Migration of soil microbes may promote tree seedling tolerance to drying conditions.

Soil-microbe interactions have the potential to mediate the ability of tree populations to persist in their current location or establish in new areas. Immigration of microbial taxa from drier conditions may promote seedling tolerance to drying climates. In a greenhouse experiment, we determined seedling performance of Ostrya virginiana and Betula nigra seedlings after experimentally swapping sterilized soils and local and foreign microbial inocula from nine sites over a gradient of precipitation and soil types, in well-watered and water reduced conditions. Swapping microbial inocula relative to abiotic soils along latitudinal, but not longitudinal, gradients resulted in reduced seedling biomass. Additionally, growth in water reduced conditions was maximized when pots were inoculated with microbes from drier sites. These results suggest that extirpation of local microbial taxa, and/or immigration of novel microbial taxa to a site may be detrimental to plant growth due to mismatches between microbes and soil conditions. However, immigration of drought adapted microbial taxa may provide additional drought tolerance to plant populations facing drying conditions. This work contributes to the understanding of how microbial interactions may potentially exacerbate or mitigate challenges to plant populations caused by climate change.