Effects of Mixing Ratios on Branch Development in Young Mixed Plantations of Betula alnoides and Castanopsis hystrix

Branch characteristics (quantity, morphology, and distribution) are critical determinants of tree growth and wood quality. However, the influence of species mixing, particularly mixing ratios, on branch development remains poorly understood. This study examined the branch attributes of Betula alnoides and Castanopsis hystrix in a six-year-old mixed-species trial plantation including monoculture of each species, and three mixtures at ratios of 1:1, 1:3, and 1:5 (B. alnoides–C. hystrix) in Pingxiang, Guangxi, China. Branch quantity (number, proportion, and density), morphology (diameter, length, and angle), and distribution (vertical and horizontal) were measured or recorded from 40 sampled dominant or codominant trees (20 B. alnoides and 20 C. hystrix). The results showed that mixing significantly increased the number and density of branches over 124.2% and 53.2%, respectively, in the lower crown (below 10 m) of B. alnoides, with these metrics positively correlated to the proportion of C. hystrix, while mixing exerted limited effects on branch quantity and size of C. hystrix. The 1:3 and 1:5 mixtures yielded more small branches (diameter < 10 mm) as well as more large branches (>25 mm) for B. alnoides. Branch distribution was almost uniform in different horizontal directions for both species, while variations in branch quantity and morphology along the stem were primarily species-specific; and both aspects remained consistent across the different mixing ratios. In conclusion, mixing B. alnoides with a low proportion of C. hystrix is proposed to produce high-quality solid wood for both species. Future studies should investigate alternative mixing patterns and higher proportions of B. alnoides in mixture with C. hystrix to optimize large-size and high-quality timber production.