Are Elaeagnus umbellata invasions limited by microbial mutualists?

Mutualistic interactions with soil microbes are a key component of plant invasions. Mutualists such as Rhizobia bacteria often limit invasive plant establishment and performance, but the role of nitrogen-fixing Frankia bacteria in invasions is much less explored. We quantified the net effects of plant-soil feedbacks, including Frankia nodulation, on invasive Elaeagnus umbellata (autumn olive) seedling performance and allocation. The inoculation experiment used soil collected at varying distances from focal shrubs and used live soil and sterilized (autoclaved) soil to separate biotic from abiotic effects. We expected seedling performance would decrease with further distance from shrubs as Frankia abundance and nitrogen inputs decreased. Contrary to expectations, seedling biomass increased with distance from focal shrubs in live soil. The observed distance effects were driven by Frankia colonization as both nodule biomass and nodulation likelihood increased with distance from focal shrubs. Nodule formation appeared beneficial to plant growth, as biomass was correlated with nodule biomass. Seedling performance and allocation did not respond to distance in sterile soil, suggesting abiotic effects play a minimal role in inhibiting nodulation near focal shrubs. These data demonstrate that, while Frankia nodulation positively impacts E. umbellata performance, the abundance of Frankia does not limit E. umbellata establishment at local scales. Future studies should evaluate the specificity of the E. umbellata- Frankia mutualism and the availability of these strains in uninvaded landscapes to understand why these mutualisms do not appear to limit invasion