Scale matters: the arbuscular mycorrhiza - root hair trade-off from individual plants to communities

1. Plant roots employ multiple strategies to acquire soil resources, including the outgrowth of root hairs and the association with arbuscular mycorrhizal fungi (AMF). These structures represent alternative pathways to access soil resources. Evidence is derived mostly from studies within single species or multi-species experiments under controlled conditions. It remains unclear whether this trade-off holds under natural conditions and whether it manifests consistently across different scales of biological organization. 2. We analysed 81 grassland species representing grasses, forbs and legumes, sampled under heterogeneous field conditions in three regions across Germany, allowing for analyses (i) across different species (interspecific), (ii) across communities (intercommunity), (iii) within plant species (intraspecific) and (iv) within root systems of individual plants (intra-individual). Fine root traits related to AMF colonization (including scoring of fungal hyphae and arbuscules) and root hair growth (including length and incidence) were quantified. 3. Comparing plant species, we observed a strong negative correlation between mycorrhizal colonization and root hair traits, confirming previous studies working under controlled conditions. The negative relationship between root hair length and mycorrhization was also evident when comparing mean trait values at the plant community scale. Among different individuals of the same species (i.e. intraspecific scale), there was no consistent pattern of trait-trait relationship. When comparing within roots of the same plant (i.e. intra-individual scale), co-occurrence of root hairs and mycorrhiza was less likely than what would be expected by chance, with forbs showing the most consistent negative pattern. 4. Our study demonstrates that the hypothesized trade-off between AMF and root hairs is a robust pattern under field conditions at the community and interspecific scale. At the intraspecific scale, no uniform trade-off pattern can be discerned, and within single root systems the negative trade-off pattern is widespread but weak. This indicates that ecological and evolutionary constraints shape trait-trait relationships at higher scale, whereas physiological limits do not restrict trait co-occurrence at lower scales.