Regional and local patterns of riparian flora:comparison between insular and continental Mediterranean rivers

We studied the species distribution patterns of the riparian flora along four Mediterranean rivers at regional and local level, in order to determine the most important gradients and to clarify the role played by biogeography. Floristic data were ordinated to identify the main floristic gradients, with or besides the geographical position of the rivers. Variance partitioning was performed to quantify the relative contributions of variable subsets (biogeography, longitudinal and lateral gradient) to the explained variation, and to test whether and to what extent the influence of biogeography could be distinguished from the effects of other environmental variables. The regional differentiation always emerged as the main variables affecting the distribution of the riverbed floras, thus confirming the primary role of geography in driving riparian plant species distribution at the regional scale comparing insular and continental rivers. After removing the “river effect”, the main gradient at the regional level was the structural transition from mountain to lowland areas, following the longitudinal gradient and the increasing human disturbance. At the local level, longitudinal and lateral gradients were the most important for riparian vegetation. Despite the high value of unexplained variance, variance partitioning revealed that the longitudinal gradient strongly affected riverbed flora patterns, followed by biogeography, while the lateral gradient, determinant at the local scale, had a weak effect at the regional scale. Our study shows how biogeographical differences influence variations in riparian plant species, due in part to the lack of truly aquatic and hygrophilous vegetation found in the study areas.