Tree species with limited geographical ranges show extreme responses to ectomycorrhizas

Aim: At continental scales, abiotic factors such as climate are typically used to explain differences in plant ranges. Although biotic interactions also underlie the biogeography of plants, the importance of plant-associated microbes is often overlooked when predicting ranges. In particular, symbiotic microbes may influence the distribution of plants that engage in strong interactions with them. We tested whether seedling response to inoculation by ectomycorrhizal fungi explains range size of trees. To examine mechanisms underlying the relationship between range size and response to inoculation, we also examined to what extent sympatry between host and fungi influenced this relationship. Location: Global. Time period: Contemporary. Major taxa studied: Trees and fungi forming ectomycorrhizas. Methods: Using a dataset of 1,275 observations from 126 papers, we calculated mean biomass response (effect size) of 59 tree species to fungal inoculation. We extracted host range area from digitized maps of native distributions, and determined whether hosts were naturally sympatric with fungal species used as inoculum by searching herbaria databases with geospatially referenced data. Results: Tree species with seedling effect sizes falling above or below the average response tended to have small ranges and those with average responses, large ranges. Moreover, hosts inoculated with fungi whose ranges were allopatric to their own had higher biomass compared to those that were inoculated by sympatric fungi, suggesting that the extent of geographical overlap between trees and symbiotic fungi may attenuate the mutualism. Main conclusions: We demonstrate that mycorrhizas may underlie host biogeographical patterns at the continental scale. Our study is novel in the scope of species and scale tested, and points to a possible mechanism underlying this pattern related to the process of mutualism breakdown accruing over time at local geographical scales. For ectomycorrhizal tree species, performance may increase when exposed to fungal partners without a recent shared evolutionary history.