Of Mutualism and Migration: Will Interactions with Novel Ericoid Mycorrhizal Communities Help or Hinder Northward Rhododendron Range Shifts?

Rapid climate change imperils many small-ranged endemic species as the climate envelopes of their native ranges shift poleward. In addition to abiotic changes, biotic interactions are expected to play a critical role in plant species’ responses. Below-ground interactions are of particular interest given increasing evidence of microbial effects on plant performance and the prevalence of mycorrhizal mutualisms. We used greenhouse mesocosm experiments to investigate how natural northward migration/assisted colonization of Rhododendron catawbiense, a small-ranged endemic eastern U.S. shrub, might be influenced by novel below-ground biotic interactions from soils north of its native range, particularly with ericoid mycorrhizal fungi (ERM). We compared germination, leaf size, survival, and ERM colonization rates of endemic R. catawbiense and widespread R. maximum when sown on different soil inoculum treatments: a sterilized control; a non-ERM biotic control; ERM communities from northern R. maximum populations; and ERM communities collected from the native range of R. catawbiense. Germination rates for both species when inoculated with congeners\u27 novel soils were significantly higher than when inoculated with conspecific soils, or non-mycorrhizal controls. Mortality rates were unaffected by treatment, suggesting that the unexpected reciprocal effect of each species’ increased establishment in association with heterospecific ERM could have lasting demographic effects. Our results suggest that seedling establishment of R. catawbiense in northern regions outside its native range could be facilitated by the presence of extant congeners like R. maximum and their associated soil microbiota. These findings have direct relevance to the potential for successful poleward migration or future assisted colonization efforts

Migration versus mutualism : can the small-ranged endemic plant Rhododendron catawbinse form specialized ericoid mycorrhizae with soil fungi north of its native range?

Climate change is one of the most serious issues facing small-ranged, endemic species whose ability to migrate to is unclear. Rhododendron catawbiense, an endemic shrub species limited to the Southern Appalachian mountains in the eastern U.S., is thought to rely on its mutualist, ericoid mycorrhizal fungi (EMF) to facilitate nutrient uptake and survival on soils with low pH and low nutrients. In this experimental study, we planted seeds of R. catawabiense in mesocosms that had been inoculated with soil organic material from below two widespread Rhododendron species native to New England to test whether the endemic could establish beneficial connections with EMF from beyond its native range. As a control, R. catawabiense seeds were also planted in similar mesocosms that had been microwaved to destroy living EMF and other wild soil biota. Seed germination rate, growth rate, root:shoot ratio, root staining for EMF colonization imaging, and soil chemical analysis were assessed as response variables. Seed germination rate was significantly higher on the live-inoculated soil. However, plant growth during the first three months, as measured by largest leaf size, was significantly greater on the microwave-sterilized soil. In the subsequent 5-6 months, the plants on the microwave-sterilized soil showed signs of nutrient stress and reduced above-ground plant growth compared to those on live-inoculated soils. In the final survey of plant leaf size (in month 10), no significant differences were found between the two treatments, as the growth rate of plants on live-inoculated soil had increased substantially compared to those on sterilized soil. I found that root:shoot ratio on sterilized soil mesocosms was significantly higher than live inoculated mesocosm soils. Consistent with this pattern, only 27% of plants sampled from the sterilized mesocosms had evidence of colonization by EMF, while 100% of samples from the live-inoculated mesocosms were colonized by EMF. The results demonstrated that R. catawbiense could be successfully colonized by soil fungi approximately 700 km outside of its native range, which provides great potential value to the future conservation of the species