Ecological Assembly Rules and Soil Legacy Effects in the Restoration of an Invaded Plant Community

Understanding the composition of ecological communities that arise from potential species pools has implications for community assembly and applications for restoration. Invasive species pose special challenges to restoration by contributing to ecosystem degradation as well as resisting restoration efforts. In the face of such challenges, understanding the complex of mechanisms working together to enable an invasive species to establish and spread may lead to better management strategies and greater restoration success. The overall objective of this dissertation is to understand mechanisms contributing to the success of a Mediterranean annual grass, Bromus diandrus, through the use of both field and greenhouse studies, and to use this understanding to inform restoration of invaded ecosystems. More specifically, I consider three potential mechanisms of invasion: 1) plant functional traits, 2) plant-soil feedback, and 3) soil legacy effects. The results of the studies of this dissertation demonstrate that multiple mechanisms of invasion promote Bromus diandrus success. First, functionally similar native plant communities did not demonstrate biotic resistance to B. diandrus invasion during restoration studies. Rather, earlier germination and larger seed mass of B. diandrus allows this invasive grass to establish even in the presence of morphologically similar native species with greater relative growth rates. Second, positive plant-soil feedback in B. diandrus attributed to the fine arbuscular mycorrhizal fungi contributes to its overall success. Lastly, strong soil legacies in abandoned agriculture also contribute to B. diandrus invasion and inhibit successful reestablishment of native plants. Root fungal pathogens found in abandoned agricultural fields result in decreased biomass of some native species as well as B. diandrus. A greater understanding of the mechanisms contributing to B diandrus invasion success suggests that restoration attempts should seed with functionally similar natives while manipulating germination cues and utilize facilitated microbial inoculations to reduce Bromus diandrus establishment. However, many mechanisms contribute to the overall success of this invasive species making it competitively superior, and eradication of B. diandrus on a large scale is unlikely