The mutualistic interactions of plants with below-ground soil biota, such as mycorrhizal fungi, appear to be crucial for many species’ performance and local distribution. Such specialized relationships are also of concern at larger geographic scales in the face of anthropogenic climate change. For example, will rare or range-restricted plant species be able to successfully colonize new regions where co-adapted mutualistic partners from their native range might be absent?
In this study, we investigated the ecological interactions of a small-ranged endemic plant, Rhododendron catwabiense (Ericaceae), with soil biota from sites in the Northeast US, 800-1000 km beyond its native range in the Southeast. Members of the Ericaceae form highly specialized ericoid mycorrhizae with fungal mutualists, often allowing them to grow on nutrient-poor soils. To test how R. catwabiense might interact with soil biota beyond its native range, we inoculated 30 experimental mesocosms with organic layer material from the bases of two widespread, Northeast native Rhododendron species. Soil in half of the mesocosms was then sterilized to eliminate fungi and other soil biota. Fifty seeds were sown into each mesocosm, and seed germination, seedling growth rate, leaf tissue nutrient status, and colonization of roots by ericoid mycorrhizae were compared.
Results/Conclusions
Germination rate was significantly higher (75%) in mesocosms with live inoculum compared to those that had been sterilized (55%). In the first two months of growth, sterilized soil seedlings grew significantly larger (mean largest leaf = 6.2 mm length) than those on live inoculum (4.1 mm). However, the high growth rate of seedlings on sterilized soil subsequently declined, while that of seedlings on live inoculum soil increased, resulting in comparable leaf size by the end of the growing season (10.3 vs. 10.6 mm). In addition, many seedlings on sterilized soil began to show qualitative signs of nutrient stress, such as leaf discoloration. Harvested and stained root samples revealed that seedlings on the live inoculated soil had successfully partnered with ericoid mycorrhizal fungi, despite the inoculum being sourced from habitats far beyond the native range of R. catwabiense. Overall, it appears that R. catwabiense can readily partner with new mycorrhizal mutualists far outside its native range. This result is important, as several recent studies have otherwise shown high degrees of local co-adaptation between plants and mycorrhizal fungi. The presence of other Rhododendron and Ericaceae plant species in new regions, and their associated ericoid mycorrhizal communities, might facilitate colonization by migrating species.