Seasonal natural pruning and branch growth dynamics in young Betula alnoides: Effects of planting density and crown layer

• Natural pruning of Betula alnoides peaked in the season of leaf senescence and shedding and increased with planting density. • Branch emergence is independent of planting density. • Branch size increment is negatively correlated with planting density and crown layer depth. • Branch length growth is more sensitive to planting density than branch diameter growth. Branch development is critical for tree growth and timber quality, and has received considerable research attention in recent decades. Although competition strongly influences branch morphology, the seasonal dynamic response of branch development to planting density across different crown layers remains poorly understood. To address this, a two-year field study was conducted since April 2017 in a five-year-old spacing trial plantation of Betula alnoides in southern China, with planting densities ranging from 625 to 2,500 stems per hectare (sph). Over the study period, the size and status of 1,885 branches distributed across various crown layers (from 1-2 m to 5-6 m in height) were periodically monitored in situ on 54 dominant or co-dominant trees. Apical branch emergence was also recorded during each measurement. The rate of natural pruning in B. alnoides was positively correlated with planting density, with most abscission occurring during the leaf senescence and abscission season. While branch emergence appeared to be independent of planting density. Both branch diameter and length increments exhibited an obvious increasing trend as planting densities decreased, and this trend was more pronounced in the middle and upper crown layers. Branch elongation was much more sensitive to planting density than diameter growth. Furthermore, the growth dynamics of branch diameter and length were closely synchronized with tree phenology, exhibiting a notable increment during leaf expansion and maturation periods. These findings highlight the importance and effectiveness of using appropriately high planting densities to control branch development in the most valuable stem sections of B. alnoides , and provide practical guidance for silvicultural interventions aimed at promoting high-quality solid wood production for this species.