The results indicate that the illumination conditions and background have a strong effect on the urban individual tree fAPAR, especially when considering the differences between sunlit and shaded leaves separately. The shaded fAPAR was relatively stable with varying diffuse ratio, while sunlit fAPAR decreased with the increase of the proportion of diffuse radiation. fAPAR was also affected by the solar zenith angle, with lower values obtained at an oblique angle. The results also indicate that fAPAR and fIPAR produce different results in an urban setting with individual trees; although, they were considered equivalent when applied to homogenous vegetation and continuous canopies. The differences can be greater than 14% in certain scenarios. Furthermore, both fAPAR and fIPAR are affected by different backgrounds by between 1.5% and 5%, depending on illumination conditions. The differences between these two parameters were influenced by the condition of the leaves (C\({}_{\text{ab}}\) content) as well as by the urban setting. The reported results can deepen the understanding of fAPAR/fIPAR of urban individual trees. They also might facilitate the formulation of guidelines for the estimation of fAPAR during field measurement using traditional portable instruments in an urban setting. In addition, the approach to assessing the radiative budget of individual trees proposed in this work can be a benchmark for evaluating in situ fAPAR measurement techniques. Conceptualization, S.W. and W.W.; methodology, S.W., W.W. and W.L. ; software, T.Y. ; investigation, S.W. and W.W.; resources, A.J.W. and T.Y. ; data curation, W.W.; writing--original draft preparation, W.W.; writing--review and editing, A.J.W., X.-X.L., G.L.F.O. and M.L.M.Y. ; visualization, W.W.; supervision, S.W. and W.L. ; project administration, A.J.W. ; funding acquisition, A.J.W. All authors have read and agreed to the published version of the manuscript. The project \"Computerised Management of Urban Trees: Tree Inspection Using Satellite Data\" is funded by the Ministry of National Development Research Fund awarded to the National Parks Board, Singapore. * [PERSON] et al. (2020) [PERSON]; [PERSON]; [PERSON] Proton Transfer Reaction Mass Spectrometry for Plant Metabolomics. _Trends Plant Sci._**2020**, _25_, 313-314. [CrossRef]
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