The aerial surfaces of plants, the phyllosphere, harbors a diverse community of microorganisms. The increasing awareness of the potential roles of phyllosphere microbial communities calls for a greater understanding of their structure and dynamics in natural and urban ecosystems. To do so, we characterized the community structure and assembly dynamics of leaf bacterial communities in natural temperate forests of Quebec by comparing the relative influence of host species identity, site and time on phyllosphere bacterial community structure. Second, we tested the value of characterizing a tree’s complete phyllosphere microbial community through a single sample by measuring the intra-individual, inter-individual and interspecific variation in leaf bacterial communities. Third, we quantified the relationships among phyllosphere bacterial diversity, plant species richness, plant functional diversity and identity and plant community productivity in a biodiversity-ecosystem function experiment with trees. Finally, we compared tree leaf bacterial communities in natural and urban environments, as well as along a gradient of increasing anthropogenic pressures. The work presented here thus offers an original assessment of the dynamics at play in the tree phyllosphere.