When aliens are cousins: The influence of species relatedness on plant-soil feedbacks in California

Plant-soil feedbacks are increasingly studied as mechanisms of community dynamics, including coexistence and invasion. Although Darwin’s naturalization hypothesis suggests relatedness should be a critical factor governing such feedbacks, this is rarely accounted for in these studies. This issue seems especially important in the California grasslands where many exotic grasses are closely related both to each other and to native species. We investigated the effects of soil collected from experimentally-established grassland communities on three native and three exotic grass species grown in the laboratory. Species phylogeny was the most important factor in determining both biomass and root-to-shoot ratio of grasses. Grasses had significantly lower biomass when grown in active field soil compared to sterile soil, with a significant interaction between phylogenetic group and soil treatment. In contrast, soil treatment did not significantly affect root-to-shoot ratio of grasses, while life history did have an effect, with perennials exhibiting significantly higher allocation to root biomass than annuals. The composition of the plant community from which soil was taken had variable effects on biomass and root-to-shoot ratio across species. These results suggest that grouping species by provenance and/or life history may not accurately predict species responses to soil biota and researchers must consider phylogeny when comparing soil’s effects on native and exotic species.