Invasion by alien plant species is currently one of the greatest threats to biodiversity and natural functioning of many types of ecosystems worldwide.  Superior resource competition by alien grass species has been shown as the fundamental mechanism for invasion in many ecosystems.  In this study, I tested the hypothesis that superior competition for soil moisture, light, and nutrient availability promotes invasion of a large bamboo-like grass, Arundo donax L., in riparian ecosystems.  I established a large-scale field experiment with several competition plant groupings (mixed and single-species groupings), soil moisture, light, and nutrient treatments in a riparian ecosystem in Southern California.  I planted 168 four-plant groupings in 12 blocks perpendicular to shallow groundwater flow in a full factorial randomized block design.  Effects of treatments on survivorship and aboveground biomass of A. donax were compared to native Baccharis salicifolia, Salix laevigata, and Populus balsamifera ssp. trichocarpa.  Results show A. donax only suppressed one native species (P. balsamifera ssp. trichocarpa) under the most stressful environmental conditions.  Competition from B. salicifolia and S. laevigata had a strong negative effect on A. donax biomass under high soil moisture, light, and nutrients.  Furthermore, B. salicifolia was facilitated by A. donax in half of the conditions tested.  Despite negative effects of competition, A. donax produced a much higher biomass than native species under high soil moisture, light, and nutrient levels, and A. donax biomass in monoculture was much higher than all species within all treatment levels.  Results suggest that high resource levels and the clonal growth form, not superior resource competition by the invasive A. donax, promote observed preemptive competitive exclusion of native riparian species.  Its unique morphology and physiology, absent in this system before its introduction, allows of A. donax to readily displace natives in many resource levels.