Abstract: Six Quercus mongolica plots with an area of 0.1 hm² were thinned in 2018. A field survey was carried out in 2020 to examine the effects of different stand densities (high: 900 trees·hm^-2; medium: 720 trees·hm^-2; low: 600 trees·hm^-2) on growth and regeneration of stands and understory species diversity of secondary Q. mongolica forests in Qingyuan, Liaoning Province. Due to the short interval after thinning, there was no significant difference in tree height and diameter at breast height under different densities. However, the crown symmetry index under low stand density was significantly higher than that of high stand density, indicating that crown growth was more sensitive to stand density than trunk growth. The abundance of seedlings was the highest in the medium density, and the basal diameter of the seedlings with the same height was significantly higher, and the seedling regeneration and growth at the medium density were much better than the other two densities. A total of 70 species were recorded, belonging to 41 families and 67 genera. Quercus mongolica, Lespedeza bicolor, Melampyrum roseum, and Potentilla freyniana were the dominant species of trees and herbs, respectively. Simpson index, Pielou index and Shannon index of shrub layer and herb layer were the highest at the medium density. It indicated that the stand density of 720 trees·hm^-2 could help maintain the sustainable development of Q. mongolica secondary forest in the mountainous area of eastern Liaoning.

Key words: stand density, Quercus mongolica, natural regeneration, species diversity