Effects of stand structure, individual dominant species and environment on herb diversity in a temperate forest region

• Stand structural attributes and environment shape herb diversity. • Define the herb dominance index ( HDI ) to quantify herb dominance. • Individual dominant herb species reduces herb diversity. • Stand structural attributes regulate plant–plant and plant–soil interactions. Herbaceous plants are a vital component of biodiversity in forest ecosystems. Herbs are, however, often excluded from forest diversity experiments, which limits our understanding of the structure and dynamics of entire ecosystems. We measured the diversity of understory herbaceous communities and stand structural attributes. The herb dominance index ( HDI ) was used to quantify herb dominance, incorporating both the cover and height of individual dominant species. Using linear mixed-effects models and structural equation modelling, we examined the direct effects of stand structure (stand density and variation of tree heights), dominant herb species and environmental factors on herb diversity, and the indirect effects via HDI . The diversity of herbaceous communities is reduced by the presence of a few dominant herbs. Increasing forest density reduced the HDI and thus indirectly enhanced herb diversity. The diversity of herbaceous communities was negatively correlated with an increase in the variation of tree heights. Environmental factors directly and indirectly influenced herb diversity by affecting the variation of tree heights and the HDI . Significant interactions between stand density–soil nutrients and the variation of tree heights– HDI suggest that stand structural attributes have significant effects on the soil–herbaceous plant and interspecific relationships. Stand structure, local environment, and individual herb dominance jointly affected the diversity of herbaceous plant communities in our temperate forests. Unraveling the complex interactions between forest density and structure and environmental factors improves our understanding of understory herb community assembly.