Abstract: Leaf-associate microbiome, including both bacteria and fungi, is an important part of forest ecosystems. Like other ecological communities, leaf microbial communities are shaped by both niche-based deterministic processes and neutrality-based stochastic processes. However, the relative importance of these two processes in determining phyllosphere microbial assembly is still poorly understood. Moreover, the vertical structure of forest provides diverse habitats for microorganisms. From the forest floor to the canopy layers, the assembly mechanism of leaf microbial communities might be different since both biotic and abiotic factors could change dramatically along the vertical dimension of the forest. Understanding the vertical distribution of the diversity and structure of leaf microbial communities in forest ecosystems is essential, but has not been explored. In this study, we collected leaf samples from a subtropical evergreen broad-leaved forest in Southern China. Both bacterial and fungal communities from leaves at the forest floor, shrub layer, under-canopy layer, and canopy layer were extracted, and their diversity and community structure were quantified using high-throughput sequencing. By analyzing patterns of bacterial and fungal diversity of samples, and the compositional difference in community structure among samples, we aimed to (1) identify leaf bacterial and fungal diversity patterns along the vertical gradient of the forest, (2) explore the ecological drivers of variation in leaf bacterial and fungal diversity and community structure, and (3) examine how the relative importance of deterministic and stochastic processes differ at different layers in the forest. Our study revealed three important findings. First, we found clear vertical patterns of increasing bacterial and fungal diversity from the forest floor to the canopy layers, which were mainly influenced by plant height. Second, leaf bacterial and fungal community structures also change along the vertical gradient, with lower layers being more similar to soil microbiome. Third, from the forest floor to the canopy layer, the relative importance of deterministic processes increases in shaping bacterial communities, but decreases in shaping bacterial communities. Our study identifies the vertical distribution of diversity and structure of leaf microbial community, while emphasizing different assembly mechanisms of leaf bacterial and fungal communities.
