Linking below- and above-ground interactions: Ericoid mycorrhizae enhance floral rewards and pollinator visitation in highbush blueberry, Vaccinium corymbosum

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Alison K. Brody, Joanna Santoro, Gretchen Saveson and Erin O'Neill, Biology Department, University of Vermont, Burlington, VT

Background/Question/Methods
Most flowering plants interact with organisms above- and below-ground and do so simultaneously. Although the evolution of floral traits is often strongly driven by pollinators, mycorrhizal fungi may affect flowering and floral rewards, thus altering the patterns of pollinator visitation and selection on floral traits and rewards.
We investigated the effects of inoculation with ericoid mycorrhizal fungi on flowering and floral traits in Vaccinium corymbosum (highbush blueberry) at Waterman Orchards in Johnson, VT, USA. Our previous results demonstrated that inoculation with ericoid mycorrhizal fungi enhanced the number of inflorescences produced by plants and overall floral display size. Here, we conducted further experiments to examine if floral rewards and attractiveness to pollinators differed between inoculated and non-inoculated plants at two sites—Waterman Orchards and the George D. Aiken Forestry Laboratory, Burlington, VT. Vaccinium corymbosum at both sites were originally obtained from Hartmann’s Plant Company, Lakota, MI, as 2- or 4-year-old saplings and inoculated at planting (or not) with BioTerraPLUS™—a commercially available mycorrhizal spore preparation. Plants were inoculated at planting in 2011 at the Waterman Orchards and in 2015 at the Aiken Laboratory. Measurements of flowering, pollinator rewards, and pollinator observations were conducted from 16 May—25 June 2021.
Results/Conclusions
Plants of one of four cultivars tested at Waterman Orchards produced 50% more nectar per flower than those that were not inoculated (t34 = 4.093; P = 0.007). Inoculated and non-inoculated plants of the same cultivars at the Aiken Laboratories did not differ in pollinator rewards. Pollinator behavior differed between inoculated and non-inoculated plants, although the differences depended on the type of visitor. The orange-banded bumblebee, Bombus ternarius, visited, on average, 114% more flowers on inoculated plants than on non-inoculated plants (F1,9 = 9.451; P = 0.013) at Waterman Orchards, while large-bodied bumblebees, combined across species, spent more time on flowers of inoculated plants than control plants (F1,22 = 4.614; P = 0.043), but visited equal numbers of flowers at the Aiken Laboratory site.
Our results demonstrate that inoculation with ericoid mycorrhizal fungi can affect floral and flowering traits and pollinator visitation. Although the strength of the links between below- and above-ground interactions may vary with resource availability as well as the pollinator community context, it is important to account for those links in describing selection on floral traits.