Identifying and Remediating the Microbial Legacy Effects of Invasive Grass for the Purpose of Improved Restoration

Biological plant invasions impact the function and biodiversity of ecosystems across the globe by displacing native plant species and altering the physical and chemical soil environment. In California, invasive grasses have displaced native plants, transforming much of the endemic coastal sage scrub (CSS) to nonnative grasslands. This has occurred for several reasons including increased competitive ability of invasive grasses and long-term alterations to the soil environment, called legacy effects. Despite the magnitude of this problem, however, it is not well understood how these legacy effects have altered the soil microbial community and, indirectly, native plant restoration. Chapter one explores how invasive plants change the abundance and diversity of three important fungal symbionts (arbuscular mycorrhizal fungi, ectomycorrhizal fungi, and fungal pathogens), as well as the implications these changes may have for ecosystem health. I finish off the chapter by discussing restoration efforts designed to ameliorate fungal legacy effects of invasive plants. Chapter two assesses the microbial composition of soils collected from an uninvaded coastal sage scrub (CSS) community (uninvaded soil) and a nearby 10ha site from which the invasive grass, Harding grass (Phalaris aquatica L.), was removed after 11 years of growth (post-invasive soil) in order to better understand the long-term impact of invasive grasses on soil microbes. Our findings indicate that Harding grass may create microbial legacy effects in the soil that likely cause soil conditions inhibitory to the germination rate, biomass, and length of California sagebrush, but not the other two native plant species. Chapter three seeks to understand if differences in native plant growth can be explained by biotic legacy effects and if these legacy effects can be reversed. We measure the growth of three CSS species inoculated with either uninvaded soil or sterilized uninvaded soil and planted into a site with known microbial legacy effects. Our findings indicate that differences in native plant growth can be explained by changes in the soil microbial community and that remediation of the soil microbial community through inoculation can improve restoration in post-invasive sites