Technical Abstract: In semiarid western North American riparian ecosystems, increased drought and lower streamflows under climate change may reduce plant growth and recruitment, and favor drought-tolerant exotic species over mesic native species. We tested whether elevated atmospheric CO2 might ameliorate these effects by improving plant water use efficiency. We examined effects of CO2 and water availability on seedlings of five native and exotic western North American riparian species (Elaeagnus angustifolia, Populus deltoides spp. monilifera, Salix exigua, Tamarix sp., Ulmus pumila) in a CO2-controlled greenhouse, using 1-m deep pots with different water-table decline rates. Low water availability reduced seedling biomass by 70-97%, and hindered the native species more than the exotics. Elevated CO2 increased biomass by 15%, with similar effects on natives and exotics. Elevated CO2 increased intrinsic water use efficiency ('13Cleaf), but did not increase biomass more in drier treatments than wetter treatments. The moderate positive effects of elevated CO2 on riparian seedlings are unlikely to counteract the large negative effects of increased aridity projected under climate change. Instead, increased aridity may reduce riparian seedling growth despite elevated CO2, particularly for native Salix and Populus species and less for drought-tolerant exotics (Tamarix sp., Elaeagnus angustifolia and Ulmus pumila).