Abstract: Understanding the dynamics of understory plant diversity and biomass in plantations is of great significance for assessing the effectiveness of sandy vegetation restoration and plantation management. We analyzed the characteristics of understory communities in Pinus sylvestris var. mongolica plantations with different stand ages (Ⅰ: ≤10 a, Ⅱ: 10 a<age≤15 a, Ⅲ: 15 a<age≤20 a, Ⅳ: age>20 a) in Mu Us Sandy Land. The impact of stand factors on understory plant communities was analyzed using structural equation modeling path analysis. A total of 57 species from 47 genera and 20 families were recorded, including 3 shrub species. Asteraceae, Gramineae, Leguminosae and Chenopodiaceae dominated herbaceous layer, accounting for 50% of the total herbaceous species. With increasing stand age, the number of families, genera and species of herbaceous layer decreased first and then increased. With increasing stand age, the perennial herbaceous plants increased, the annual and biennial plants decreased relatively, the xerophytes decreased, and the mesophytes increased. The Shannon diversity index, Simpson dominance index and Margalef richness index of herbaceous layer decreased first and then increased with increasing stand age, and the Pielou evenness index increased with increasing stand age, but there was no significant difference among different stand age classes (P>0.05). There was a negative relationship between tree layer coverage and shrub layer coverage and biomass, and a positive relationship between tree layer coverage, shrub layer coverage and understory herb layer coverage, aboveground biomass, species richness and Shannon diversity index. There was a negative correlation between aboveground biomass of shrub layer and herb coverage, aboveground biomass, species richness and Shannon diversity index. In summary, the vegetation restoration model of Pinus sylvestris var. mongolica plantation in Mu Us Sandy Land is conducive to the restoration of understory herbaceous plants. Trees and shrubs should be rationally allocated in the future restoration of sandy vegetation to improve species diversity and community stability.

Key words: Pinus sylvestris var. mongolica plantation, understory vegetation, species diversity, aboveground biomass, Mu Us Sandy Land