Knot-related defects are the major cause of timber quality degradation, and diminishing this kind of defects is an important issue in forest management. For the purpose of clear-wood production, knot attributes and branch occlusion of Betula alnoides under natural pruning were investigated in a 14-year-old experimental plantation with five planting densities ranging from 500 to 3333 stems per hectare in southern China, and a total of 1325 occluded branches from 30 trees were sampled and dissected. The mean occluded branch diameter (OBD), radius of knots and branch insertion angle (IA) decreased significantly with increasing planting density. Planting with high stocking density significantly reduced the frequency of thick occluded branches (diameter ≥ 20 mm) while increasing the frequency of small ones (diameter < 10 mm). Branch occlusion time (OT) also tended to increase with decreasing planting density. The results of generalized linear mixed models showed that OBD was the major factor influencing OT, radius of dead portion of knot (RDP), total radius of knot (TRK) and IA. In addition, OT was positively correlated with RDP but negatively correlated with stem diameter growth rate during branch occlusion (SDGR). Silvicultural strategies with appropriate planting density for large-diameter clear-wood production of B. alnoides were discussed.

Branch development in the lower part of stem is critical to both early stem growth and wood quality of the most valuable section of tree, and its regulation through planting density has always been greatly concerned. Here the effect of planting density on branch development was examined in a five-year-old plantation of Betula alnoides with six planting densities (625, 833, 1111, 1250, 1667, and 2500 stems per hectare (sph)) in Guangdong Province, South China. Branch quantity (number, proportion, and density), morphology (diameter, length, and angle), position (height and orientation), and branch status (dead or alive) were investigated for 54 dominant or co-dominant trees under six treatments of planting density after the growth of each tree was measured. Factors influencing branch development were also explored by mixed modelling. The results showed that the mean tree heights of 1250 and 1667 sph treatments were higher than those of other planting density treatments. The quantity of live branches decreased with increasing planting density. However, planting density had no significant effect on the number of all branches, and there existed no remarkable difference in branch number and proportion among four orientations. As for branch morphology, only the largest branch diameter had a significantly negative correlation with planting density. In addition, high planting density significantly increased the height of the largest branch within the crown. Mixed effects models indicated that branch diameter, length, and angle were closely correlated with each other, and they were all in positively significant correlation to the branch height at the stem section below six meters. It was concluded that properly increasing planting density will promote natural pruning, improve early branch control, and be beneficial for wood production from the most valuable section of the stem.

The rainy seasons have been becoming irregular and unpredictable under global warming now. This usually makes seedling preparation more difficult to match transplanting. Here applications of plant growth regulators, paclobutrazol (PBZ) and gibberellins (GA₃) were studied with two trials to regulate growth of Betula alnoides Buch. Ham. ex D. Don container seedlings for the purpose of improving their quality when transplanting delayed. In the first trial five treatments (coded as P1 to P5) were laid out with total dosages of 0 mg (control), 2 mg, 6 mg, 10 mg and 20 mg PBZ per seedling to assess inhibitory effects of PBZ on growth of B. alnoides seedlings. After four and a half months, the second trial was carried out on the seedlings from the optimized PBZ treatments to detect growth recover of the seedlings by GA₃, in which four treatments were arranged with GA₃ dosages being 0, 1/10, 1/5 and 1/3 of the optimized paclobutrazol amounts applied. PBZ applications resulted in a significant decrease of height and root collar diameter growth since approximately one and two months after being treated, respectively. Among these treatments, P5 was the best one in controlling seedling growth, and its inhibitory effects lasted more than four and half months. PBZ treatments differed obviously in total seedling biomass from the control at 96 and 132 days after treatment (dat), and allocated more ratio of biomass into roots. Their root/shoot ratios increased and leaf areas decreased considerably with increasing PBZ dosage, while only P5-treated seedlings showed remarkable difference from the control in chlorophyll contents. The GA₃ recovery showed that 15-day height increments of seedlings treated with GA₃ were twice more than that without GA₃ treatment, and did not differ significantly from the control without PBZ and GA₃ treatment. It is concluded that the combined application of PBZ and GA₃ can be recommended as a potential tool for regulating growth of B. alnoides seedlings, making it flexible to growing seedlings in the nursery.