Abstract:Pinus massoniana is a widely planted native tree species in China. The simple community structure and low biodiversity of P. massoniana plantation are common ecological problems. Exploring the influence of different forest gap sizes on the understory vegetation community of P. massoniana plantation can provide a theoretical basis for the nearnatural management of P. massoniana plantation. To explore the distribution of shrub and herb layer plant composition, niche breadth and niche overlap of dominant species in the gap after natural regeneration for 1 year after cutting, we set four different sizes of gaps A (50 m2), B (100 m2), C (200 m2) and D (667 m2) in 45yearold P. massoniana plantation, with the untreated P. massoniana plantation as control in this study. The results showed: (1) except that there was no significant difference in the number of species in the shrub layer under the 100 m2 gap, the number of species in the shrub and herb layer in the other gaps was significantly higher than that in the control (P < 0.05), and the number of species in the shrub and herb layer under the 200 m2 gap was the most, with 35 and 20 species, respectively. The species richness index of the shrub and herb layer under the four forest gaps significantly increased compared with the control (P < 0.05), the maximum values all appeared under the 200 m2 gap, and their values were respectively 1.5 times and 2.6 times that of the control. (2) The species of sun plants of the shrub and herb layer increased in the forest gap, and there were the most species under the 200 m2 gap. There were 13 species of sun plants in the shrub layer and 5 species of sun plants in the herb layer. (3) Among the dominant species in the shrub and herb layer under the four forest gaps, the niche breadth of the sun plants was larger, the average niche width of the shrub and herb layer under the 200 m2 gap was the smallest, the degree of resource utilization was low, and there was no significant correlation between the important value and the niche breadth (P > 0.05). (4) The niche overlap index of the dominant species in the shrub and herb layer under the forest gap was small. Under 667 m2 gap, the average niche overlap indexes of dominant species in shrub and herb layer were the lowest, 0.029 and 0.024, respectively. The proportions of high niche overlap of dominant species in 200 m2 gap were the highest, 20% and 23.8%, respectively. Studies have shown that cutting and opening windows promote the development of understory vegetation in the P. massoniana plantation, enriche the diversity of understory plants, which is conducive to the stable and sustainable development of the P. massoniana forest, and is of great significance to accurately improve the quality of the P. massoniana plantation.