The influence of stand spatial structure on understory sapling species diversity in natural forests of Qujiang District

Understory sapling regeneration is crucial for maintaining the stability and functioning of forest ecosystems. To investigate the mechanisms through which stand spatial structure influences understory sapling species diversity and to identify key driving factors, this study was conducted in natural evergreen broad-leaved forests within Qujiang District, Quzhou City, Zhejiang Province. A 1-ha permanent plot, subdivided into twenty-five 20 m × 20 m quadrats, was established. The relationships between stand spatial structure indices (aggregation index, competition index, mingling degree, openness) and understory sapling (DBH ≤ 5 cm) species diversity indices (Margalef's richness index, Shannon-wiener index, Simpson index, Pielou's evenness index) were systematically analyzed using Pearson correlation analysis, canonical correlation analysis, and multiple stepwise regression analysis.Pearson correlation analysis revealed that mingling degree was significantly positively correlated with sapling species richness, Shannon-wiener diversity index, and species evenness, while it was significantly negatively correlated with the Simpson dominance index. Canonical correlation analysis further indicated an extremely significant relationship between the two sets of variables (stand spatial structure and sapling species diversity), with the first canonical correlation coefficient being 0.793 ( P < 0.001). Mingling degree emerged as the dominant factor affecting sapling diversity. Multiple stepwise regression analysis confirmed that mingling degree was the key driver explaining the variation in all sapling species diversity indices. Additionally, openness had a significant negative impact on species richness.In conclusion, mingling degree within the stand spatial structure is the most critical factor influencing understory sapling species diversity in the natural forests of Qujiang District. Enhancing the tree species mingling degree in the canopy layer can effectively promote understory sapling species richness, evenness, and overall diversity • Stand spatial structure has a significant influence on understory sapling species diversity. • Mingling is the key stand spatial structure factor affecting understory sapling diversity. • This study provides a scientific basis for the sustainable management of natural forests in Qujiang District.