Experimental drought has been shown to delay the development of the root microbiome and increase the relative abundance of bacteria with thick cell walls, or monoderms, especially within the phylum Actinobacteria. However, the generalizability of these findings to natural systems or other diverse plant hosts remains unknown. Leveraging a natural gradient of water-availability across the coast redwood (Sequoia sempervirens) range, we tested three hypotheses: (1) that site-specific water-availability is an important predictor of bacterial community composition for redwood roots and rhizosphere soils; (2) that there is relative enrichment of Actinobacteria and other monoderm bacterial groups within the redwood microbiome in response to drier conditions; and (3) that bacterial growth morphology is an important predictor of bacteria response to water-availability, where filamentous taxa will become more dominant at drier sites compared to unicellular bacteria. We find that diversity (both α- and ß) of redwood bacterial communities is partially explained by water-availability and that Actinobacterial enrichment is a conserved response of land plants to water-deficit. Further, we highlight how the trend of Actinobacterial enrichment in the redwood system is largely driven by the Actinomycetales. We propose bacterial growth morphology (filamentous versus unicellular) as an additional mechanism behind the increase in Actinomycetales with increasing aridity. A trait-based approach including cell-wall and growth morphological may explain the distribution of bacterial taxa across environmental gradients and help to predict patterns of bacterial community composition for a wide range of host plants.

There are 3 missing values for percent carbon and nitrogen (and derived C:N ratios); there was not enough soil remaining from these after soil chemistry and DNA sequencing.