How plant mycorrhizal traits are estimated?

As plant trait measurements are increasingly available, more complete plant trait datasets are compiled allowing to formulate more mechanistic and complex questions in plant ecology. One of the plant functional aspect that are lagging behind is related to plant’s biotic interactions. Mycorrhizal symbiosis is increasingly recognized as a key plant biotic interactions, directly involved in plant survival and distribution. However, little advances have been done in the way these traits are estimated, and thus relative little amount of data is available. 

Recently, an alternative method (taxonomic approach) to the classical collation and revision of empirical evidences (hereafter empirical approach) was proposed. The taxonomic approach is based on an expert-based extrapolation from often unreferenced background mycorrhizal information at the species-level, to plant family level, or exceptionally extrapolated to genus level. Despite the taxonomic approach has been claimed to be almost definitive, no quantitative comparisons have been made to support this claim. A group of researchers from the University of Tartu have prepared the first quantitative comparison between these two approaches to clarify their differences, identify their weaknesses and discuss further improvements in their estimation. This comparison has been just published in the Journal Mycorrhiza. The main findings are that the key differences in the output of both approaches are due to two main causes: 1) the taxonomic extrapolation to family level might not be appropriate systematically, as some of the plant mycorrhizal traits can be highly variable within plant families. Second and even more relevant, 2) both approaches use different conceptual frameworks. For instance, the concepts of arbuscular mycorrhizal, facultatively mycorrhizal or non-mycorrhizal plants significantly differed between the approaches. Overall this paper highlights the need to discuss basic concepts in the field for paving the way for more solid plant mycorrhizal trait approaches to come.

Figure. Graphs with the percentages of matching (blue) and mismatching plant species (red) in a comparison of plant mycorrhizal traits derived using taxonomic and empirical approaches (the empirical approach is used as reference), more details in the paper.

Citation: Bueno, C. G., Gerz, M., Zobel, M., & Moora, M. (2018). Conceptual differences lead to divergent trait estimates in empirical and taxonomic approaches to plant mycorrhizal trait assignment. Mycorrhiza, https://doi.org/10.1007/s00572-018-0869-1 (link to full text)

Abstact:

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.