Accurate stem-level volume estimation at large scale is highly desired in temperate natural forests due to their economic and ecological significance. Mobile Laser Scanning (MLS) systems (e.g., handheld or backpack) offer the ability to efficiently capture high-density point clouds over large areas, creating opportunities for automated, large-scale individual stem volume estimation. However, effective algorithms that can automatically and accurately analyze the dense and complex MLS point clouds of temperate natural forests are lacking. To address this issue, we propose a novel three-stage method to automatically detect, segment, and reconstruct individual stems, enabling direct volume estimations from MLS point clouds of temperate natural forests. First, a deep learning model is employed to separate understory vegetation from overstory trees, reducing point cloud complexity. Next, we introduce a Bidirectional Section Growing (BSG) method for individual stem detection and segmentation, specifically for the segmentation of merchantable logs and multi-stem scenarios, using a novel Least Squares with Similarity Optimization (LeSSO) algorithm. Finally, the Sector Median Points (SMP) method is developed to reconstruct stem shapes for precise volume estimation. Our method is evaluated on four datasets collected in temperate natural forests across the U.S. and Europe. Experimental results demonstrate its superior performance compared to state-of-the-art algorithms, achieving 89.2% Intersection over Union (IoU) for understory removal, 99.4% F-score for stem detection, 91.5% IoU for stem segmentation, and reconstruction accuracy with a Point-to-Mesh distance of 0.0004 m2 and a Chamfer distance of 0.05 m. Moreover, we record 42 stem locations in the field for one of the U.S. datasets and conduct destructive measurements of section-wise diameters for each of them to serve as independent reference data to evaluate stem detection and volume estimation. Our method is able to detect all 42 trees from the point cloud, and reconstructed stem models yield the most accurate section-wise diameter estimates with a Root Mean Square Error (RMSE) of 2.27 cm and R2 of 0.96, and best volume estimation with RMSE of 0.18 m3 and R2 of 0.97. Our method paves the way for automated and accurate estimation of merchantable stem volume from MLS point clouds collected in complex temperate natural forests.
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