FUNCTIONAL SIGNIFICANCE OF THE TRIPARTITE ASSOCIATION AMONG SAGEBRUSH SEEDLINGS, ARBUSCULAR MYCORRHIZAL FUNGI, AND DARK SEPTATE ENDOPHYTES

the sagebrush steppe is a common habitat in western north america. this semiarid habitat is characterized by a community of perennial grasses, forbs, biological soil crusts, and several subspecies of the shrub artemisia tridentata nutt (big sagebrush). over the past century, invasion by annual grasses has affected sagebrush communities by increasing the frequency of wildfires. the altered fire frequency tends to eliminate a. tridentata and other components of the native vegetation ultimately leading to plant communities dominated by exotic annual grasses. in undisturbed sagebrush habitats, a. tridentata is dominant and contributes to the development of a heterogeneous landscape that provides habitat and forage for local animals including the greater sage-grouse and pygmy rabbit, which depend on a. tridentata for most of their winter diet. due to the critical roles of a. tridentata in sagebrush habitats, there is considerable interest in reestablishing it following fires. in particular one subspecies is of much interest, a. tridentata ssp. wyomingensis beetle & young (wyoming big sagebrush). this subspecies occurs throughout western north america where it usually occupies xeric locations that receive 200-300 mm of annual precipitation. the xeric environment makes reestablishment of wyoming big sagebrush particularly difficult. major factors responsible for low establishment include slow growth and summer drought. a key factor to improve establishment of wyoming big sagebrush may be a better knowledge of the symbiotic associations between sagebrush seedlings and root fungal endophytes. the long threads that form the body of these fungi grow through the soil and penetrate roots tissues. the threads extending outside the roots increase the surface area from where plants can extract nutrients and water from the soil. fungal endophytes that form associations with sagebrush roots include arbuscular mycorrhizae (amf) and the dark septate (dse). low abundance of beneficial amf or dse may limit seedling establishment, especially in sites that have been invaded by exotic annuals. for amf, prior results support this notion, which will be further tested in this project. in addition, we will begin to assess whether dse have beneficial, parasitic, or no effect on sagebrush seedlings. from seedlings of wyoming big sagebrush, we have isolated a dse within the darksidea genus. in preliminary experiments, this fungus colonized up to 70% of the root length without any apparent negative effect. the ecological significance of this association is, however, unknown. a combination of greenhouse and field experiments involving inoculation of seedlings with darksidea sp. and native amf will be conducted to assess the effect of darksidea on amf colonization and seedling survival under drought. moreover, we will continue isolating and testing other dse with the ultimate goal of identifying taxa that form beneficial symbioses with sagebrush seedlings. identification of such symbioses could readily lead to more effective practices to reestablish sagebrush. this, in turn, would help to maintain biodiversity and enhance the ecological services that the sagebrush steppe provides.