Mapping bark bacteria: initial insights of stemflow-induced changes in bark surface phyla

Abstract

Life on and within bark surfaces is likely affected by many factors, including bark moisture, bark microrelief, bark pH, and the climatology of forests. The complex set of mutually interacting biotic and abiotic factors operating on bark surfaces presents a challenge in disentangling the linkages and connections between bark-dwelling organisms and biosphere-atmosphere interactions that modulate life in the canopy. Microbial communities on bark surfaces are under-represented in the literature. Given our knowledge of microbial diversity in the phyllosphere and rhizosphere and the impacts of climatology on that diversity, we hypothesized that tree bark would create a microenvironment that selects for specific microbes and that climatological and atmospheric influences from different land uses may further influence such microbial communities. Unfortunately, to our knowledge, no such study to test this hypothesis has been conducted. Recognizing this knowledge gap, samples were collected from bark and stemflow from two northern red oak trees in exurban and suburban forest fragments in the mid-Atlantic region before and after a rain event and subjected to DNA extraction and amplicon sequencing. Major phyla present on bark at both sites include Acidobacteria, Actinobacteria, Proteobacteria, and Bacteroidetes. Significant differences in amplicon sequence variants were observed between the two trees and between the northerly and southerly aspects of the sampled trees. The results of this pilot study highlight the critical need for future work examining the interplay among stemflow and microbial community composition and function (as a part of the larger ecosystem) in relation to varying land use. IMPORTANCE Compared with the phyllosphere, bacteria inhabiting bark surfaces are inadequately understood. Based on a preliminary pilot study, our work suggests that microbial populations vary across tree bark surfaces and may differ in relation to surrounding land use. Initial results suggest that stemflow, the water that flows along the bark surface, actively moves bacterial communities across a tree. These preliminary findings underscore the need for further study of niche microbial populations to determine whether there are connections between the biodiversity of microbiomes inhabiting corticular surfaces, land use, and hydrology.