Of mutualism and migration: will interactions with novel ericoid mycorrhizal communities help or hinder northward Rhododendron range shifts?

Abstract

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' 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.

Ericoid mycorrhizal fungi as biostimulants for improving propagation and production of ericaceous plants

“…Seeds of ericaceous plants are small with an average length of 1.5 mm and a mean width of 0.5 mm, and they generally have a poor germination rate due to the limited supply of nutrition from the endosperm. In a mesocosm study conducted for determining if novel ErM communities could assist or hamper the shift of northward species of Rhododendron , Mueller et al. (2022) reported that germination rates of R. catawbiense and R. maximum after inoculation with novel soils containing ErM fungi were significantly greater than those of controls (without mycorrhizal fungi) or inoculated with conspecific soils, 75.2% vs. 54.5% for R. catawbiense and 65.7% vs. 54.4% for R. maximum .…”

“…Ericoid mycorrhizal fungi improve seed germination and rooting of cuttings Mueller et al (2022). reported that germination rates of R. catawbiense and R. maximum after inoculation with novel soils containing ErM fungi were significantly greater than those of controls (without mycorrhizal fungi) or inoculated with conspecific soils, 75.2% vs. 54.5% for R. catawbiense and 65.7% vs. 54.4% for R. maximum.…”

“…Oidiodendron maiusRhododendron fortunei Enhanced rooting and root growth of microcuttingsWei et al, 2020 Unspecified ericoid mycorrhizal fungi Rhododendron catawbiense and R. maximum Increased seed germination ratesMueller et al, 2022 Wei et al 10.3389/fpls.2022 Rhododendron,…”

Plant Species’ Capacity for Range Shifts at the Habitat and Geographic Scales: A Trade-Off-Based Framework

“…Allee effects may occur if animal pollinators are less likely to visit more isolated individuals or low-density populations, such as at a range edge, which could limit the seed set [ 176 , 300 , 301 , 302 ]. Changes in the density or composition of root mutualists, such as mycorrhizal fungi, can also affect range shifts [ 260 , 303 , 304 , 305 ]. For example, reduced densities of mycorrhizal mutualists of native plant species have been shown to enhance the population growth of invading non-native plant species [ 306 ].…”

“…Ultimately, climate change will not only affect plants via direct effects on the abiotic dimensions of the ecological niche, but it will also affect the organisms with which plants have mutualistic and antagonistic interactions, likely causing changes in their population densities and ranges. Complex decoupling of the distributions of plants from their mutualists and antagonists could inhibit or promote range shifts in ways that depend on the species’ ecological strategy and unavoidable trade-offs across life stages [ 263 , 304 , 305 ].…”