Contrasting patterns of herb-layer diversity and their environmental correlates in evergreen coniferous and deciduous broadleaf forests: evidence from Mount Lao, China

Abstract Key Message Evergreen coniferous forests (ECF) and deciduous broadleaf forests (DBF) exhibit fundamentally distinct mechanisms driving understory herbaceous diversity across α- and β-diversity dimensions. Canopy cover dominates α-diversity in ECF (peaking at 0.6–0.7), while soil organic matter governs richness in DBF. Forest-type-specific management strategies are required to maintain biodiversity and ecosystem multifunctionality. Context Understory herbaceous layers harbor over 80% of vascular plant diversity in temperate forests and serve as early indicators of ecosystem dynamics. While ECF and DBF differ fundamentally in microenvironmental conditions (light availability, litter dynamics, and soil properties), few studies have systematically compared how these contrasting systems shape herbaceous diversity mechanisms within homogeneous climatic regions. Aims This study aims to quantify the divergent drivers of α- and β-diversity in understory herbaceous communities between warm-temperate ECF and DBF, and to identify forest-type-specific management implications for biodiversity conservation. Methods Using vegetation surveys, soil sampling, and advanced modeling (GAMM, GDM, JSDM) across 69 forest plots on Mount Lao, China, we assessed how forest type modifies ecological filters governing herbaceous community assembly across multiple diversity dimensions. Results The two forest types harbored distinct compositions (15 species exclusive to ECF, 23 to DBF). In ECF, species richness followed a significant unimodal response to canopy cover (optimal at 0.6–0.7), whereas DBF richness declined with increasing soil organic matter. Overstory diversity positively influenced herbaceous richness in both systems, consistent with the resource heterogeneity–diversity hypothesis. For β-diversity, soil pH served as the primary driver in ECF, while available potassium heterogeneity dominated in DBF. Both forest types displayed significant distance–decay relationships strongest within 0–15 km, indicating joint effects of environmental filtering and dispersal limitation. Conclusion Divergent canopy-soil interaction mechanisms necessitate differentiated management approaches: ECF requires optimized canopy cover (0.6–0.7) to maximize richness while maintaining spatial connectivity, whereas DBF management should emphasize nitrogen mitigation and soil carbon regulation through mixed-species plantations to prevent competitive exclusion and maintain abundance equitability.