## 3 Results
### The Relationship between \(SIF_{canopy}\) and \(APAR_{chl}\) Based on the Simulated Data
#### 3.2.2 Effect of LAI on the Relationship between \(\mathrm{SIF}_{canopy}\) and APAR\({}_{chl}\)  
Using the simulated dataset, the response of \(\mathrm{SIF}_{canopy}\) to APAR\({}_{chl}\) at both bands was also investigated for canopies with different values of the LAI (see Figure 6). It can be seen that, at the O\({}_{2}\)-A band, this relationship varies very little as the LAI changes, especially for dense vegetation (LAI \(>\) 3). However, for the O\({}_{2}\)-B band, plotting the relationship between \(\mathrm{SIF}_{canopy}\) and APAR\({}_{chl}\) produces a very scattered distribution (Figure 6b) because it is dominated by Cab as illustrated in Figure 5b. Figure 5: The ratio of \(\mathrm{SIF}_{canopy}\) to APAR\({}_{chl}\) for canopies with different Cab content at the O\({}_{2}\)-A (**a**) and O\({}_{2}\)-B (**b**) bands based on the simulated dataset. Figure 6: The relationship between \(\mathrm{SIF}_{canopy}\) and APAR\({}_{chl}\) at the O\({}_{2}\)-A (**a**) and O\({}_{2}\)-B (**b**) bands for different LAI values, as obtained using the simulated dataset. The ratio of \(\mathrm{SIF}_{canopy}\) to APAR\({}_{chl}\) at the O\({}_{2}\)-A band increases with LAI (Figure 7a). An opposite trend is observed at the O\({}_{2}\)-B band (Figure 7b). Except for the low Cab condition at the O\({}_{2}\)-B band, the ratio of \(\mathrm{SIF}_{canopy}\) to APAR\({}_{chl}\) at both bands is almost saturated for values of the LAI greater than 3 and the influence of the LAI decreases as Cab increases. This can be contributed to the effects of reabsorption and scattering at different wavelengths. At the O\({}_{2}\)-B band, the reflectance is very low (<5%) and the reabsorption effect of \(\mathrm{SIF}_{canopy}\) increases with larger LAI values; therefore, the ratio of \(\mathrm{SIF}_{canopy}\) to APAR\({}_{chl}\) decreases with increased LAI. At the O\({}_{2}\)-A band, the leaf reflectance and transmittance is high (40-60%) and the reabsorption effect due to \(\mathrm{SIF}_{canopy}\) can be neglected. Therefore, the \(f_{esc}\) increases with larger LAI and is determined by the scattering mechanism at the near infrared (NIR) band. 2.3 Effect of Plant Structure Type on the Relationship between \(\mathrm{SIF}_{canopy}\) and APAR\({}_{chl}\)  
To explore whether the relationship between \(\mathrm{SIF}_{canopy}\) and APAR\({}_{chl}\) depends on the plant type, a simulation using different leaf angle distribution functions was carried out to investigate the relationship between \(\mathrm{SIF}_{canopy}\) and APAR\({}_{chl}\) at both the O\({}_{2}\)-A and O\({}_{2}\)-B bands. The results are illustrated in Figure 8. Six plant types, described in terms of different values of LIDFa and LIDFb, were used in the simulation. LIDFa determines the leaf inclination whereas LIDFb describes the variation in leaf inclination. At the O\({}_{2}\)-A band, the slopes for the relationships shown in Figure 8 range from 0.0035 \(\mathrm{\SIUnitSymbolMicro m}^{-1}\) sr\({}^{-1}\) for erectophile-type plants to 0.0067 \(\mathrm{\SIUnitSymbolMicro m}^{-1}\) sr\({}^{-1}\) for planophile-type plants. Similarly, for the O\({}_{2}\)-B band, the slope values are 0.0017 \(\mathrm{\SIUnitSymbolMicro m}^{-1}\) sr\({}^{-1}\) and 0.0046 \(\mathrm{\SIUnitSymbolMicro m}^{-1}\) sr\({}^{-1}\) for erectophile-type plants and planophile-type plants, respectively. This means that the fraction of APAR\({}_{chl}\) re-emitted by chlorophyll fluorescence and observed at the top of the canopy is higher for planophile-type plants than erectophile-type plants when the same amount of PAR is absorbed by chlorophyll. For planophile plants, the fraction of fluorescence photons that escape from the lower leaves is much lower than from the top leaves. Therefore, the increase in the slope of \(\mathrm{SIF}_{canopy}\) against APAR\({}_{chl}\) as the LAD (leaf angle distribution) becomes more horizontal can be considered due to the different PAR distribution within canopies with different canopy structures.