Anthropogenic climate change is predicted to imperil many species, as the climate envelopes their native 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, we investigated how seed germination and establishment of Catawba Rhododendron (Rhododendron catawbiense), an endemic plant of the Appalachian Mountains in eastern US, might be influenced by the presence of local versus novel soil microbial communities, particularly specialized ericoid mycorrhizal fungi. Through a series of greenhouse mesocosm experiments, we compared germination, leaf size, mortality, and ericoid mycorrhizal colonization rates of Catawba Rhododendron and the widespread Great Laurel (Rhododendron maximum) when sown on three soil inoculum types: i) neutral, non-mycorrhizal forest soil as a control, ii) ericoid mycorrhizal fungi (ERMF) communities from beneath a Great Laurel population in Massachusetts, ~800 km north of the range of Catawba Rhododendron, and iii) ERMF communities collected from beneath a Catawba Rhododendron population within its native range in West Virginia.
Results/Conclusions
Across this experiment we found significant evidence of higher germination rates for seeds sown on soils inoculated with novel ERMF communities collected from beneath the heterospecific Rhododendron species. We also found no difference between each species’ local ERMF inoculum and the control forest soil treatments. This reciprocal effect of each species’ increased germination with the other species’ ERMF was not transitory, with no difference in subsequent mortality rate across treatments. Leaf size was significantly higher for plants grown in the Catawba Rhododendron inoculum than the control soil for both species. Mycorrhizal colonization rate followed a similar pattern, with both species demonstrating the highest ERMF colonization rates in the Catawba Rhododendron inoculum, followed by Great Laurel inoculum. The results suggest that the endemic Catawba Rhododendron could successfully form relationships with novel ERMF in areas far north of its small native range, and that the presence of congeneric Rhododendron species might even facilitate seedling establishment with natural or “assisted” northward migration following climate change. The causes of greater germination with novel ERMF could result from an increased capacity to exploit novel partners with which species are not locally-coevolved; however, it is unclear whether the plants or fungi are driving this pattern.