Effects of non-native riparian plants in riparian and fluvial ecosystems: a review for the Iberian Peninsula

Riparian zones are among the natural habitats more prone to be invaded by exotic plants. In this study we review the causes and consequences of these invasions on fluvial and riparian ecosystems, as well as the effects described for the Iberian Peninsula so far. Riparian zones receive a high propagule pressure of exotic plants, their abiotic conditions are benign for plant life, and their biotic resistance from native vegetation is released by natural (floods) and anthropic (hydrological changes) disturbances. The convergence of these factors explains the high invasion rate of riparian zones. An eventual replacement of native by non-native vegetation might alter the fire regime, the depth of the water table, nutrient cycles and organic matter processing, soil properties, communities of detritivore invertebrates and vertebrates dwelling in rivers and riparian zones. In the Iberian Peninsula we found that the effects of non-native riparian plants were more often negative (e.g. alteration of the structure and activity of microbial communities) than neutral (e.g. similar decomposition rate between native and non-native litter), and rarely positive for primary production (e.g. faster litter decomposition). Our review also highlights a strong bias in the selection of target non-native species (mostly Eucalyptus globulus, Ailanthus altissima and Robinia pseudoacacia) and in the studied effects (mostly litter decomposition and structure and activity of microbial communities). Therefore, a reliable evaluation of the effects of non-native plants in Iberian riparian ecosystems requires extending the number of studied species and of response variables.