Long-term effects of planting density and site quality on timber assortment structure based on a 41-year plantation trial of Chinese fir

•Growth data of a 41-year trial of Chinese fir plantations.•Long-term effects of five different densities and four different site classes on timber assortment structure are explored.•Promoting effect of low density on large-diameter timber is enhanced as site quality increases.•The interaction between density and site index significantly affects the formation of large-diameter timber.•Significant effect of site index on large-diameter timber persists longer.