Objective The mechanisms by which stand type regulates understory diversity in plantations within climatic transition zones remain poorly understood, particularly the layer−specific differences between shrub and herb assemblages across stand types. Here, we investigate five typical broad−leaved plantation types in the subtropical–warm temperate transitional zone of eastern China to identify the drivers of understory species diversity (species richness, Shannon index, Simpson index, and Pielou evenness) and phylogenetic diversity (Faith’s PD, NRI, and NTI), aiming to inform regional plantation conservation strategies. Methods We established grid-based nested plots across five core ecological zones in Jiangsu, surveyed vegetation composition, measured topographic and soil variables, reconstructed a species-level phylogenetic tree, and quantified the effects of candidate drivers using correlation analysis, redundancy analysis (RDA), and linear mixed-effects models. Result The shrub layer was dominated by temperate genera, whereas the herb layer comprised a mixture of temperate and cosmopolitan genera. Both species and phylogenetic diversity were significantly higher in the herb layer than in the shrub layer, with diversity following a clear gradient: mixed forests non-pioneer pure forests pioneer pure forests. Shrub-layer diversity correlated primarily with topographic factors, herb-layer diversity with soil properties and latitude, and phylogenetic conservatism was predominantly regulated by stand type. Conclusion Stand type emerges as the core driver of understory diversity, operating through the regulation of microhabitat heterogeneity, with pronounced layer-specific mechanisms—topographic filtering for shrubs, and soil–latitude co-regulation for herbs. Our findings clarify the maintenance mechanisms of understory diversity in transitional-zone plantations and provide theoretical support for regionally tailored plantation management.
Ge et al. (2026) studied this question.