<p>Tree bark hosts a unique community of microorganisms, distinct from that of other plant tissues and from the microbial associates of macrophytic epiphytes. These corticolous communities are important components of tree-associated food webs. However, the factors influencing corticolous microbial diversity are understudied. We used amplicon sequencing to characterise and compare the bacterial and aerophytic algal communities colonising the bark of birch trees at three sites in southern Norway with differing histories of nitrogen deposition. Within each site, we investigated associations between the corticolous community and variation in placement on the trunk, including north-facing vs. south-facing aspect, height and trunk diameter. While the most abundant bacterial and algal taxa were similar across the three sites, there were clear differences among sites. Aspect had a strong effect at the most oceanic and open site, with increased abundance of cyanobacteria at northern compared to southern aspects. We observed a higher abundance of nitrogen-fixing cyanobacteria at the site where historical nitrogen deposition had been lowest and correspondingly higher abundance of non-diazotrophic green algae where historical nitrogen deposition had been highest. Generally, the bacterial and algal communities followed similar patterns and co-occurrence network analysis revealed that algal taxa were interspersed among bacterial clusters. Our study provides the first characterisation of both the bacterial and aerophytic algal bark-associated communities of birch, one of the most common and ecologically important trees in Norway. Our results add to the growing body of literature demonstrating that tree bark supports a complex microbial community that varies among sites.</p>