The relative competitive abilities of plants are determined by growth rates, morphological traits such as size and plasticity, physiological traits such as nutrient acquisition rates, and secondary metabolite biochemistry. However, recent research indicates that endophytic symbionts can also increase or decrease the competitive effects of plants on neighbors. Here we explore the role of fungal endophytes as a contributor to the competitive ability of an invasive species. Centaurea maculosa, a highly successful invasive species in North America, hosts a high diversity of fungal endophytes. We tested the role of two fungal endophytes, Alternaria CID120 and Alternaria CID73, on the performance of C. maculosa in competition against both North American and European perennial bunchgrass species.
Our results suggest that one endophyte, Alternaria CID120, has strong direct positive effects on the growth of C. maculosa, both when grown alone and when in competition with other species. Also, Alternaria CID120 has a direct negative effect on grass species, regardless of the continental origin, suggesting that the endophyte may provide both a direct and indirect competitive advantage to C. maculosa. Further, we found that when the competitive effects of C. maculosa were particularly strong, such as against North American species, endophytes had no effect on the outcome of the interactions. However, when the competitive effects of C. maculosa were weak, such as against European grasses, endophyte presence greatly changed the balance of competition in favor of C. maculosa. Our results indicate that symbiotic fungal endophytes enhance the competitive effects of an invasive weed in some cases, primarily when interacting with strong competitors.