In a study published in New Phytologist, researchers show that the upper and lower surfaces of a single oak leaf function as two fundamentally different microbial habitats. Its sun-drenched upper surface and shaded, humid underside harbor vastly different microbial empires, and their microbiomes show striking seasonal divergence.
Leaf surface represents one of the largest microbial habitats on Earth. However, most previous studies have treated leaves as homogeneous units, pooling samples from both surfaces. This oversight has masked a critical dimension of microbial ecology.
To fill this gap, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences 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 starkly contrasting microenvironments drive 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 of 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 surged significantly higher, whereas the lower surface experienced notable declines in both diversity and abundance.
Furthermore, they found that the abundance of core community members differed early in the season but later converged. The upper-surface microbiomes remained remarkably resilient across all seasons, while their lower-surface counterparts suffered from strong seasonal filtering, with diversity waning over time.
Crucially, lower-surface microbiomes were tightly coupled with the host plant’s shifting physiological traits. In contrast, upper-surface communities appeared largely decoupled from host traits, and molded by relentless abiotic pressures that favored evolutionarily conserved, stress-hardy lineages.
“Our results highlight that within-leaf heterogeneity is a fundamental axis of microbial ecology,”said YIN Xiangbo. “By distinguishing between stable and dynamic leaf traits, we can better understand how host phenotypes shape microbial communities and their responses to environmental change.”
First published: 02 August 2026
