University of California Water Resources Center

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 this project is that the current abundance of nutrients, water, light, and fire in riparian ecosystems of Mediterranean-type climate promotes A. donax invasion. 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 response to 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 cover 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.