Effects of nutrient addition on endophyte-associated grass invasion in a long-term, old-field community experiment

Strictly vertically transmitted (hereditary) Epichloë spp. fungal endophytes are symbionts with cool-season pooid host grasses. Such endophytes may increase host invasiveness in the non-native, introduced ranges. However, the costs and benefits for the host can vary with the abiotic and biotic growing conditions, and the endophyte may become locally or temporally extinct when costs outweigh benefits. Our long-term field experiment involved seven Schedonorus pratensis (meadow fescue) cultivars hosting Epichloë uncinata endophyte, which represent host-grass populations with different genetic background and Epichloë infection levels. In the first six years, the host grasses persisted but did not become invasive in an old-field plant community, regardless of their endophyte infection frequency. We hypothesized that increasing nutrient availability would decrease endophyte costs and thus increase the host’s success and abundance. We fertilized half of the plots for four additional years and re-examined S. pratensis invasiveness. We predicted that increased nutrient availability would increase S. pratensis abundance and E. uncinata frequency and concentration, as well as decrease plant community diversity, relative to unfertilized plots. Fertilization increased endophyte concentrations in three host populations with low endophyte infection frequency. However, E. uncinata did not enable S. pratensis populations to achieve high abundance or to reduce plant community diversity, with or without fertilization.