Soil mutualists mediate interactions of Trifolium species: Implications for fine-scale co-occurrence
Plant species exhibit varying degrees of spatial co-occurrence in natural communities. Species pairs may be segregated, aggregated, or independently distributed on local to regional spatial scales. These patterns may be driven by multiple factors, including interactions between plants species, interactions between plants and soil microbes, and plant adaptation to local abiotic heterogeneity. Here we, we investigated the effects of soil mutualists, including rhizobial and fungal symbionts, on interactions between Trifolium species with varying degrees of fine-scale co-occurrence within coastal prairie communities. We grew three focal species—T. fucatum, T. gracilentum, and T. microdon—in a factorial greenhouse experiment manipulating competitor (alone, conspecific, or heterospecific), soil source (home or away), and soil microbes (sterile, small, or small plus large microbes) and tested the effects of these factors and their interactions on focal plant performance and rhizobial associations.
Responses to competition and soil microbes were strongest in T. fucatum, which is spatially segregated from the other experimental species in natural communities. T. fucatum was strongly suppressed by conspecific competitors and benefit from the presence of familiar soil rhizobia. In contrast, T. gracilentum and T. microdon, which frequently co-occur together at fine scales, showed limited responses to competition and soil microbes. Across all species, soil microbes modified responses of focal plants to competition. Focal plants were less suppressed by conspecific and heterospecific neighbors in the presence of familiar rhizobia. The presence of plant neighbors in turn modified the response of focal plants to soil microbes. Focal plants experienced a larger benefit of familiar soil microbes when growing with competitors versus alone. These results demonstrate that plant-plant and plant-mutualist interactions interact to determine plant performance, suggesting mechanisms that drive observed patterns of species co-occurrence in natural communities.