Understory diversity are driven by resource availability rather than resource heterogeneity in subtropical forests

Understory vegetation accounts for most of the plant diversity, particularly in plantation forests, and contributes significantly to energy flows and nutrient cycling. However, we lack a mechanistic understanding of understory diversity associated with resource availability and heterogeneity across forest types over a wide range of stand development stages. We sampled the understory vegetation of Chinese fir plantations and natural secondary forests (4–40 years old) and used primary forests as a reference in subtropical China. The percent covers of all understory plant species were estimated at the shrub layer (height >1.3 m) and herb layer (≤1.3 m). We quantified understory light availability and heterogeneity, substrate diversity and soil nutrients and employed principal component analysis to reduce multiple physical and chemical soil properties to PC1 and PC2, with PC1 positively correlating with soil carbon and total nitrogen, and PC2 with available soil nitrogen and phosphorus. Structural equation modelling (SEM) was employed to test the direct and indirect influences of forest type, stand age, light availability and heterogeneity, substrate diversity and soil nutrients on the diversity of understory species. We found that shrub layer richness was higher in the secondary natural and primary forests than in the plantations, but plantations had higher bryophyte richness. Shrub cover was positively correlated with shrub richness and had a positive indirect effect on shrub richness but a negative indirect effect on herb richness via its effect on soil carbon and nitrogen. Stand age has a positive direct effect on bryophyte richness. There were strong positive correlations among the richness of understory layers and life forms. Our results suggest that physical and chemical soil properties, specifically soil nutrient availability, rather than light availability and heterogeneity or substrate diversity, were the main drivers that affected subtropical forests' understory richness.