To investigate the effects of stand densities on the structure of understory plant communities, species diversity and soil nutrient content in Juglans mandshurica forests, and to reveal the intrinsic relationship between plant diversity and soil nutrients.

This study focused on natural secondary forests of Juglans mandshurica on Mao'er mountain with different densities (840, 680 and 480 trees·hm^-2). Through field community surveys and indoor soil analysis, the species composition, importance values, diversity index of understory woody and herbaceous plants were investigated, and soil nutrient indicators at different soil layer depths were measured for correlation analysis.

1) A total of 65 plant species were surveyed under the Juglans mandshurica tree forest throughout the study area, including 16 woody species belonging to 11 families and 14 genera, and 49 herbaceous species belonging to 28 families and 45 genera; 2) The dominant species of the woody layer is Syringa reticulata subsp. amurensis, with a maximum importance value of 85.23% at a stand density of 480 trees·hm^-2; the dominant species of the herbaceous layer is Beckmannia syzigachne, with a maximum importance value of 50.49% at a stand density of 680 trees·hm^-2; 3) Species richness is highest when the stand density is 680 trees·hm^-2, with the richness indices of woody and herbaceous plants being 11 and 31, respectively; the Simpson and Shannon-Wiener diversity indices show a trend of first increasing and then decreasing as stand density decreases, reaching their highest values at a stand density of 680 trees·hm^-2; the evenness index increases as stand density decreases, and the highest evenness index for herbaceous plants is 0.81 at a stand density of 480 trees·hm^-2; 4) The nutrient content in the soil is highest in the 0-10 cm layer. When the stand density is 680 trees·hm^-2, the contents of organic matter, total nitrogen, alkaline nitrogen and available phosphorus in this soil layer are significantly higher than those in other density treatments. Woody plant diversity is extremely significantly positively correlated with total soil nitrogen content and significantly negatively correlated with soil pH, while herbaceous plant richness is extremely significantly positively correlated with the content of available phosphorus.

A stand density of 680 trees·hm^-2 is the optimal density in the study area for maintaining understory plant diversity in Juglans mandshurica forests and enhancing soil fertility. Stand density indirectly regulates the composition and diversity of plant communities by altering the soil environment, this study will provide a reasonable management density for secondary forests of Juglans mandshurica and offer a scientific basis for the sustainable management of Juglans mandshurica.