The Role of Native Riparian Vegetation in Resisting Invasion by Giant Reed, Arundo donax

Several decades of giant reed (Arundo donax L.) management in coastal riparian plant communities of southern California have provided many opportunities to observe the long-term trajectories of restoration projects. The two objectives of the current research were 1) to test if invasion resistance to A. donax could be achieved through native restoration, where carbon limitation through shading was the resource limitation facilitating the resistance, and 2) to determine which factors were most important in restoration success of A. donax removal sites. In the first objective, physiological responses of A. donax to shading revealed that a 94% reduction in total plant mass occurred with extreme shade (5% light), where moderate shading (65% light) was found to facilitate plant establishment. Next, a simulated restoration planting tested the effects of competitive shading from two native shrub species on A. donax establishment. Invasion resistance was only found with one species, mulefat (Baccharis salicifolia), and was positively correlated with the level of shade generated. Complete inhibition of establishment was only observed in the oldest and most densely planted competitive environments. These findings were supported in the wildland where under various native riparian competitive canopies mulefat provided the greatest invasion resistance to A. donax establishment. In the second objective, aged A. donax restoration sites (59 total), from 5 to 20 yr since invasive removal, were surveyed across southern California for restoration success. On a regional scale, active restoration provided early, dense native cover and potentially invasion resistant habitats, whereas passive restoration provided delayed native recruitment. Restoration age was positively correlated with native woody cover and negatively with exotic cover for both restoration types, and total species richness was equivalent regionally. Small passive restoration sites found close to the watercourse were most apt to recruit native species and provide successful habitat restoration. Management methods that leave viable A. donax rhizome fragments in Mediterranean riparian plant communities need to insure that fragments are less than 60 g to minimize invasion potential, as propagule size, especially rhizome fragments greater than 240 g, may limit the competitive effects imposed by existing or actively restored native vegetation