Abstract: We acquired the terrestrial laser scanning (TLS) point cloud data of five permanent plots of Larix olgensis plantations with different thinning intensities [control (CK), four low-intensity thinning treatments (T₁), two medium-intensity plus one low-intensity thinning treatments (T₂), two medium-intensity thinning treatments (T₃), and two high-intensity thinning treatments (T₄)] in the Mengjiagang Forest Farm. Then, we verified the use of TLS to quantify the individual tree Hegyi competition index (CI) in plantations and analyzed the effect of thinning intensities on stem and crown competition in L. olgensis. The results showed that the thinning treatment reduced the competition among L. olgensis trees, with the average competition indices of T₁, T₂, T₃ and T₄ plots being decreased by 0.5%-10.7%, 15.1%-20.6%, 28.6%-34.8% and 39.2%-47.9%, respectively. There were significant differences in the stem-based CI (diameter at breast height, tree height) among different treatments. CI based on crown volume and crown surface area showed significant differences among the thinning treatments. In contrast, the differences in CI based on maximum crown diameter and crown projection area were not significant between the control and low-intensity thinning treatments, though differed significantly between all other treatments. In summary, TLS-based point clouds could quantify individual tree stem and crown competition, and CI based on crown volume and surface area could effectively reflect the competitive relationships within thinned forest stands.

Key words: Larix olgensis plantation, terrestrial laser scanning (TLS), thinning, Hegyi competition index, stem competition, crown competition