The Variable Invasive Success of Cortaderia Species in a Complex Landscape

It is likely that plant invasions result from complex interactions between species traits, community traits, and abiotic conditions. Despite this, there have been few experimental studies that directly examine these interactions. Two morphologically similar perennial grass species in the genus Cortaderia are currently invasive pests in California, but their patterns of invasion differ. In this study I experimentally compared the performance of the two species across four vegetation types in central California. I examined how the different invasive patterns of the species result from differential responses to environmental factors that vary over relatively small spatial and temporal scales. Cortaderia selloana had consistently higher seed germination than C. jubata, but germination rates interacted significantly with soil disturbance and habitat type, and these interactions varied between experimental years. When mammalian herbivores were excluded, the principal cause of transplant mortality was physiological stress, and C. selloana transplants had greater survivorship than C. jubata transplants in all habitats except maritime chaparral. In the presence of mammalian herbivores, however, herbivory became the dominant source of transplant mortality, and the survivorship of both species was indistinguishable in all habitats. At the seasonal wetland site, patterns of transplant survivorship varied significantly between years, probably as a result of increased competition for water during the second experimental year. Transplants of the two species did not differ significantly in growth except at the seasonal wetland site where C. selloana transplants added significantly more leaf area than C. jubata transplants. These results indicate that the relative invasive potential of C. selloana and C. jubata is strongly moderated by the presence of generalist herbivores. The influence of herbivory is in turn dependent on the community type. In addition, plant competitors can modify the invasive success of the two species, but the effect of competitors varies with seasonal differences in resource supply. Although important mechanisms that influence the invasion process can be identified, the complex invasion dynamics exhibited by these morphologically similar species across relatively small spatial and temporal scales suggest that overly broad invasion rules will be of little practical utility in the regional management of invasive species.