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 the effects of CO₂ and water availability on seedlings of two native (Populus deltoides spp. monilifera, Salix exigua) and three exotic (Elaeagnus angustifolia, Tamarix spp., Ulmus pumila) western North American riparian species in a CO₂-controlled glasshouse, 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 (Δ¹³C(leaf) ), 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. Our results suggest that increased aridity will reduce riparian seedling growth despite elevated CO₂, and will reduce growth more for native Salix and Populus than for drought-tolerant exotic species.
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