Restoration of invaded ecosystems in California: nitrogen deposition and rhizosphere interactions as drivers of invasion

California wildlands are impacted by two major factors, anthropogenic nitrogen (N) deposition from internal combustion and agriculture, and invasive species from the Mediterranean Basin. Together these have caused major vegetation-type conversion of native grasslands and shrublands to exotic annual grassland. The negative impacts of type-conversion include loss of native biodiversity in a region known as a global biodiversity hotspot, and increased fire frequency when persistent exotic grass fuels replace less flammable native fuels. Exotic grass fuel load increases under N deposition, which may be as high as 30 kg/ha/yr, levels equivalent to agricultural fertilizer inputs. Exotic grasses are more productive than native species under high N, so regions with higher N deposition are experiencing greater type-conversion. Vegetation that historically burned at < 30 year intervals may burn at < 5-year intervals, making recovery by natural succession impossible. Land managers spend increasingly more time in invasive species control and restoration of native biodiversity. Alternatively, legislative reductions of N inputs would likely enable reduced land management costs. The proposed research falls into three areas: 1) to study the impacts of anthropogenic N deposition and provide the scientific basis for legislation to reduce N deposition. Experiments are designed to test critical loads of N that negatively impact native diversity, invasion, and ecosystem dynamics. Critical loads are values of N deposition in kg/ha/yr that regulators can use to set air quality standards. 2) To test new techniques for ecological restoration that include a) using native species with competitive abilities equivalent to invasive species. Many native plants are poor competitors, so by selecting competitive traits it may be possible to restore native vegetation; b) To reduce soil N by carbon additions to immobilize N thus making it unavailable to plants. Exotic plants have greater soil N requirements for growth than native species. By reducing soil N, native species will be more competitive with exotics; and c) To manipulate soil fungi that may be pathogens or mutualists of native and exotic plants. Exotic species have feedbacks on soil microorganisms that may inhibit establishment of native species. By using a technique of soil transfer to manipulate soil organisms, native plant establishment may be improved; and 3) To study soil and root pathogens of Mediterranean invasive species with an eventual goal of developing biocontrols. Invasive species may be abundant in their home environment because they have been released from pathogens that were not transferred with their host plant. By comparing root pathogens of species (red brome and medusahead grass) that occur in both the Mediterranean and California, we may be able to detect pathogens that control the plants in their home environment. Outcomes/impacts of the work will be to provide managers with new techniques for restoration, to provide regulators with N critical load values for air quality standards, and to identify pathogens that may form the basis of biocontrol strategies for the future.