Leaf Area Index Retrieval for Broadleaf Trees by Envelope Fitting Method Using Terrestrial Laser Scanning Data

Most conventional leaf area index (LAI) retrieval methods using terrestrial laser scanning (TLS) data are based on Beer's law and are severely affected by the effects of leaf occlusion and aggregation. Moreover, the correction of LAI using the clumping index (CI) relies on assumptions and is generally not robust. This letter exploits the high spatial resolution and penetration capability of TLS to explore the physical meaning of point cloud data sampling and then model the leaf cluster envelope by the alpha-shape algorithm. Subsequently, the canopy LAI is obtained by counting the surface area of the envelope of each leaf cluster within the canopy and combining it with the projected area of the canopy. The entire process is physically based and introduces a new LAI inversion approach based on the TLS. We tested the approach by simulating the TLS data of 25 synthetic trees with different leaf areas and morphologies to evaluate its robustness. Four strategies were adopted for parameter selection in the envelope modeling step to automate the process of finding the optimal envelope radius and improve the inversion accuracy of LAI. In comparison with the traditional LAI retrieval method based on Beer's law (RMSE% is 47.3%), we found that the method proposed in this letter has a higher inversion accuracy with a minimum RMSE% of 27.7%. Our method is also significantly more robust for high LAI scenes and performs well in scenes with high occlusion and aggregation.