Branch architectural features of Populus euphratica in the lower reaches of the Tarim River using terrestrial laser scanning.

Branch architecture is the spatial structure formed by branching within the canopy, which directly affects tree growth, function, and ecological adaptation. To reveal the morphological adaptation mechanisms of Populus euphratica to extremely arid environments, we used terrestrial laser scanning to scan 51 individual P. euphratica trees in the lower reaches of the Tarim River. We reconstructed the branch structure using the quantitative structural model TreeQSM, extracted the geometric parameters of first- to third-order branches, and analyzed the allometric relationships among different branch orders, vertical distribution patterns of branches, as well as the correlations between architectural parameters (number of branche, length, and angle), and tree structural parameters (tree height, diameter at breast height, crown base height, crown diameter, crown area, crown volume, and crown height ratio). The results showed that the mean numbers of first-, second-, and third-order branches of individual trees were 9.2, 68.1, and 567.1, respectively; the corresponding mean branch lengths were 2.70, 1.18, and 0.30 m; and the mean branch angles were 53.30°, 61.33°, and 62.61°, respectively. With increasing branch order, branch number increased, branch length decreased, and branch angle increased slightly. The ratios of branch number of first- to second-order branches and second- to third-order branches were 1:8 and 1:9, respectively, while the corresponding length ratios were 2.15:1 and 4:1. Branches were mainly distributed within the 3-5 m canopy height range. The lengths of first- and third-order branches decreased with increasing canopy height, whereas second-order branch lengths varied only slightly with canopy height. Among the first- to third-order branches, all architectural parameters except the number of first-order branches increased with increasing diameter at breast height (DBH). The numbers and lengths of first- to third-order branches in P. euphratica exhibited clear allometric growth patterns and vertical distribution characteristics, and DBH was an important structural parameter for characterizing branch architecture. These results would provide support for fine-scale structural inversion and quantitative functional studies of P. euphratica branches.