Proposed trait-based frameworks for AM fungi
(Van Der Heijden & Scheublin, 2007) conducted the first comprehensive review on AM fungal traits to predict plant growth and ecosystem functioning. The authors provided a list of 13 AM fungal functional traits, which they categorized into morphological traits (e.g.,hyphal length, rate and extent of root colonization, spore production) and physiological traits (e.g. , fungal carbon acquisition, host preference, nutrient uptake efficiency). Subsequently, (Behm & Kiers, 2014) noted substantial intraspecific trait variation among AM fungal species (also see (Koch et al. , 2017)), complicating the characterization of traits and their incorporation into functional trait models. To address this issue, they proposed a five-part framework for characterizing intraspecific trait variation of AM fungal species within the context of nutrient cycling, based on experimental design and trait measurement considerations. According to Behm and Kiers (2014), AM fungal genetic units should be subjected to diverse environmental conditions (e.g., host plants, soil nutrient concentrations). Subsequent measurements would encompass the degree of variation, trait reversibility, relationships among traits, the adaptive nature of variation, and the potential for variation to evolve. Through these five dimensions, researchers could map traits onto an evolutionary tree and incorporate them into functional models for predicting nutrient cycling dynamics.
Chaudhary et al., (2022) highlighted the challenges in defining traits for organismal networks such as the mycorrhizal symbiosis. They proposed a unified trait framework, complemented by a standardized vocabulary, with the objective of establishing a clear connection between trait-based mycorrhizal ecology, AM fungal niches and community assembly rules. The authors categorized traits into three main groups: morphological, physiological, and phenological. Within each of these categories, they pinpointed distinctive mycorrhizal traits specific to both the host plant (such as root:shoot ratio, growth form, photosynthetic pathways) and the fungal partner (e.g., spore size, hyphal length, and melanin content). Beyond these bifurcated traits by plant or fungal traits, (Chaudhary et al. , 2022) introduced the concept of mycorrhizal traits. These are unique attributes that emerge during symbiosis and are co-dependent on both partners. They encompass aspects such as root colonization-induced structures, plant mycorrhizal response, and resource exchange rates. This novel framework provides an enriched understanding of mycorrhizal ecology and serves as a basis for the empirical framework proposed here.
(Chagnon et al. , 2013) put forth an AM fungal trait-based framework building on Grime’s CSR (competitive, stress-tolerant, ruderal) framework - which identifies stress, disturbance and competition as the major filters driving trait selection and evolution in natural communities. By allowing speculative connections to be made regarding potential linkages between fungal traits (e.g., hyphal fusion, sporulation phenology, carbon sink strength, growth rates, etc.) and environmental filters (e.g., soil disturbances, scarce C transfer from host, low soil pH), this framework could tentatively identify priority traits for measurement, and combinations of host and fungal traits that may lead to the highest mutual benefits. Building on the apparent family-level conservatism of many traits or responses to environmental filters, parallels were drawn between AM fungal major families and C, S and R strategies. However, as stressed by Chagnon et al. (2013), this family-to-strategy association is simplistic and unlikely to stand the test of time. In addition, it several AM fungal families (e.g., Pacisporaceae, Entrophosporaceae, Diversisporaceae, or more basal lineages in the Paraglomerales or the Archaeosporales). It also fails to consider the relative distribution of different AM fungal families in certain biomes or at certain latitudes. For example, Acaulosporais a common genus in the tropics, where it can be dominant both in natural forests and under intensive land-use where ruderal traits are crucial (e.g., (González-Cortés et al. , 2012). The primary significance of the CSR framework in AM fungal trait-based ecology should not be considered as merely a framework for associating families with strategies. Instead, it should be recognized as a tool for leveraging well-established life-history trade-offs in plant ecology to pinpoint pertinent fungal traits that should be incorporated into our research agenda.
We build upon prior frameworks, emphasizing that the discrepancies observed among studies, stemming from non-standardized experimental approaches and the absence of a comprehensive database on AM fungal traits, represent significant barriers to achieving a more predictive understanding of AM fungal ecology. Moreover, the validity and relevance of the isolates and species employed in these studies are reliant on the taxa available in culture collections or from a few natural communities. A deliberate inclusion of numerous uncultured taxa, or other taxa hitherto overlooked fungal mutualisms in conjunction with AM fungi—such as Mucoromycotina, as suggested by (Hoysted et al. , 2023) —remains an important task. Given the existing data showing large variability in plant and soil responses to the AM symbiosis both among and within AM fungal species, we must address these issues to assess if, and to what extent, AM fungal traits determine growth responses or effects on ecosystems.