Vertical stratification of biodiversity and productivity relationships and their genesis in South Subtropical Evergreen Broad-leaved Forest

Exploring how biodiversity and productivity are related in different vertical layers of forests, and understanding the role of various biotic and abiotic factors, can shed light on the heterogeneous distribution of forest productivity and guide sustainable forest management. Based on the South Subtropical Evergreen Broad-leaved Forest, we divided woody plants into understory and overstory based on individual diameter at breast height and found that (1) the relationship between biodiversity and forest productivity varied among the understory, overstory, and whole community. The absolute value of the correlation coefficient between biodiversity and forest productivity tended to decrease from the understory to the overstory. (2) Soil organic carbon (SOC), species mingling index, and altitude were the dominant factors affecting understory productivity. Soil pH, uniform angle index (reflecting the spatial distribution of trees), and SOC were the dominant factors affecting overstory productivity. Soil pH, altitude, and SOC were the dominant factors affecting the whole productivity. (3) In the understory, only the soil factors and species diversity can directly and significantly affect understory productivity. In the overstory, soil factors, species diversity, and structural diversity can contribute significantly to overstory productivity directly. In the whole community, only the soil factors and topographic factors can directly and significantly affect whole productivity. Phylogenetic diversity is the primary factor affecting understory productivity, and soil attributes is the primary factor affecting overstory and whole productivity.