## 4 Discussion  
### The Effects of ALS Data Characteristics  
#### 4.1.1 Scan Angle and Flight Path  
The scan angle range of acquiring ALS data had obvious effects on the estimation accuracy of the four forest component proportions due to different spatial coverage of point cloud data in a 3D forest stand. For example, obvious differences were found for the estimation result of the four forest component proportions obtained using the ALS- and 3 DSMax-based methods. The total RMSEs of the four forest component proportions were 5.9% and 10.3% for WPA-P2 (-29\({}^{\circ}\) --7\({}^{\circ}\), -3\({}^{\circ}\) -5\({}^{\circ}\)) and WPA-P4 (7\({}^{\circ}\)-19\({}^{\circ}\)), respectively, as reported in Tables 3 and 4. This difference could be attributed to the incomprehensive sampling of a forest plot due to the limited penetrability of the laser beam and occlusion effects among tree crowns based on a single ALS flight line. Similarly, the retrieval accuracy of crown diameter in WPA-P2 (R\({}^{2}\) = 0.67, RMSE = 1.94 m) was better than the result in WPA-P4 (R\({}^{2}\) = 0.60, RMSE = 2.43 m). However, the values of R\({}^{2}\) and RMSE between the ALS- and visual-based tree height results in WPA-P2 (R\({}^{2}\) = 0.89, RMSE = 1.77 m) were similar to the results in WPA-P4 (R\({}^{2}\) = 0.85, RMSE = 1.78 m). It means that the scan angle range had few effects on the segmentation of tree height. This was consistent with the study of [PERSON] et al. (2016). However, more scan angles are preferable to collect a comprehensive forest point cloud data for retrieving crown diameter and estimating their four forest components. For example, the points belonging to the shaded component were identified as sunlit components owing to the incomplete point cloud data and affected the estimation accuracy of the four forest component proportions.  
Moreover, the directions of flight paths have apparent effects on the estimation accuracy of the four forest component proportions. For example, the RMSE of overstory was 7.3% for plot PC-P2 with two crossed flight paths. It was less than the corresponding one at 15.3% for plot PC-P3 with two parallel flight paths (Table 4). Therefore, we recommend two or multiple crossed flight paths to acquire  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Plot\#** & **WPA-P4** & **PC-P2** & **PC-P3** \\ \hline \multirow{7}{*}{Vertical stratification} & Height threshold (m) & 6.4 & 3.2 & 2.5 \\ \cline{2-4}  & Original gap size (m) & 5.0 – 17.0 & 3.0 – 10.0 & 3.0 – 9.0 \\ \cline{2-4}  & Overstory gap size (m) & 4.0 – 16.0 & 1.0 – 7.0 & 1.0 – 6.0 \\ \cline{2-4}  & Original gap size (m) & 143.0 & 75.0 & 53.0 \\ \cline{2-4}  & Overstory gap size (m) & 102.0 & 56.0 & 36.0 \\ \cline{2-4}  & Identification percentage (\%) & 71.0 & 72.0 & 68.0 \\ \cline{2-4}  & Gap size RMSE (m) & 3.9 & 2.5 & 2.9 \\ \hline \multirow{7}{*}{Root mean square error (\%)} & Shaded background & 5.6 & 5.9 & 4.7 \\ \cline{2-4}  & Sunlit background & 9.7 & 8.1 & 8.9 \\ \cline{2-4}  & Shaded overstory & 15.1 & 7.5 & 15.9 \\ \cline{2-4}  & Sunlit overstory & 11.1 & 7.2 & 14.7 \\ \cline{2-4}  & Forest background & 7.6 & 6.9 & 6.8 \\ \cline{2-4}  & Forest overstory & 13.1 & 7.3 & 15.3 \\ \cline{2-4}  & Background and overstory & 10.3 & 7.2 & 11.1 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparisons between the gap sizes obtained from stratified overstory and original ALS data and the root mean square errors of four forest component proportions between ALS- and 3 DSmax-based in three forest plots (WPA-P4, PC-P2, and PC-P3).  
a more comprehensive forest canopy ALS data to ensure the accuracy of vertical forest stratification and further the estimation accuracy of the four forest components.