Enhancing species diversity through mixed plantations of native tree species mytilaria laosensis and michelia macclurei in southern China

Mixed-species planting promotes forest growth and enhances ecological functions. However, it remains unclear whether mixed forests are more conducive to fostering species diversity than monocultures over long time scales. To address this, we established fixed plots of 140 m × 40 m, 70 m × 50 m, and 70 m × 40 m in plantations of Mytilaria laosensis (Lecomte) and Michelia macclurei (Dandy) in southern Guangxi, China. We analyzed species composition and diversity characteristics using the Jaccard similarity index, principal component analysis and four traditional diversity indices. The results indicated: 1) The species composition similarity between mixed forest and monocultures was higher than that observed between monocultures. 2) As plot area increased, the species richness (SR) and Shannon diversity index (Hʹ) of stands and regenerations followed a power-law distribution, while species abundance increased linearly and evenness (EH) gradually decreased. 3) Regeneration was sparse in the M. laosensis stand but denser in the other two forests. 4) The majority of regenerations had a ground diameter (GD) smaller than 5 cm. Both SR and Hʹ decreased rapidly at first and then more slowly as GD increased, approaching 0, while the relationship between EH and GD was weak. Three out of four species diversity indices of the mixed forest were higher than those of the M. laosensis stand and the mean values of the two monocultures. These results suggest that planting patterns significantly alter the species composition and diversity of plantations, while emphasizing the importance of tree species selection in promoting species diversity.