Resolving the mycorrhizal status of important northern hemisphere trees

Aims: Knowledge about mycorrhizal associations is important for understanding mineral nutrition, stress tolerance and regeneration dynamics of trees. Here we address the mycorrhizal status of 940 species of important trees growing in ecosystems or cultivated in temperate regions of the Northern Hemisphere by resolving conflicting mycorrhizal trait information. Results: Using 3800 observations from the FungalRoot database, we show that mycorrhizal status is highly consistent within species, genera and most families. Most contradictory mycorrhizal designations result from putative diagnosis errors, such as reported ectomycorrhizas EcM) in otherwise arbuscular mycorrhizal (AM) trees (10% of records). Furthermore, the most commonly studied species are more likely to have incorrect designations in databases due to accumulation of errors. Here we provide a definitive mycorrhizal status based on careful evaluation of records, with additional support from detailed anatomical observations and physiological data. We also identify common causes for errors, such as complex root anatomy in the Cupressaceae and Rosaceae. We also present detailed microscopic images of root structural features in trees with EcM or AM associations. Most AM roots have a suberised exodermis, which forms a permeability barrier around plant-fungus interfaces in the cortex and also protects roots from unwanted fungi. EcM short roots also have highly specialised anatomical features. Conclusions: Tree root atatomical features demonstrate convergent evolution that is presumably linked to more efficient and specific mycorrhiza formation. We recommend that future metastudies use corrected databases and new errors be avoided by using appropriate methodology and consistent definitions of association types.