Assessing habitat susceptibility and resistance to invasion by the bioenergy crops switchgrass and Miscanthus × giganteus in California

One potential externality of the bioeconomy is the unintentional introduction and widespread dissemination of bioenergy crops that become invasive species. As one component of parameterizing risk assessment of invasiveness, we evaluated the colonization, survival, and establishment potential of switchgrass (Panicum virgatum) and giant miscanthus (Miscanthus × giganteus) in a riparian and dryland habitat in central California, where both species are non-native, under varying levels of soil moisture availability and competition over two years. Survival (<11%) and performance (≤3 tillers) of transplants of both species were poor in the dry upland environment regardless of competitive regime. In contrast, the lowland riparian habitat supported switchgrass (34–61% survival) and giant miscanthus (5–19% survival) transplants, with survival varying based on soil moisture availability and competitive environment. After two years, switchgrass grown without competition in the first year in the lowland habitat produced about six-fold more tillers that were twice as tall and yielded eight times more aboveground biomass than switchgrass growing in an intact resident plant community. In a second experiment, introduced switchgrass seed and giant miscanthus rhizome survival was extremely poor (<20%) in the upland habitat. Our results indicate that dryland regions of California have very high resistance to either switchgrass or giant miscanthus. However, riparian areas, particularly disturbed low competition areas, are capable of supporting the establishment of switchgrass. Though limited in habitat representation, this is the first study to evaluate experimental introductions of bioenergy crops into potentially susceptible habitat.