Intraspecific variation in mycorrhiza-associated traits predicts plant responses to resource limitation.

Variation in arbuscular mycorrhizal fungal (AMF) root colonization and associated root traits among plant species plays a crucial role in determining plant community responses to environmental change. However, species mean trait values often obscure substantial variation among individuals within a species. Because trait-based ecology has focused predominantly on interspecific differences, the intraspecific responses of AMF-associated traits to resource limitations remain poorly understood. To address this gap, we examined intraspecific variation in AMF-associated traits among genetically diverse individuals of alfalfa (Medicago sativa) and sainfoin (Onobrychis viciifolia) under drought and nutrient limitation. Drought induced the greatest intraspecific variation in plant performance and AMF-associated traits. In both species, baseline levels of hyphae and arbuscule colonization predicted subsequent responses to drought stress. However, species-specific patterns emerged following stress. In alfalfa, plant individuals with lower shoot mortality were associated with greater vesicle colonization and increased average root diameter, indicating a shift toward carbon storage and altered root morphology during stress recovery. Conversely, despite higher overall arbuscule and hyphae colonization in sainfoin, intraspecific variation in drought tolerance was primarily linked to root tissue density, highlighting the importance of root structural investment rather than AMF abundance. These findings highlight that, while AMF-associated trait responses to stress are species-specific, substantial variation exists among individuals within a species, and this variation strongly influences plant performance under stress. Our results underscore the importance of incorporating intraspecific variation into trait-based frameworks for understanding plant community responses to environmental change and for integrating AMF-associated traits into cultivar selection for sustainable agricultural systems.