Plant community invasibility in riparian landscapes: Role of disturbance, geomorphology, and life history traits

Riparian landscapes are highly susceptible to invasion by non-native plant species. High productivity and frequent disturbances associated with flooding contribute to invasibility of riverbanks and floodplains. The hydrogeomorphology of riparian landscapes is intricately tied to plant community composition and structure. Plant invasions often coincide with the period directly following regulation because novel riparian habitat is created. Native species lag behind introduced species in colonization of new riparian habitat due to life history constraints imposed by adaptation to the previous disturbance regime. Plant invasions often coincide with the period directly following river regulation, but non-native plant species also spread along rivers that have not been hydrologically altered.;Tamarix spp. are invasive shrubs that have spread prolifically throughout riparian landscapes of the southwestern US. Tamarix has a long period of seed release and high salinity and drought tolerance relative to native pioneer shrubs and trees. These characteristics combined with the ability to form a thick litter layer contribute to competitive exclusion of native shrubs. Tamarix control is currently conducted with a variety of methods (herbicide application, mechanical removal, biocontrol, burning, flooding). I researched the potential for prevention of Tamarix establishment using controlled flooding along one of the most regulated rivers in the US, the Colorado River through the Grand Canyon National Park.;In addition to hydrologic and climatic regimes, bi-trophic interactions (e.g., herbivory, pathogens) also influence riparian plant community composition and structure. I investigated the potential for selective foraging by beavers to affect coarse-scale spatial patterns of riparian vegetation along the Colorado River in the Grand Canyon. Spatial associations of beaver occurrence and Salix and Tamarix cover were analyzed using multiple linear regression models after accounting for relationships with geomorphic variables (geomorphic reach, sinuosity, and rock resistivity). Beaver presence had a strong positive association with Tamarix cover and a slight, positive association with Salix cover. This suggests that Tamarix and beavers occupy similar habitats even at spatial scales less than 4.5 ha, beavers prefer habitats with high Tamarix cover, or that beavers promote Tamarix dominance through selective foraging of Salix.;To explore the hypothesis that river regulation increases dominance of non-native species I conducted a survey of riparian shrubs and trees along 20 river segments across the southwestern US. I created an index of flow alteration based on hydrologic conditions prior to and after dam completion or prior to and after 1962 for non-dammed rivers. Regressions of dominance of native (Salix exigua and Populus spp.) and nonnative (Tamarix spp. and Elaeagnus angustifolia) woody species with the degree of flow alteration revealed a positive relationship between Tamarix and flow alteration, a negative relationship between Populus and flow alteration, and no significant relationship between S. exigua and E. angustifolia and flow alteration. Native and non-native species respond to hydrologic characteristics based on their life history strategies, not based on their native status. Therefore, river regulation does not necessarily increase cover of nonnative woody plant species.;I organized seven guidelines for river restorationists based on a comprehensive review of riparian research. These guidelines focus on restoration principles relevant to woody riparian vegetation and are particularly applicable for regulated rivers where flow regime can be altered for restoration purposes. I advocate formulation of alternative flood regimes based on knowledge of natural variability, consideration of effects of increased fire frequency in regulated systems and potential opportunities to use fire and floods to reinvigorate plant establishment and geomorphic processes, and use of functional groups and consideration of multi-trophic species interactions to predict effects of management scenarios. Watershed-scale and site-scale restoration approaches are often required for restoration of connectivity, variability, and geomorphic processes. The most promising strategy for riparian restoration planning, implementation, and monitoring remains adaptive management. However, the temporal scale of adaptive management must extend to incorporate climate change scenarios. (Abstract shortened by UMI.)...