Measuring and estimating ectomycorrhizal fungal diversity: A continuous challenge

Thousands of ectomycorrhizal (ECM) fungal species exist, but estimates of global species richness of ECM fungi differ widely. Many genera have been proposed as being ECM, but in a number of studies evidence for the hypothesized ECM habit is lacking. Progress in estimating ECM species richness has therefore been slow. Recently, we have published evidence for the ECM habit of fungal species and for the identification of the mycobiont(s) in specific ECM associations, using published and web-based mycorrhiza literature. The identification methods considered were morpho-anatomical characterization of naturally occurring ECMs, pure culture synthesis, and molecular identification. In addition, stable isotope data of C and N, and phylogenetic information were also considered as relevant criteria to assess ECM habit. Our survey indicated that for 343 fungal genera an ECM status has been alleged, and for about two thirds (236 genera) of these supportive evidence of ECM status exists or can at least be entertained as the more reasonable hypothesis. On the basis of our literature search we conservatively estimated ECM species richness around 7750 species. However, on the basis of estimates of knowns and unknowns in macromycete diversity, we suggested that a final estimate or ECM species richness between 20,000 and 25,000 would be more realistic. Recent updates, taking into consideration evidence that became available after our study was released, have not changed figures substantially (234 genera, 7950 species), confirming that current knowledge of ECM fungal diversity, as supported by experimental evidence, is only partly complete, and that inclusion of many fungal genera in this trophic and ecological category is not verified at this stage. Care must thus be used when compiling lists of ECM and saprotrophic species on the basis of published information only. We reflect on the status of the various sources of evidence. We also discuss interesting avenues of future research, including a wider assessment and understanding of the ubiquity and diversity of the secondary root-associated fungi (root endophytes, especially Ectomycorrhiza-Associated Ascomycetes).