Conceptual differences lead to divergent trait estimates in empirical and taxonomic approaches to plant mycorrhizal trait assignment.

Empirical and taxonomic approaches are the two main methods used to assign plant mycorrhizal traits to species lists. While the empirical approach uses only available empirical information, the taxonomic approach extrapolates certain core information about plant mycorrhizal types and statuses to related species. Despite recent claims that the taxonomic approach is now almost definitive, with little benefit to be gained from further empirical data collection, it has not been thoroughly compared with the empirical approach. Using the most complete available plant mycorrhizal trait information for Europe and both assignment approaches, we calculate the proportion of species for each trait, and model environmental drivers of trait distribution across the continent. We found large degrees of mismatch between approaches, with consequences for biogeographical interpretation, among facultatively mycorrhizal (FM; 91% of species mismatched), non-mycorrhizal (NM; 45%), and to a lesser extent arbuscular mycorrhizal (AM; 16%) plant species. This can partly be attributed to the taxonomic precision of the taxonomic approach and the use of different AM, NM, and FM concepts. Our results showed that the extrapolations of the taxonomic approach do not consistently match with empirical information and indicate that more empirical data are needed, in particular for FM, NM, and AM plant species. Clarifying certain concepts underlying mycorrhizal traits and empirically describing NM, AM, and FM species within plant families can greatly improve our understanding of the biogeography of mycorrhizal symbiosis.