Large populations of bacteria inhabit leaf surfaces and these leaf-associated (phyllosphere) bacterial communities can have important effects on plant health and they represent an important source of bacteria to the atmosphere. Although phyllosphere communities have been examined for decades using culture-based techniques, the full diversity of these communities has not been well studied. As a result, we do not know if a given plant species harbors similar bacterial communities regardless of location or if different plant species from a single location harbor distinct bacterial communities. We used a novel high-throughput barcoded pyrosequencing technique to characterize and compare the bacterial communities on leaves from 65 different tree species found on the University of Colorado campus in Boulder, Colorado. We also examined geographic variability in the bacterial communities found on a single tree species, Pinus ponderosa, across a range of distances (<1 km to >10,000 km).

Results/Conclusions

We found that individual trees harbored surprisingly high levels of bacterial diversity. The dominant bacterial phyla on the leaves were Proteobacteria, Acidobacteria, and Firmicutes, and some of the more abundant groups were groups that are rarely detected in culture-based surveys. Shifts in community composition were predictable; trees that were more closely related harbored more similar bacterial communities. Likewise, the inter-specific variability in leaf-associated bacterial communities surpassed the  intra-specific variability as the bacterial communities found on a single tree species were relatively similar to one another regardless of the sampling location. Together these results suggest that the structure of leaf-associated bacterial communities is strongly regulated by leaf characteristics raising questions about the role of these bacterial communities in leaf function and susceptibility to disease.