Competition between young exotic invasive and native dominant plant species: implications for invasions within riparian areas

QuestionsThe high competitiveness of exotic invasive species has often been demonstrated, but usually with respect to native species known to have low competitive ability. Considering five exotic and five native riparian species with close characteristics regarding competitive ability, habitat and growth form, we addressed the following questions: (i) do the selected invasive plants produce more biomass than the selected native dominants under competitive pressure; and (ii) are the selected invasive species better competitors than the selected native dominants? LocationCommon garden experiment at the Henri Gaussen Botanical Garden, Toulouse, France. MethodsWe selected five native dominant species and five exotic invasive species co-occurring along a riparian successional gradient of the middle Garonne River (SW France). Young plants of each species were planted in pots in ten intra- and 17 inter-specific combinations in conditions of high water and nutrient availability. To simulate the effects of hydrological disturbance during earlier growth stages, a partial cutting of plants was applied 6weeks after planting. We measured above-ground and below-ground biomass of individuals of each species after 6mo of growth. ResultsThere were large disparities among species performances, regardless of whether the species were exotic or native. The exotic species produced more above-ground and below-ground biomass than the natives species for 73% of the selected species pairs. The exotic species had higher competitive ability than the native species, mainly related to the high competitive effect of I.glandulifera. The two species with the highest biomass production and competitive ability were invasive exotics, whereas the two species with the lowest were dominant natives. ConclusionsOur results predict that competition among young individuals could play a major role for the invasion success of the studied exotic species in European riparian areas.