RAPID INVASION OF FRAXINUS-ORNUS L ALONG THE HERAULT RIVER SYSTEM IN SOUTHERN FRANCE - THE IMPORTANCE OF SEED DISPERSAL BY WATER

To document the range extension of a plant species in a region in which it was previously absent, we have examined the distribution pattern of Fraxinus ornus L. 65 years after its introduction to the Herault River in southern France. Censuses of flowering individuals showed that Fraxinus ornus successfully colonized, but remained mainly restricted to, flood-disturbed riparian habitats in the Herault River system. The present distribution of this species showed that it spread along the river system at an average rate of 970 m/yr. We suggest that this very rapid spread, of a usually wind-dispersed species, has resulted from water transport of seeds during periodic flooding in autumn.