Objective The effects of site quality and stand density on the growth and timber assortment structure of Chinese fir plantations were investigated to provide theoretical basis for the cultivation of large-diameter timber of Chinese fir.

Method According to the upper, middle and lower slope positions and different slope directions, a total of 63 plots of Chinese fir plantations with the age range from 28 to 36 years were set up. Based on the data of growth and basic parameters measured forest growth, correlation analysis and regression analysis were performed in the study.

Result When the stand density was similar, the higher the site index was, the higher the individual volume, stock volume and large-diameter timber yield rate of the same cultivation years were. Under the same site conditions, the stand mean height, DBH, tree volume and ratio of large diameter timber yield decreased with the increase of stand density, whereas the stand volume increased with the increase of density within a certain density range. Stand density had a greater effect on differentiation of DBH than that of tree height when the site index was the same. However, the effect of stand density on differentiation of tree height and DBH varied when the site index was different.

Conclusion When cultivating large-diameter timber of Chinese fir under site conditions with a site index of 22 or above, the density can be controlled by three thinnings, and the final retention density of the stand should be 650-1000 trees per hectare.