Under the canopy: disentangling the role of stemflow in shaping spatial patterns of soil microbial community structure underneath trees.

Stemflow is a spatially concentrated input of rainwater at the base of trees, resulting from precipitation draining down tree branches to the stem. Depending on tree shape, stemflow can represent a significant fraction of the total rainfall that contacts the tree's canopy area, and can become chemically enriched along its drainage path. As a result, stemflow has been hypothesized to influence microbial communities in the receiving soil proximal to the stem. However, previous studies have (i) yielded conflicting results on the significance of stemflow as a driver in bacterial community composition, and (ii) not directly compared communities in soils with and without stemflow receipt. In this study, a stemflow diversion system was employed on Quercus virginiana trees in Skidaway Island (Georgia, USA) to directly compare soil bacterial communities receiving no stemflow to those beneath trees with no diversion system in place. In both treatments, sample distance from the stem significantly influenced bacterial community structure. However, the absence of stemflow resulted in increased bacterial community diversity across all samples. Stemflow diversion also significantly altered longitudinal patterns in the abundance of multiple taxonomic groups. These results support the hypothesis that Q. virginiana stemflow has a significant impact on bacterial soil inhabitants and is a key factor in taxon selection in stem-proximal communities.