FungalRoot v.2.0 - an empirical database of plant mycorrhizal traits: A response to Bueno et al. (2021) 'Towards a consistent benchmark for plant mycorrhizal association databases': A response to Bueno et al. (2021) 'Towards a consistent benchmark for plant mycorrhizal association databases'.

Mycorrhizal symbiosis, comprising functionally distinctive plant–fungus associations, mediates key plant population and community processes, and ultimately the functioning of terrestrial ecosystems (Tedersoo et al., 2020). It is estimated that c. 90% of the world’s vascular flora forms mycorrhizal symbioses with soil fungi (Smith & Read, 2008; Brundrett & Tedersoo, 2018). Although this general estimate is probably adequate, there is a severe shortage of empirical information about mycorrhizal associations at the plant species level, with only c. 5% of the world’s flora explored (Moora, 2014; Bueno et al., 2019b). Several database developments have emerged since the seminal work of Harley & Harley (1987; HH); extending both the number and the geography of plant species covered (Wang & Qiu, 2006, WQ; Akhmetzhanova et al., 2012, MID), and defining and describing some key mycorrhizal traits of plant species – ‘mycorrhizal type’ and ‘mycorrhizal status’ (Hempel et al., 2013, MF; Moora, 2014). Nonetheless, this expansion poses new challenges connected with the compilation of global data based on heterogeneous sources with different practical and conceptual frameworks (Bueno et al., 2019b; Kattge et al., 2020). Careful work developing consistent definitions and standardizing field and laboratory protocols is essential for harmonizing database content and avoiding critical inconsistencies (P erez-Harguindeguy et al., 2013; Schneider et al., 2019). Recently, Soudzilovskaia et al. (2020) presented the largest compilation to date of empirical information about mycorrhizal associations in plants based on scientific literature (FungalRoot). Although this is a valuable and unique effort, we note three critical aspects that seriously hamper consistent data harmonization and should be addressed before the FungalRoot database can be considered as a standard reference in the field. Namely: (1) conceptual inconsistency in the designation of plant mycorrhizal associations; (2) incoherent application of plant mycorrhizal trait concepts; and (3) limited transparency in the incorporation of expert opinion. As we explain below, these issues appear particularly problematic in the cases of arbuscular mycorrhizal (AM) and nonmycorrhizal (NM) plants, and perhaps of lesser concern in the cases of other mycorrhizal types. However, given the high share of AM and NM plant species in the previous largest mycorrhizal database (73.1% and 18.0%, respectively, in WQ) and in the FungalRoot database (76.6% for AM, 15.5% for NM), these are critical issues to be resolved.