Effects of Planting Density on Branch Development and Spatial Distribution in Two Elite Chinese Fir (Cunninghamia lanceolata (Lamb.) Hook.) Clones

The development of branches on the lower part of the trunk plays a decisive role in the early growth of trees and influences the wood quality of their most valuable sections. This study investigated the effects of planting density on the horizontal and vertical spatial distribution, branch morphology, and branch quantity characteristics of two elite clones of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), Yang 061 and Yang 020. The results revealed the following distinct responses between the two clones: Yang 061 exhibited significant reductions in branch base diameter and length under a higher planting density, alongside an increased frequency of small branches (<15 mm), but there was no significant effect on branch angles. In contrast, Yang 020 was primarily influenced by genetic factors, with planting density exerting minimal impact. This clone optimized resource allocation by promoting the mortality of lower branches through angle adjustments, resulting in negligible changes to branch development across different planting densities. Pronounced differences in vertical and horizontal branch distributions were observed in areas with significant light disparities, such as the middle crown and northeast direction, for both clones, with increased planting density exacerbating these differences. In conclusion, increasing the planting density of Yang 061 from 2500 to 3333 trees·ha−1 can effectively reduce its branch size; whereas, increasing the planting density of Yang 020 from 3000 to 5100 trees·ha−1 can slightly decrease its branch size without affecting its early rapid growth, thereby achieving a synergistic improvement in both timber yield and quality. These findings highlight the importance of interactions between genetic traits and planting density in shaping branch development and spatial distribution patterns, providing insights to optimize planting density for improving the productivity and wood quality of Cunninghamia lanceolata plantations.