Giant reed, Arundo donax L., is one of the greatest threats to riparian ecosystems of Mediterranean-type climate regions, including California. Forming extensive monotypic stands, A. donax increases the risks of flooding and fire, uses prodigious amounts of water, and reduces habitat value for wildlife. Urban and agricultural development adjacent to riparian ecosystems may contribute to its invasion success. The main hypothesis of my dissertation is that the current abundance of nutrients, water, light, and fire in riparian ecosystems of Mediterranean-type climate promotes A. donax invasion. A two-year field experiment in a riparian ecosystem on the Santa Clara River in California showed that A. donax produced greater biomass than native species under high soil moisture, light, and nutrient levels, and its biomas-s in monoculture was much greater than most species and treatment levels. However, results suggest that high resource levels and this new plant functional group in these riparian ecosystems, rather than superior resource competition, promote A. donax’s competitive exclusion of native riparian species. Analysis of the influence of nutrient enrichment from residential and agricultural land use types on A. donax infestation throughout several coastal watersheds in Southern California revealed that floodplains with enriched soil NO3-N supported A. donax infestations whereas sites with lower N levels did not. Unlike the native red willow (Salix laevigata Bebb.), A. donax may take advantage of anthropogenically enriched N (and K) levels in riparian ecosystems. Regarding fire, A. donax began regrowth from rhizomes immediately after being burned in October 2003 along the Santa Clara River whereas native riparian plants remained dormant for several months, and A. donax grew 3–4 times faster than native riparian plants. A year after the fire, A. donax dominated these burned areas (99% relative cover and a 24% increase in relative  over compared to pre-fire conditions). Arundo donax infestations appear to create an invasive plant-fire regime. These results help elucidate the optimal conditions for A. donax invasion of riparian ecosystems, which in turn can help prioritize control strategies and revegetation of riparian ecosystems.