Tripartite Plant-Soil-Microbial Relationships May Impact Changing Tree Species Distributions.

Plant soil feedbacks may have profound effects on seedling establishment and fitness due to escape from pathogens or disruption of mutualist interactions. Little is known about how changing biotic interactions from shifting plant species distributions due to changing climate may influence range boundaries. In particular, associations with new soil microbial communities could significantly alter seedling establishment and fitness. We measured differences in biomass and ectomycorrhizal fungi colonization rate for Carpinus caroliniana seedlings grown under controlled conditions using four potential migration scenarios by manipulating plant genotypes, microbial inocula, and sterilized soils in greenhouse pots. The migration scenarios included a status quo condition and simulations where plant genotypes and soil microorganisms migrated alone or together into sterilized soil from sites at higher latitudes. Lower biomass was observed for seedlings migrating outside the current species range; however, this reduction was partially restored when “home” soil microbes co-migrated with the plant genotype. In contrast, within the current species range, C. caroliniana seedlings grew best when migrating into new soils, while soil microbes remained matched to their ancestral soils. No difference in ectomycorrhizae colonization rate was observed among the migration treatments, suggesting that composition, and not abundance, of ectomycorrhizae was important in determining seedling success. Based on molecular analysis of fungal communities in soil and C. caroliniana roots collected from each site, we found a significant positive relationship between seedling biomass and the number of ectomycorrhizal fungi genera shared between the microbial inocula and on roots collected from the sterilized soil source.  No correlation was detected for shared non-mycorrhizal fungi or for additional ectomycorrhizal genera in the microbial inocula. Our findings suggest that shifts in distributions of plant populations with climate or other disruptions is highly dependent on uncertain responses in soil microbial taxa.