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

Abstract

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.

Keywords: agriculture; bacteria; cortisphere; ecohydrology; urban.

Fig 1

Stacked bar chart of relative percentages of amplicon sequencing variants. In the Banning Park stemflow sample (B SF), the greatest percentages are those of Bacteroidetes and Firmicutes. In the Fair Hill stemflow (FH SF) and Open (rainfall), the largest percentages of the sample are those of Gammaproteobacteria. The largest populations of bark samples are those of Acidobacteria, Alphaproteobacteria, and unassigned bacteria.

Fig 2

Heatmap of abundance for both bark tissue and stemflow and rainfall. (A) Abundance in stemflow from Banning Park and Fair Hill and the open precipitation sample from Fair Hill. Primary abundance in FH SF and Open Pg is P. agglomerans, followed by the Gammaproteobacteria genus and Bacteroidetes. (B) Unassigned bacteria are most abundant in bark tissue samples, as well as Acidobacteria and Proteobacteria. (C) Bark sample group primarily by site more than by height, direction, or time, although these factors are critical for individual phyla. Red lines indicate a lack of statistical significance. Heights labeled 1 through 4 represent 0.5 m, 1.52 m, 2.44 m, and 3.96 m, respectively.

Fig 3

Non-multidimensional scaling plot of samples. Fair Hill samples are red, and Banning Park samples are blue. Stemflow collected from Fair Hill is brown, and stemflow collected from Banning Park is grey. Lab blanks are black. A clear separation is seen by site, also by sample type. The low biomass stemflows are close to the lab blank but not identical, with greater variation in Fair Hill stemflow.

Fig 4

Contour plot of phylum Δabundance. Phylum Δabundance was visualized as a contour plot with the y-axis representing the height of the sampling location (m) and the x-axis representing the aspect. The north direction is shown at both ends to represent the closing of the cylinder, which is the tree bole. Data were smoothed using a third-order polynomial spline to aid in the interpretation of bark surface patterns of abundance. The blue vertical line represents the predominant wind direction for the storm event sampled, while the horizontal dashed line represents the height of the stemflow collar. Green areas show where a given phylum is more abundant after the storm event, white shows areas of no difference, and the brown end of the color map represents areas where phylum is less abundant after the storm event. Note the differences in color ramp values for different phyla.

Fig 5

Difference plots show the changes in the populations above the stemflow collar before and after rain events: (A) Δabundance on bark surface below the stemflow collar after a rain event. Values below the zero line indicate depletion of the population on bark after a rain event; values above the zero line indicate an increase in the population on bark after a rain event. (B) Δ abundance on the bark surface above the stemflow collar after a rain event. Values below the zero line indicate depletion of the population on bark after a rain event; values above the zero line indicate an increase in the population on bark after a rain event. (C) Most abundant phyla remain in stemflow after the subtraction of bulk precipitation. Values above the zero line indicate greater abundances of respective phyla in stemflow than rainfall.