Role of phylogenetic relatedness in plant–soil feedback dynamics of sympatric Asclepias species

Eric Duell and Gail Wilson, Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK, James D. Bever, Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS

Background/Question/Methods
Plants affect associated biotic and abiotic edaphic factors, with reciprocal interactions collectively known as plant-soil feedbacks (PSFs). While PSFs are well-documented in the field of terrestrial plant ecology, many knowledge gaps remain. The direction and magnitude of PSFs are influenced by a number of factors, including soil biota, herbivory, and environmental conditions. The role of phylogenetic relatedness has recently emerged as a potential driver of PSFs, though the strength and direction of feedbacks among sympatric congeners is not well-studied. Milkweeds (Asclepias) have gained much recent attention, due at least in part to their role as the primary food source of larval monarch butterflies, though relatively little is known regarding the interactions between milkweeds and belowground soil symbionts, such as arbuscular mycorrhizal (AM) fungi. In the Great Plains of central North America, several milkweed species can be found growing in relative close proximity to one another, though very little is known regarding how these co-occurring congeners may be interacting with one another. In our study, we examined plant-soil feedback responses of a common, clonal milkweed species (A. syriaca) and several sympatric congeners across a gradient of increasing phylogenetic distances (A. tuberosa, A. viridis, A. sullivantii, and A. verticillata, respectively).
Results/Conclusions
Biomass of A. syriaca was significantly less when grown in soil conditioned by A. verticillata, relative to conspecific-trained soil, as well as soil conditioned by the three other peripheral species. Additionally, A. verticillata was the only one of the four peripheral species which produced significantly greater biomass in conspecific-conditioned soil, relative to soil trained by A. syriaca. Pairwise feedback interaction coefficients revealed a negative plant-soil feedback between A. syriaca and A. tuberosa, whereas positive feedbacks were detected between A. syriaca and the three remaining peripheral congeners. No differences were observed in the colonization of A. syriaca roots by AM fungi, regardless of identity of soil-conditioning species. Among the four peripheral species, both A. sullivantii and A. verticillata displayed significantly less colonization by AM fungi in soils conditioned by A. syriaca, relative to conspecific-trained soils. Pairwise interactions between A. syriaca and the four peripheral species indicate positive feedbacks with three of the four, with neutral PSF in the fourth. Our results suggest that PSF dynamics between a common, clonal milkweed species and sympatric congeners do not follow the general patterns often observed in PSF among related species.