We aim to improve the predictive mapping of stem volume with airborne laser scanning (ALS) data acquired in Laos by adapting the area-based approach (ABA) to a tropical context. Separating laser returns of bushes from main stories with a cut-off threshold is a step very important to the ABA. The adaptation focused here on applying global and plot-adaptive cut-off thresholds to improve the extraction of canopy metrics. In order to select the optimal global cut-off threshold for removing understory bushes and ground objects, a sensitivity analysis of the modeling efficacy to the global cut-off threshold was conducted in the range from 0 to 5 m at 0.1-m intervals. To account for structural variation between plots, a simple plot-adaptive method was proposed for adjusting the threshold of each specific plot. The results showed that the optimal global cut-off threshold, which implicitly assumed the forest structure being homogeneous for all plots was 3.6 m. A model based on the plot-adaptive cut-off thresholds achieved better accuracy (RMSE 28%) than did the optimal global threshold-based model (RMSE 30%). It is concluded that the ALS-based canopy metrics extracted using the plot-adaptive method describe the structural heterogeneity of tropical forests adequately, whereas the global thresholding method is contingent on the forest structure being simple.
Bibliographical noteFunding Information:
The authors thank the SUFORD project and the Finnish Ministry of Foreign Affairs for arranging for the data acquisition. This research was supported by a Ponsse Grant 2010 managed by the Foundation for European Forest Research (FEFR), with Dr. Chao Zhang''s participation supported by the National Natural Science Foundation of China (Grant No. 31500392). We are grateful to Dr. Nicholas Coops for his constructive suggestions during the development of this study. Last but not least, review comments provided by the editor and anonymous reviewers are sincerely acknowledged.
- airborne laser scanning
- area-based approach
- feature extraction
- forest inventory
- tropical forests