## 4 Discussion  
In this study, we investigated the use of MISR High-Resolution (HR) data products at 275 m spatial resolution--the 36 bi-directional reflectance factor data channels and 12 Rahman-Pinty-Verstraete model parameters (\(\rho_{0}\), \(k\), \(\Theta\) in each of the 4 spectral bands)--to retrieve vegetation structural variables for different forest types, using Random Forest models. This section discussed several findings regarding the performance of multiple scenarios including the BRF multi-spectral nadir (Scenario 0 and 1), multi-angular and single spectral (Scenario 2), single off-nadir viewing angle (Scenario 3), combined multi-spectral and multi-angular (Scenario 4), as well as the RPV models (Scenario 5) based on the combined samples from the Lowveld and iSimangaliso St-Lucia sites. First, the RF model results based on the multi-spectral MISR-HR BRF measurements at nadir (Scenario 1 indicated considerable improvement over the traditional two bands approach (Red and NIR only, Scenario 0)) for both vegetation structural parameters. However, the multi-spectral nadir only (Scenario 1) compared to the single-spectral multi-angular model (Scenario 2) showed that equivalent results can be obtained with either spectral or directional data for both mean tree height and canopy cover. This finding was consistent with findings reported by other studies, that single band multi-angular information is not sufficient to estimate vegetation structural parameters [90; 102; 103]. The results obtained for the single off-nadir viewing angle and multi-spectral model (Scenario 3) for both vegetation structural parameters generally showed comparable results to the multi-spectral nadir only looking model (Scenario 1). We also observed that small off-nadir angles (i.e., Af) yielded lower errors than the off-nadir models with larger angles. This could be attributed to the fact that at nadir, or small off-nadir viewing angles, a greater proportion of gaps is visible which would result in increased reflectance as well as more structural information due to the higher visibility of the shading associated to woody components closer to the ground. Inversely, at larger viewing angles, the canopy gaps and shadows are less visible due to a change of perspective (side looking) and the canopy surface could be perceived as being smoother and more homogenous irrespective of height and cover [28; 104]. The low accuracies obtained for both the single band multi-angle or single off-nadir multi-spectral models supported findings from other studies [90; 103; 105; 106], that suggested that using only spectral or single angular information is less accurate in the estimation of vegetation structural parameters, this despite the retrieval of the MISR data at 275 m pixel size. On the one hand spectral reflectance data does not directly measure three-dimensional canopy structure hence it cannot provide explicit information on it [107; 108], but rely on indirect effects such as shadowing. On the other hand single view data is limited in the estimation of vegetation structural data due to the forests' three-dimensional characteristics (i.e., size, shape of canopies, position of trees) which result in varying reflection and distinctive pattern of shadows with varying view angles [23; 25]. Various studies highlighted that single-view is not adequate in capturing forest structure as compared to multi-angular information [44; 90; 103]. The use of all the spectral and directional data (the 36 MISR data channels) in a single model yielded the highest estimates for both the mean tree height and canopy cover. This result was found to support several studies, which found improved retrieval of vegetation structural parameters when using multiple off-nadir angles with multi-spectral bands compared to using spectral data only at nadir [44; 50; 109].