Title: How Plant-Soil Feedbacks at Varying Levels of Plant Community Diversity Influences the Re-establishment and Performance of Native Sagebrush Steppe Species

Program: Biology MS

Committee Chair: Marie-Anne de Graaff

Committee: Marie-Anne de Graaff, Allison Simler-Williamson, Leonora Bittleston

Abstract: Increased frequency of wildfire has become a significant problem in the sagebrush steppe, altering soil biogeochemical properties that promote the establishment of invasives at the expense of native species. To reduce annual grass invasion, restoration efforts often include seeding with non-native bunch grasses such as Agropyron cristatum (L.) Gaertn (crested wheatgrass [AGCR]) that outcompete cheatgrass; however, this strategy does not lead to the return of a native sagebrush steppe ecosystem. Restoring ecosystems to higher levels of native species diversity may reduce invasion and promote sustained success of restoration efforts since biodiversity can improve ecosystem functioning and promote resilience. In 2019 a plant diversity treatment field experiment was successfully established in Owyhee County, Idaho 3 months following a fire. Plots were seeded with a diversity gradient of 2, 4, and 8 native species treatments, as well as AGCR monocultures and non-seeded control plots. Over 4 years I observed changes in plant community composition and soil microbial community composition. In year four, I collected field soil to evaluate how microbial inocula associated with Control, AGCR, 2 species and 8 species treatments impact sagebrush seedling growth. After 6 months, sagebrush seedlings were harvested for aboveground biomass, root biomass, specific root length (SRL), and fungal colonization analyses. My study yielded five main results: (1) Seeding with high native plant species diversity supported a more diverse community of plants and reduced invasion. (2) AGCR monocultures suppressed invasion by cheatgrass, but did not support diverse plant communities. (3) Increasing plant species diversity did not cause different, biologically significant increases in bacterial and fungal diversity. However, fungal and bacterial community composition did differ among diversity treatments. (4) There were no differences in aboveground biomass of sagebrush seedlings grown in treatment soils, but root biomass was the lowest in plants grown in 2 species plot soils in concert with higher proportions of fungal colonization. (5) Microbes associated with Control and AGCR treatments immobilized more N than microbes associated with the 2 and 8 species treatments.