XVI International Botanical Congess

Symbiotic microbes are potential determinants of plant diversity, and ecological processes responsive to diversity. Tall fescue (Festuca arundinacea), the dominant grass over the eastern U.S., is frequently infected by a fungal endophyte that enhances host fitness. A 4-year experiment compared diversity and productivity of large grassland plots established from seed banks supplemented with infected (E+) or E- fescue seeds. Vegetation structure was greatly affected by the endophyte. From initially similar communities, species richness declined 40% in E+ plots relative to E- plots. While total biomass was not affected, the proportion comprised of tall fescue increased significantly higher in E+ plots. These results indicate that the host-specific endophyte, totaling only a few grams per hectare, significantly suppressed plant diversity and suggest that endophyte symbiosis is a primary determinant of diversity in other grasslands.