Investigating the nature of mutualisms between Rhododendron seedlings and local versus novel communities of ericoid mycorrhizal fungi

Anthropogenic climate change is predicted to imperil many species as the climatic conditions their ranges currently occupy shift poleward rapidly in coming decades. Models forecasting these range shifts focus almost exclusively on change in abiotic factors, although it is expected that biotic interactions will also play a significant, but often unpredictable, role in species’ responses. In this study I investigated the performance of a narrow endemic Appalachian plant Rhododendron catawbiense (Catawba Rosebay) to explore how seed germination and establishment might be influenced by the presence of local versus novel microbial and fungal soil communities, including specialized ericoid mycorrhizae thought to be crucial to the success of many plants in the Ericaceae. Through a series of greenhouse mesocosm experiments, we compared germination rates of R. catawbiense when sown on three soil inoculum types: i) neutral, non-mycorrhizal forest soil, ii) ericoid mycorrhizal fungi (ERMF) communities from beneath the widespread Rhododendron maximum (Great Laurel) in a predicted area of northern range shift for R. catawbiense in Massachusetts, and iii) ERMF communities collected from beneath R. catawbiense within its native range in West Virginia. Across this experiment, and earlier trials, I repeatedly found significant evidence of higher germination rates (P < 0.01) for seeds sown on soils inoculated with novel ERMF communities collected from beneath the other Rhododendron species, but no difference between the each species’ local ERMF inoculum and the neutral forest soil treatments. These results suggest interesting dynamics at play in the mutualistic interaction between Rhododendron seeds and their mycorrhizal fungi partners, possibly including overexploitation of novel partners, but the direction of this interaction is not yet fully understood. The results also strongly suggest that R. catawbiense could successfully establish mutualisms with novel ERMF in areas outside of its native range, and that the presence of congeneric Rhododendron species might even facilitate establishment.