Exploring 3D spatial structure variations of a forest stand caused by Moso bamboo expansion using terrestrial laser scanner

• TLS was an effective instrument for acquiring point clouds of a mixed stand of bamboo and fir. • Highest classification accuracy of 93.08% was achieved combining three type structure features. • S and K were the most sensitive spatial structure indices for monitoring 3D structure variations. • With intrusion of Moso bamboo, mean S and K of stand substantially decreased to 0.28 and 0.16. • Space-for-time substitution was workable for analyzing long-time structure variations of a stand. The rapid expansion of bamboo worldwide has been profoundly impacting on the structure, function, and biodiversity of native forest communities. In studies on these variations, traditional field surveys appear not only time-consuming and laborious, but also insufficiently precise. In this paper, we applied Terrestrial laser scanner (TLS) to analyze spatial structure changes in local forest stand caused by the intrusion of Moso bamboos using the space-for-time substitution method. Three sample plots including a China fir stand, a mixed stand and a pure Moso bamboo stand were selected at an ecotone to represent the different phases of bamboo expansion in Hutou Village, Chongqing, China. Their point clouds were first scanned using TLS, and then classified via combining the generic structure features and the morphology features of crown and stem surfaces of individual plants. Subsequently, six indices, including mingling (M), neighborhood comparison (U), uniform angle index (W), forest layer index (S), intersection angle competition index (UCI), and opening degree (K), were combined to quantify the structural characteristics of those stands and analyze the variations. As a result, the overall classification accuracy of trees and bamboos was improved to 93.08%. The spatial structure indices of the three stands exhibited varying degrees of difference, and S and K were proved most sensitive. When Moso bamboo was at the underground infiltration phase, the mean S of China fir stand was 0.48, indicating the distinction in vertical stratification, and the mean K was 0.21, showing lack of growing space. With the continues intrusion of Moso bamboo, the understory plants nearly died out, resulting in the mean S and K decreasing to 0.28 and 0.16, respectively. When Moso bamboo progressively established its advantage, and entered the dominance maintenance phase, the mean S and K further decreased to 0.22 and 0.12. These demonstrated that the growing space had been seriously reduced, forming a relatively closed bamboo forest structure. Therefore, to effectively manage the expansion of Moso bamboo, it is beneficial to long-term monitoring of forest structure variations using TLS point cloud-derived indices and adopt appropriate intensity and frequency of selective logging.