Effects of mycorrhizae on growth and demography of tallgrass prairie forbs

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet den sities, vegetative growth rates, and flowering of three forb species were s tudied in native tallgrass prairie in northeastern Kansas. Mycorrhizal acti vity was experimentally suppressed for six growing seasons on replicate plo ts in an annually burned and an infrequently burned watershed with the fung icide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% c olonization). Mycorrhizae influenced the population structure of these forb s. Although mycorrhizal suppression had no long-term effect on genet densit ies and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to l ong-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth an d reproduction varied among species. Ramet growth rates, biomass, and flowe ring of Salvia azurea were greater in plots with active mycorrhizal symbios is, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced. The pattern of responses of these three species to mycorrhizae differed con siderably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorr hizal symbiosis together modulates plant responses and the intensity and pa tterns of interspecific competition between and among tallgrass prairie gra ss and forb species. Further, the results indicate that effects of mycorrhi zae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant per formance and indirect negative effects mediated through altered competitive interactions.