XVI International Botanical Congess

Patterns of plant species richness (S.R.) and community composition were studied along several riparian zones of rivers from France, USA and Spain. The longitudinal trends found under oceanic climate were very similar and could be related to the intermediate disturbance hypothesis, whereas streams under semi-arid climate showed irregular patterns along their course, as a consequence of the irregularity of the water availability. Exotic and ruderal species were more sensitive to stream zonation and to man-influenced factors than other groups. The percentage of exotics reached up to 60% of the species in oceanic streams. The permanency of surface water was a determinant factor in the control of community structure in semiarid regions, but resulted in less exotic species able to invade the riparian zone. Spatial patterns at the local scale of a stretch of the middle Adour River (SW France) showed a very complex mosaïc of 45 different plant communities. As a result, the S.R. was high (702 species on 200 ha). The highest S.R. was found in fragmented patches along the stream, which were also the most invasible (up to 25% of aliens). The relationships between i) community structure and invasibility by allochtonous species and ii) disturbance regimes and landscape patterns are discussed.