A single leaf may look like a simple patch of green, but to the microscopic life that inhabits it, it is a vast landscape divided into different worlds.

In a study published in New Phytologist on August 2, researchers found that the upper and lower surfaces of a single oak leaf function as two fundamentally different microbial habitats. The sun-exposed upper surface and the shaded, humid underside harbor distinct microbial communities that show striking seasonal divergence.

Leaf surfaces represent one of the largest microbial habitats on Earth. However, most previous studies have considered leaves as a whole, combining samples from both surfaces, which has overlooked an important dimension of microbial ecology.

To fill this gap, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences (CAS) and their collaborators conducted a field experiment in a mixed forest in central Germany. They tracked bacterial communities on both surfaces of the same Quercus robur (oak) leaves from spring to autumn.

They found that the upper (adaxial) surface endures relentless sunlight, ultraviolet radiation, and wind, while the lower (abaxial) surface remains shadier, more humid, and dotted with stomata. These contrasting microenvironments support completely different microbial survival strategies.

"The upper surface selects for stress-tolerance, while the lower surface fosters host- and insect-mediated interactions in a sheltered niche," said YIN Xiangbo from XTBG, first author of the study.

To eliminate genetic variability in the host plant, the researchers worked with genetically identical oak trees and repeatedly swabbed the very same leaves from spring through autumn. They combined high-throughput 16S rRNA sequencing with detailed measurements of leaf traits, including wettability, pigment content, and stomatal distribution.

Their results showed marked divergence. The upper surface hosted stress-tolerant genera, but the lower surface favored sugar-fermenting taxa and insect-associated bacteria. Bacterial richness remained comparable between the two sides until autumn, when a striking split occurred late in the season: richness on the upper surface rose sharply, whereas the lower surface experienced notable declines in both diversity and abundance.

Furthermore, the researchers found that the abundance of core community members differed early in the season but later converged. The upper-surface microbiomes remained remarkably resilient across seasons, while their lower-surface counterparts experienced strong seasonal filtering, with diversity declining over time.

Crucially, lower-surface microbiomes were tightly coupled with the host plant's shifting physiological traits. In contrast, upper-surface communities appeared largely independent of host traits and were instead shaped by persistent abiotic pressures that favored evolutionarily conserved, stress-tolerant lineages.

"Our results highlight that within-leaf heterogeneity is a fundamental axis of microbial ecology," YIN said. "By distinguishing between stable and dynamic leaf traits, we can better understand how host phenotypes shape microbial communities and their responses to environmental change."