Objective To study the effects of different thinning retention densities on the branch growth of Chinese fir plantations, and to provide scientific theoretical basis and technical support for directed cultivation of Chinese fir knotless-wood.

Method Based on the 9-year-old Chinese fir plantations in Guanzhuang state-owned forest farm, Shaxian County, Fujian province, three kinds of thinning retention densities (1200 trees·hm⁻²(thinning intensity: 67% , L) , 1800 trees·hm⁻²(thinning intensity: 51% , M) and 2250 trees·hm⁻²(thinning intensity: 27% , H) were set up. In the fifth year after thinning, standard trees were selected in different plots to study the effect of thinning treatments on the branch growth of Chinese fir.

Result 1)The total number of branches, the number of branches below 8 m, the number of branches above 8 m, the number of dead branches, the number of living branches all under L treatment were the largest, followed by treatment M and H, and the percentage of dead branches under treatment H were the largest, followed by treatment M and L. 2)The basal diameter increased with the decrease of thinning intensity which showed L > m > H, and there was significant difference between L and H (p < 0.05) , which indicated that the lower retention density was more conducive to the branch growth. 3)There was a significant linear correlation between the basal diameter and the length of the living branch, while correlation between the basal diameter and the length of the dead branch was small. 4)The basal diameter, branch length, the number of dead branches and the number of living branches increased with the increase of vertical height, while the basal diameter and the length of living branches and the branching angle of H and M decreased with the increase of vertical height of the trunk. 5)The basal diameter, branch length and branching angle of dead and living branches were larger in the southern region (90～225 °) than in the northern region, but the number of branches was larger in the eastern region (0～180 °) than in the western region.

Conclusion The thinning intensity should be adjusted according to different cultivation purposes. The lower retention density can inhibit the growth of basal diameter and number of branches, thus reducing the size and number of knots and improving the wood quality. In this study, the retention density1800 trees/ha is more suitable to cultivate knotless large-diameter timber.