The phyllosphere is a challenging microbial habitat in which microorganisms can flourish on organic carbon released by plant leaves but are also exposed to harsh environmental conditions. Here, we assessed the relative importance of canopy position—top, mid, and bottom at a height between 31 and 20 m—and tree species identity for shaping the phyllosphere microbiome in a floodplain hardwood forest. Leaf material was sampled from three tree species—maple (Acer pseudoplatanus L.), oak (Quercus robur L.), and linden (Tilia cordata MILL.)—at the Leipzig canopy crane facility (Germany). Estimated bacterial species richness (Chao1) and bacterial abundances approximated by quantitative PCR of 16S rRNA genes exhibited clear vertical trends with a strong increase from the top to the mid and bottom position of the canopy. Thirty operational taxonomic units (OTUs) formed the core microbiome, which accounted for 77% of all sequence reads. These core OTUs showed contrasting trends in their vertical distribution within the canopy, pointing to different ecological preferences and tolerance to presumably more extreme conditions at the top position of the canopy. Co-occurrence analysis revealed distinct tree species-specific OTU networks, and 55–57% of the OTUs were unique to each tree species. Overall, the phyllosphere microbiome harbored surprisingly high fractions of Actinobacteria of up to 66%. Our results clearly demonstrate strong effects of the position in the canopy on phyllosphere bacterial communities in a floodplain hardwood forest and—in contrast to other temperate or tropical forests—a strong predominance of Actinobacteria.

叶际是一个具有挑战性的微生物栖息地，微生物可以依靠植物叶子释放的有机碳而繁衍生息，但也暴露在恶劣的环境条件下。在这里，我们评估了树冠位置（高度在 31 至 20 m 之间的顶部、中部和底部）以及树种特性对于塑造漫滩阔叶林叶际微生物组的相对重要性。叶子材料是在莱比锡树冠起重机设施（德国）的三种树种中取样的：枫树（Acerpseudoplatanus L.）、橡树（Quercus robur L.）和椴树（Tilia cordata MILL.）。估计的细菌物种丰富度（Chao1）和通过16S rRNA基因定量PCR近似的细菌丰度表现出明显的垂直趋势，从冠层的顶部到中底部位置强烈增加。30 个操作分类单元 (OTU) 构成了核心微生物组，占所有序列读数的 77%。这些核心OTU在冠层内的垂直分布表现出截然不同的趋势，表明它们具有不同的生态偏好和对冠层顶部可能更极端条件的耐受性。共现分析揭示了不同树种特异性的 OTU 网络，并且 55-57% 的 OTU 对于每个树种来说都是独特的。总体而言，叶际微生物组中放线菌的比例高得惊人，高达 66%。我们的结果清楚地表明，在洪泛区硬木森林中，树冠位置对叶际细菌群落有强烈影响，并且与其他温带或热带森林相比，放线菌占主导地位。