## 4 Discussion
Further, the curve did not approach a meaningful PRI\({}_{0}\) value at \(\alpha_{S}\to 0\). Unrealistic values were also retrieved for other model parameters (Table 6). These results indicate the existence of another factor besides \(\alpha_{S}\) that depends on scatter angle and affects PRI systematically and across all \(\alpha_{S}\) values. When canopy reflectance would be averaged over all vegetation pixels (e.g., to simulate the signal of a medium spatial resolution sensor over a closed canopy), this unspecified factor would dominate over \(\alpha_{S}\) by eliminating the dependence of PRI on the sunlit fraction \(\alpha_{S}\). Although the masks that were used to exclude non-vegetated pixels were different for each scatter angle, not applying a mask (i.e., a true simulation of large-footprint sensor) would not alter the dependence of PRI on \(\beta\) in Figure 3 (data not shown). The masks did not differentiate between the tree species growing in the test site. According to the raster maps from the National Forest Inventory, a small fraction of birch trees were expected at the site. However, based on visual inspection of the AHS images (Figure 2), the presence of broadleaf was small, and the effect on the PRI-shadow fraction analysis can be neglected. When we compared the ratio of Scots pine TOC reflectance between the direction closest to hot spot (\(\beta=19^{\circ}\)) and nadir (\(\beta=55^{\circ}\)) for different sunlit fractions, we observed a high value, especially in the blue region (Figure 8). We hypothesize that this is caused by needle wax [40]. In the visible wavelengths where leaf pigments are highly absorbing, leaf scattering is dominated by specular reflectance at the surface [41]. Specular reflectance, directed mostly in backward directions, increases strongly with decreasing wavelength in the visible spectral region. To test this, we plotted the ratio of Scots pine needle reflectance to transmittance measured in Hyytiala in 2013 [42], and observed a pattern similar with our observed ratio between TOC reflectance for two scatter angles (Figure 12). The variation in leaf reflectance to transmittance ratio in Figure 12 explains the unexpectedly large PRI close to hot spot well. When calculated from laboratory-measured needle transmittance, Scots pine needle PRI in midsummer 2012 was \(-0.008\); when calculated from reflectance, PRI was \(0.014\). In the hot spot direction, canopy reflectance in the visible part of the spectrum (where multiple scattering can be ignored to a reasonable accuracy) is contributed by reflectance only, while in the dark spot, both needle reflectance and transmittance contribute. The two measurements (laboratory measurements of needle spectra and AHS data) cannot be compared directly, as laboratory-measured data are directional-hemispherical, and airborne data hemispherical-directional. However, the two curves show similar features: a local minimum at 550 nm and a local maximum close to 680 nm. The importance of specular scattering in Scots pine canopies is not unexpected. In the laboratory, [PERSON] and [PERSON] [43] have shown that shoot scattering directionality needs to be accounted for when interpreting multi-angular PRI measurements, and that the directionality is likely caused by specular scattering. Obviously, this statement is not only valid for the relatively exotic airborne multi-angular imaging spectroscopy data, but also whenever angular information is applied on spectral remote sensing data, e.g., fitting BRDF models or correcting for angular effects in instruments with wide swaths, and analysis of tower-based PRI measurements. Moreover, the specific structural effects described here need to be accounted for when comparing nadir measurements made at different solar angles, such as for example when analyzing the time series of data from a single location measured from a sun-synchronous satellite.