Abstract
Changes to the natural (historical) disturbance regime of riparian ecosystems can increase the likelihood of invasion by non-native species. Dams increase non-native plant species invasion into riparian ecosystems by altering hydrological regimes and geomorphology of riparian systems. Arundo donax L. (commonly, giant reed) is a perennial grass that is an aggressive invasive species in riparian habitat throughout California and other subtropical regions. It can displace native plant species, reduce habitat quality for native wildlife, and pose serious challenges for conservation management and ecological restoration of degraded habitats. This study tested the hypothesis that abundance of A. donax is greater in dammed streams than undammed streams. To test this hypothesis, I compared the abundance and distribution of individual A. donax clumps along one kilometer transects at three dammed and three undammed streams in Napa County, California. I conducted an initial census along each of the six transects, recording the location of all individual A. donax clumps encountered within the riparian zone (stream bed and immediate floodplain). Post-census, I sampled these data with 36 randomly located circular plots (490 m2) corresponding to locations along each stream transect (216 total). The census encountered 47 individual clumps of A. donax along dammed streams, and one A. donax clump along undammed streams (at Dry Creek). Of the 48 clumps recorded in the census, 32 occurred in random plots along dammed streams, and the single clump on the undammed streams also turned up in the random plots (at Dry Creek). Abundance of A. donax in randomly sampled plots was significantly greater along dammed streams compared to undammed streams. Results suggest that earthen dams play an important role in facilitating downstream colonization of A. donax. By altering the historical disturbance regime of riparian corridors, dams may facilitate the spread of A. donax. Understanding the factors that influence invasion of A. donax in riparian corridors will be important for effective conservation management.