Species diversity and biomass of the herbaceous layer in three forest types of Eastern Shandong, China

Introduction This study focused on coniferous forests, conifer-broadleaf forests, and broadleaf forests in eastern Shandong to investigate the characteristics of understory herbaceous species diversity and biomass, as well as their coupling relationships across different forest types. Methods Through plot surveys and harvesting methods, the species composition, diversity, biomass of the herb layer, and related environmental factors were collected and analyzed. Correlation analysis and hierarchical partitioning were used to examine the relationship between species diversity and biomass, as well as the factors affecting biomass. Results The species composition of the herbaceous layer differed significantly among the three forest types, with low community similarity. Broadleaf forests and conifer-broadleaf forests had the highest similarity (50.0%), while coniferous forests and conifer-broadleaf forests had the lowest (34.9%). Herbaceous layer biomass in broadleaf forests was significantly higher than that in the other two forest types. The relationship between species diversity and biomass varied by forest type: a significant unimodal relationship was observed in coniferous forests; species richness was positively correlated with biomass in conifer-broadleaf forests; and a significant negative correlation was found in broadleaf forests. Stand characteristics had the highest explanatory power for biomass across the three forest types (45.4%, 35.7%, and 65.5%, respectively), followed by total soil phosphorus (22.3% in coniferous forests), the richness index (18.7% in conifer-broadleaf forests), and total soil nitrogen (25.1% in broadleaf forests). Discussion The relationship between herbaceous species diversity and biomass exhibited diverse patterns among different forest types, suggesting that the underlying mechanisms governing diversity–biomass relationships are context-dependent and vary with forest composition. Stand characteristics consistently emerged as the dominant factor explaining biomass across all forest types, highlighting the critical role of forest structure in shaping understory productivity. Soil nutrients also contributed substantially, though their relative importance varied by forest type: total phosphorus in coniferous forests, richness index in conifer-broadleaf forests, and total nitrogen in broadleaf forests. These findings indicate that the relative influence of biotic and abiotic factors on understory biomass is mediated by forest type, with important implications for understanding ecosystem functioning and informing forest management practices.