## 4 Discussion  
[PERSON] et al. [38] have measured canopy reflectance in Jarveslja at two wavelengths (660 nm and 850 nm) at a solar zenith angle of 52\({}^{\circ}\) at 660 nm, and a solar zenith angle of 46\({}^{\circ}\) at 850 nm. In the 660 nm wavelength band, they reported HDRF values of 0.03, 0.02, and 0.02 for \(\beta=19^{\circ}\), \(\beta=55^{\circ}\), and \(\beta=76^{\circ}\), respectively. Our values, measured in Hyytiala, were 0.03, 0.02, and 0.02, respectively. In the 850 nm wavelength band, the HDRF values in Jarveslja were 0.18, 0.15, and 0.12 [38] for scatter angles of 19\({}^{\circ}\), 55\({}^{\circ}\), and 76\({}^{\circ}\), respectively. The corresponding values obtained by us in Hyytiala were 0.27, 0.19, and 0.19. The values are remarkably similar in the red band. The difference in canopy HDRF in the NIR region (850 nm) is likely due to different canopy structures and illumination angles. The dark spot (lowest HDRF) observed by [PERSON] et al. [38] was approximately 25\({}^{\circ}\) off-nadir, which is close to the observation geometry used in the study: with good approximation, we can consider \(\beta=76^{\circ}\) to represent the dark spot of the canopy. We found a strong non-linear relationship between canopy PRI and sunlit fraction for each view angle separately. The relationships could be modeled with the equations provided by [PERSON] et al. [29]. Our empirical findings support that the observed fraction of canopy shadow in the field of view of the sensor is an important driver for spectral BRDF. Further, the dependence between sunlit (or shadow) fraction and PRI is strongly non-linear, and the linear model that has been used by many authors [20; 27; 39] should be used with care. However, \(\alpha_{S}\) is not the only, or not necessarily even the key driver in scattering directionality. The observed maximum spectral divergence among the vegetated pixels caused by \(\alpha_{S}\) for any fixed scatter angle was 0.02 (Figure 9). At a constant \(\alpha_{S}\), we observed much larger spectral divergence values (Table 5). While the two darkest view angles (\(\beta=76^{\circ}\) and \(\beta=55^{\circ}\)) were relatively similar (maximum divergence 0.03), the angle closest to the hot spot (\(\beta=19^{\circ}\)) differed from the other two, with maximum SID values close to 0.04. The relationship between \(\alpha_{S}\) and the PRI was the strongest at \(\alpha_{S}<0.2\), and flattened out when sunlit fraction increased. The mean \(\alpha_{S}\) values for the three view angles were close to or above this threshold. Despite the strong relationship between \(\alpha_{S}\) and the PRI for each view angle separately, the stand mean PRI did not change systematically with the scatter angle. As the PRI--\(\alpha_{S}\) curves for the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & \(\mathbf{f}=\mathbf{19^{\circ}}\) & \(\mathbf{f}=\mathbf{55^{\circ}}\) & \(\mathbf{f}=\mathbf{76^{\circ}}\) & **Combined** \\ \hline PRI\({}_{0}\) & 0.042 & \(-\)0.01 & 0.017 & 0.255 \\ \(Q_{570}\) & 25.6 & 20.0 & 36.6 & 549.3 \\ \(Q_{531}\) & 19.7 & 17.1 & 29.8 & 291.5 \\ \hline \end{tabular}
\end{table} 
Table 6: Non-linear fit (Equation (11)) parameter estimates for \(\beta=19^{\circ}\), \(\beta=55^{\circ}\), \(\beta=76^{\circ}\) and all data combined. Figure 11: Fitted PRI _vs_ sunlit fraction curves for \(\beta=19^{\circ}\), \(\beta=55^{\circ}\), and \(\beta=76^{\circ}\). Symbols denote the location of the mean sunlit fraction for each scatter angle. three view angles were quasi-parallel with similar parameters (Table 6), the location of \(\alpha_{S}\) on the flat part of the curve made the PRI sensitive to other potential interfering factors. The difference between the curves for the three view angles, caused by an unspecified factor, was systematic and apparently independent from \(\alpha_{S}\) (Figure 11). When we combined all of the PRI and sunlit fraction values for all of the scatter angles (Figure 11), the curve was not parallel to the other curves.