Phyllosphere bacterial diversity is shaped through interactions between hosts and microbes. Most studies having focused on pairwise associations between host taxa and their symbionts, little is yet understood about the influence of the host community as a whole in shaping these interactions. Envisioning phyllosphere bacterial communities as a spatially structured network of communities linked by dispersal (i.e., metacommunities) can help us better understand the relative importance of species sorting among host populations and species versus dispersal from the neighboring host community for bacterial community assembly in forest ecosystems. Here we investigate drivers of metacommunity structure of epiphytic bacteria of the phyllosphere among 33 tree host species distributed across a large‐scale transition from deciduous to boreal forest. We expect the identity and traits of hosts to play an important role in determining phyllosphere bacterial composition. We further hypothesize that bacterial dispersal from neighboring host species will modulate the match between a focal host species and its microbiota, and shape opportunities for host specialization of phyllosphere bacteria at local and regional scales. We defined specialization as the level of phylogenetic similarity among hosts that a bacterial symbiont associates with. We found that host taxonomic identity and traits were important drivers of bacterial community turnover and variation in host specialization across the landscape. Dispersal from neighboring communities further played a role in homogenizing bacterial communities. The microbiota of focal hosts such as sugar maple was thus increasingly similar to that of neighboring host species along the transition from deciduous to boreal forest. Specialization of bacterial taxa on sugar maple was further positively correlated with the relative abundance of this host in the landscape, revealing a role for the host community context in shaping evolutionary relationships between phyllosphere bacteria and their tree hosts. These results overall suggest that the dispersal of phyllosphere bacteria from the dominant tree community members may be constraining the match between tree species and their symbionts, particularly at their range limits. We also demonstrate that considering host‐associated microbial communities as part of metacommunities within the host landscape is a promising tool for improving our understanding of host‐symbiont matching.

中文翻译：

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宿主邻域在横向梯度上影响细菌群落的组装和树木种类的专长

通过宿主与微生物之间的相互作用，形成了毛层细菌的多样性。大多数研究着眼于宿主类群及其共生体之间的成对关联，但对宿主社区作为整体在影响这些相互作用方面的影响知之甚少。将叶层细菌群落设想为通过扩散（即，元群落）联系起来的群落的空间结构化网络，可以帮助我们更好地了解宿主种群和物种之间的物种分类相对于相邻宿主群落的扩散对于森林生态系统中细菌群落组装的相对重要性。在这里，我们调查了从落叶林到北方森林的大规模过渡过程中分布的33个树宿主物种中，叶圈附生细菌的亚群落结构的驱动力。我们期望宿主的身份和特征在确定叶球细菌组成方面起重要作用。我们进一步假设，细菌从邻近宿主物种扩散将调节局灶性宿主物种与其微生物群之间的匹配，并在局部和区域范围内塑造叶球细菌宿主专业化的机会。我们将专业化定义为细菌共生体所关联的宿主之间的系统发育相似性水平。我们发现宿主生物分类身份和特征是细菌群落更新和整个宿主专业化变异的重要驱动力。来自邻近社区的分散在匀化细菌群落中进一步发挥了作用。因此，在从落叶林到北方森林的过渡过程中，诸如糖枫之类的焦点寄主的微生物群落与邻近寄主物种的微生物群落越来越相似。糖枫上细菌类群的专业化与该寄主的相对丰富度呈正相关，揭示了寄主社区环境在塑造叶球细菌与其树宿主之间的进化关系中的作用。这些结果总体上表明，来自优势树群落成员的叶球细菌的散布可能限制了树种及其共生体之间的匹配，特别是在它们的范围极限处。