Adaxial-abaxial leaf surface asymmetry is a key ecological driver of the phyllosphere microbiome.

Microorganism-driven ecosystems on leaf surfaces play pivotal roles in regulating plant health and fitness. While individual leaves provide distinct microhabitats on their adaxial (upper) and abaxial (lower) surfaces, most studies have assessed phyllosphere microbial community assembly and function using whole leaves, thereby overlooking their inherent spatial heterogeneity. Here, we independently analyzed bacterial and fungal communities on adaxial and abaxial leaf surfaces across seven temperate tree species over a 6-month period. We further assessed their metabolic and ecological profiles to understand how leaf surface compartmentalization shapes microbial communities. Our results revealed that compositionally distinct microbial communities were consistently established on each leaf surface. Co-occurrence network analyses showed that the abaxial communities exhibited differences in interaction structure compared with the adaxial communities, characterized by a higher proportion of negative correlations. Predicted functional profiles indicated that adaxial communities were enriched in degradation pathways, potentially reflecting nutrient-poor, high-stress conditions, whereas abaxial communities were enriched in biosynthetic and energy-generating functions, consistent with resource-rich environments. Collectively, our results highlight adaxial-abaxial leaf surface asymmetry as a key ecological driver shaping the structure, function, and interaction dynamics of the phyllosphere microbiome.