Effects of ungulate grazers on arbuscular mycorrhizal symbiosis and fungal community structure in tallgrass prairie

Abstract

Complex interactions among mycorrhizal fungi, plants, and herbivores occur in grasslands. Grazing of aboveground vegetation may influence plants directly and indirectly through the alteration of mycorrhizal symbiosis and other below-ground processes, and mycorrhizae in turn can influence plant responses to defoliation. An understanding of these interactions is important to our understanding of the dynamics of plant and fungal communities and to the sound management and conservation of grassland ecosystems. In this study, the effects of grazing in tallgrass prairie on mycorrhizal colonization of plant roots, fungal community composition, and extraradical mycorrhizal hyphal (EMH) development were examined. In October 1994 and 1995 rhizosphere samples were taken at different topographical positions in tallgrass prairie sites grazed for several years by cattle at varying intensities, in ungrazed sites, and inside and outside permanent 25-m2 exclosures at each site. Spores of 19 species of AM fungi were encountered in these sites, and Glomus heterosporum was the most abundant species present. Moderate and intense grazing increased root colonization by mycorrhizal fungi in both years of this study. Similar root colonization levels inside and outside permanent exclosures in the ungrazed sites indicated no confounding effects of the exclosure structure itself. In 1995, EMH development was also increased under intense grazing. AM fungal species diversity (based on AM fungal spores present) decreased with grazing under both moderate and high grazer densities in both years. Different fungal species varied in abundance with topographical position, however topography did not significantly affect AM fungal spore species diversity, mycorrhizal root colonization or EMH development in either year. Although overall total spore density was unaffected by grazing intensity or soil type, several individual species increased or decreased in response to these factors. These results suggest that defoliation alters plant resources which stimulates greater development of mycorrhizal symbiosis. The shifts in fungal species composition and decrease in diversity with grazing also indicates that defoliation, or alteration of the soil microenvironment by grazers, favors certain species of grazing-adapted AM fungi that increase under grazed conditions. The high mycorrhizal dependency of many tallgrass prairie grasses and these results together suggest significant interactions between plant-grazer and plant-fungal relationships in tallgrass prairie. In addition to direct effects of herbivory, our results indicate that grazers may influence grassland plants indirectly through alterations in soil communities and in mycorrhizal symbiosis.