SummaryPreferential allocation towards the most beneficial mutualist could maintain mycorrhizal mutualism. Context dependence of preferential allocation could then determine environmental patterns in abundance of mycorrhizal mutualists.We assessed the preferential allocation of carbon (C) and differential phosphorus (P) uptake across four light treatments between the host plant Allium vineale and two arbuscular mycorrhizal (AM) fungi within a split-root system. The ratios of C allocation and P uptake between the beneficial and nonbeneficial AM fungi were measured using isotopic labelling.Allium vineale preferentially allocated more C towards roots infected with the most beneficial AM fungus in high light and, in return, received more P from the beneficial fungus. Preferential allocation declined with shading, as A. vineale allocated 25% of labelled C to roots infected with beneficial AM fungi in high light, but only 15% with shading, a similar percentage to that allocated to roots infected with nonbeneficial fungi regardless of shading.Our findings demonstrate that plant preferential allocation towards the most beneficial mycorrhizal mutualist depends upon above-ground resources, suggesting that the abundance of beneficial mycorrhizal fungi will increase with amount of above-ground resources, with implications for mycorrhizal mediation of plant productivity with anthropogenic change.

Sensitivity of plant species to individual arbuscular mycorrhizal (AM) fungal species is of primary importance to understanding the role of AM fungal diversity and composition in plant ecology. Currently, we do not have a predictive framework for understanding which plant species are sensitive to different AM fungal species. In two greenhouse studies, we tested for differences in plant sensitivity to different AM fungal species and mycorrhizal responsiveness across 17 grassland plant species of North America that varied in successional stage, native status, and plant family by growing plants with different AM fungal treatments including eight single AM fungal isolates, diverse mixtures of AM fungi, and non-inoculated controls. We found that late successional grassland plant species were highly responsive to AM fungi and exhibited stronger sensitivity in their response to individual AM fungal taxa compared to nonnative or early successional native grassland plant species. We confirmed these results using a meta-analysis that included 13 experiments, 37 plant species, and 40 fungal isolates (from nine publications and two greenhouse experiments presented herein). Mycorrhizal responsiveness and sensitivity of response (i.e., variation in plant biomass response to different AM fungal taxa) did not differ by the source of fungal inocula (i.e., local or not local) or plant family. Sensitivity of plant response to AM fungal species was consistently correlated with the average mycorrhizal response of that plant species. This study identifies that AM fungal identity is more important to the growth of late successional plant species than early successional or nonnative plant species, thereby predicting that AM fungal composition will be more important to plant community dynamics in late successional communities than in early successional or invaded plant communities.

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