Rare plants have more positive plant soil feedbacks than common congeners

Background/Question/Methods

Understanding the factors that regulate species’ abundances is a central goal in ecology and essential to the conservation of biodiversity. For plants, soil microorganisms may feature as important factors regulating rarity and commonness. The theory of soil community feedback (Bever 1994) provides a conceptual framework for linking plant abundance with the belowground microbial community. For example, net negative feedback occurs when a plant species supports microbes more detrimental (or less beneficial) to itself than to other plant species in the community, and should reduce dominance of the recipient plant species. Past studies support the hypothesis that rare plant species experience stronger negative feedback with the soil community than common species. However, limitations of prior work include the use of greenhouse rather than field environments for soil conditioning, the potential for confounding plant relative abundance with plant phylogenetic relatedness, and the inclusion of common, non-native plant species, which may differ from natives in their interactions with microbes. We tested soil community feedback for six native grass species that co-occur in a Midwestern forest understory. Field observations were combined with reciprocal soil transfer experiments to address the question: do rare plant species experience a different direction or magnitude of soil community feedback than common congeners? The 3×2×2 design consisted of plant genus (3: Poa, Elymus, Sphenopholis) × plant rarity (2: rare or common congener) × soil origin (2: soil inoculum collected beneath rare or common congener) × sterilization (2: live or sterile soil). 

Results/Conclusions

If results supported the hypothesis that rare plant species experience stronger negative feedbacks with the soil community than do common species, then the performance of rare species in their own soil relative to soil from a common congener would be more negative than the performance of common species in their own soil relative to soil from a rare congener. In contrast to this expectation, the rare species performed better (greater total biomass) in their own soils than in soils collected from common congeners (positive feedback), and common species performed worse in their own soils than in the soils from rare congeners (negative feedback). The effects of soil origin largely disappeared when soils were sterilized, implicating soil microbes in these feedbacks. Our results suggest that positive feedbacks experienced by rare plant species may aid population persistence, while negative feedbacks may limit the competitive dominance of common plant species.