We studied the riparian vegetation on the agricultural floodplain of the Middle Garonne River (SW France) in order to compare native and exotic plant community patterns. In total, we investigated the vegetation of 1,824 plots during four seasons along 50-m-long transects delineated transversely from the banks of five types of water bodies (subsystems) differing by their exposure to natural and to human-induced disturbance. Exotic species accounted for 21% of the total of 726 species identified. We characterized native and exotic plant communities of each subsystem on the basis of Grime's ecological strategies, species lifespan, species richness, and species cover. The communities of each subsystem were compared with respect to the landscape structure (35 patch types) and to their distribution according to the distance to the bank of each water body, the distance to the main channel of the river, the annual duration of inundation, the total plant cover, the total species richness, and the proportion of exotics. Although significant contrasts exist between the community structures of each water body, we found a strong correlation between the attributes of native and exotic species pools. Native and exotic species covers were negatively correlated, while native species richness was positively correlated to exotic species richness, both at local and large scale. This positive correlation remained significant when comparing plots within each patch type. Few or no differences were detected between the distribution of native and exotic species according to the six variables of interest, the effect of the origin (exotic or native) of the plants being negligible as a discriminant attribute. The possible role of landscape complexity and the role of combined natural and human-induced disturbances are discussed to explain these patterns.
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