What Will Southern California Rivers Look Like When Dominated by Arundo?

Ecologically important freshwater river systems in Southern California are experiencing invasion by the non-native perennial giant “reed-like”(looks like bamboo) grass Arundo donax. Arundo donax is an aggressive invader; it produces the most above ground biomass, up 40 tons per hectare, and has a growth rate of up to 5 cm per day (Bell 1997 and Purdue 1958) as it grows tall, hollow stalks up to 9 meters high (Dudley 2000). Arundo has large rhizome-root mats that can spread out laterally for many hectares as a thick mass beneath surface, squeezing out other riparian plant life. This large root-rhizome mass forms a resistant layer approximately 1 meter thick that can be undermined during stream erosion processes. Assuming that we will not be able to eradicate this plant, what will Southern California stream ecosystems look like when Arundo is dominant? We hypothesize that a change from diverse riparian forest to a monoculture of Arundo will lead to increased flood and fire hazard on the floodplain, sediment accretion wherever large stands of Arundo are present, channel diversion, and shallow failure of portions of upper banks. For larger braided channels, we predict increased island formation or stabilization within the channel will occur due to an increase in living and dead Arundo debris creating more permanent channels in the braided river. For small single thread channels with intermittent or ephemeral flow, Arundo may nearly fill the channel during the dry season, significantly reducing channel capacity while increasing channel roughness leading to significant channel incision and also expansion of the floodplain.