Premise. Arbuscular mycorrhizal fungi (AMF) can strongly influence host plant vegetative growth, but less is known about AMF effects on other plant traits, the relative impacts of AMF on vegetative growth versus floral traits, or AMF-induced intraspecific variation in traits.

Key results. AMF species varied in their effects on host plants, from negative to positive effects. AMF often had inconsistent effects on vegetative biomass versus floral traits, and therefore quantifying one or the other may provide a misleading representation of potential AMF effects. AMF treatments generated key variation in plant traits, especially floral traits, with potential consequences for plant-pollinator interactions. Given increased intraspecific trait variation in Linum lewisii plants across AMF species compared to uninoculated individuals or single AMF treatments, local AMF diversity and their host plant associations may scale up to influence community-wide patterns of trait variation and species interactions.

Conclusions. These results have implications for predicting how aboveground communities are affected by belowground communities. Including AMF effects on not just host plant biomass but also functional traits and trait variation will deepen our understanding of community structure and function, including pollination.

In an experimental greenhouse study, we inoculated seven species of native perennial wildflowers (Achillea millefolium, Erigeron speciosus, Geranium viscosissimum, Linum lewisii, Lupinus argenteus, Monarda fistulosa, and Phacelia hastata) with six species of AMF (Acaulospora morrowiae, Claroideoglomus etunicatum, Diversispora spurca, Funneliformis mosseae, Gigaspora gigantea, and Rhizophagus clarus) in a factorial design, including un-inoculated individuals as controls. Seeds were sourced locally from the Bridger Plant Materials Center, which has selected and released large quantities of native wildflower species for pollinator enhancement plantings for the region (southwest Montana, United States). In the unlikely event that genetically-related seeds were acquired, they would have been distributed randomly among treatments by mixing within large seed batches. Plant species were selected for their known ability to grow in greenhouse conditions (e.g., Burkle et al., 2020), verified root associations with AMF (data not shown), requirement for insect pollinators for seed production (Schaal and Leverich, 1980; Cruden et al., 1984; Pardee et al., 2018), and diversity of growth forms which may reflect varying investment strategies and provide some initial insight to the potential for life history responses to AMF. We grew seven replicates of each of the species combinations in 1:1 mix of fine sand and topsoil (pasteurized), for a total of 343 pots (6.9 cm dia x 35.6 cm l). AMF species were obtained from INVAM (https://invam.ku.edu), and, because of the lack of tight association at the level of plant species between plant community composition and AMF community composition (Davison et al., 2011; Zobel and Öpik, 2014; Horn et al., 2017) species were selected based on their phylogenetic differences (the six species belonged to five different families) and potential for ecological differences  (Maherali and Klironomos, 2007; https://invam.ku.edu/species-descriptions). AMF were maintained in cultures using Sorghum sudanese as a host plant, and inoculum for the study included soil and colonized root fragments. After adding 175 mL of the soil mix to the bottom of each pot, the inoculum was added as 16 mL (1 tablespoon) mixed with 374 mL soil, and covered by an additional 25 mL of soil. Three seeds of a single species were added to each pot and allowed to germinate. Lupinus argenteus seeds were scarified to increase germination. Growth conditions included a day /night temperature regime averaging 26°C / 15°C and a 16 h photoperiod with supplemental lighting provided by high pressure sodium lamps. Seedlings were thinned to one individual between 10 and 14 days after emergence. Each pot was fertilized once per week with 20 mL of ¼ strength Hoagland’s solution, beginning 3 weeks after seeding. Pots were randomly arranged on the bench, and rotated weekly for the duration of the experiment. We controlled pests with insecticidal soap (Safer brand) as needed. This experiment ran for 22 weeks, from March 1 to August 12, 2013. We allowed the experiment to run as long as possible without plant individuals becoming root-bound; the experiment was terminated when it was clear that no additional plant individuals would flower during the growing season.

For each plant, we recorded plant survival, and, if the plant flowered, we recorded date of first bud, date of first flower, height at first flower, mean flower size (based on width or area of three flowers, depending on forb species), and flower production (the total number of flowers produced by the plant). At the end of the study, the aboveground biomass of each plant was separated into floral biomass and vegetative biomass, dried, and weighed.