Abstract: We investigated the understory vegetation, determined the physicochemical properties at 0-20 and 20-40 cm soil layers in 28-year-old Populus simonii plantations under different stand densities (450, 600, 750, 950, 1100 trees·hm^-2) in Zhangbei County, Zhangjiakou City, Hebei Province, aiming to understand the effects of stand density on species diversity of understory plants and soil physicochemical properties. The results showed that there were almost no shrubs in the understory. A total of 38 species, 30 genera, 18 families of herbaceous plants were observed in the understory vegetation. The dominate herbaceous plants were from the Cyperaceae, Fabaceae, Gramineae, Brassicaceae, and Compositae. When the stand density of P. simonii increased, Margalef richness index (D_M), Simpson dominance index (D_S), Shannon diversity index (H), and Pielou evenness index (E) of the herbaceous plant communities rose first but then declined, and peaked at the stand density of 600 trees·hm^-2. The overall trend of soil nutrients in the two soil layers increased first and then decreased with increasing stand density, and the maximum value all appeared at 600 trees·hm^-2. The contents of soil total nitrogen, total phosphorus, alkali-hydrolyzable nitrogen, available phosphorus, and available potassium positively correlated with understory herbaceous species diversity index. In the 0-20 cm soil layer, soil total nitrogen and available phosphorus contents were significantly positively correlated with D_M, D_S, H and E. Soil total phosphorus content had significant positive correlation with D_S and H. There was a significant positive correlation between alkali-hydrolyzable nitrogen and D_S. In the 20-40 cm soil layer, soil bulk density had a significant negative correlation with D_M, D_S, H, and E. Soil total phosphorus content had a significant positive correlation with D_M, D_S, H, and E. There was a significant positive correlation between soil available potassium and D_M and H. The stand density of 600 trees·hm^-2 was the optimal one for P. simonii plantations in this region, which could improve understory herbaceous species diversity, provide good growth space for understory vegetation, and enhance soil fertility.

Key words: stand density, Populus simonii, understory vegetation, species diversity, soil property