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

We studied riparian species distribution patterns along 4 rivers in 2 Mediterranean regions, an island and a continental area. Our aim was to determine the following aspects at 2 spatial scales: i) floristic patterns; ii) major gradients affecting riparian flora; iii) the role of biogeography on riparian vegetation. A stratified random sampling design with segments identified as strata was used. Univariate and multivariate statistics were applied to detect patterns of variation in the species data and main floristic gradients. At regional scale, variation in life forms and phytogeographical element composition showed strongest correlations with climatic variables. At local scale only the continental rivers had many differences in phytogeographical elements linked to their greater length with respect to insular rivers. With regard to the distributions of the riverbed flora, geography emerged as the main gradient at the regional scale. The structural gradient was important at local scale, even if the lateral gradient drove the greatest variation in vegetation composition in continental rivers. Variance partitioning underlined that the biogeographical location strongly affected riverbed flora patterns. Our results support the idea that the flora of Mediterranean riparian ecosystems is not strictly “azonal” but can be defined as “semi-azonal”. Species distribution showed a strong association with climate, which was the primary correlate of change in floristic composition between riparian sites at both scales.