INVASIBILITY OF SPECIES-RICH COMMUNITIES IN RIPARIAN ZONES

Invasibility of riparian plant communities was estimated by the percen tage of alien species found along the Adour River (Southwest France) a nd along Lookout Creek, McKenzie River, and Willamette River (Central Cascades, Oregon, U.S.A.). At the patch scale, the invasibilities of r iparian plant communities were compared between one exceptionally rich site of the Adour River and patches selected in the Hob and Dungeness watersheds (Olympic Peninsula, Washington, U.S.A.). Alien species rep resented 24% of 1396 species for the Adour and 30% of 851 species for the McKenzie. They represented 24% of 148 species for the Hob drainage and 28% of 200 species for the Dungeness drainage. Similar trends wer e found along the Adour River and along the McKenzie River for changes in total number of species per site and in percentages of alien speci es per site. These trends may be related to the intermediate disturban ce regimes and tot he physical structure of the riparian corridors. Cl imatic and human factors are also involved in these longitudinal chang es. Positive linear relationships were found between the total number of species and the percentage of aliens observed in each site. At the patch scale, most of the sampled communities contained alien species. Although mature vegetative patches appeared to be invasible, young com munities contained more alien species than older ones. For entire corr idors, a positive linear relationship was found between total species richness and percentage of alien species in each patch type for the ri chest site of the Adour River. This may be partially explained by land scape features considered in a successional context. We suggest the us e of empirical rules, and stress the importance of riparian systems fo r monitoring the conservation of local and regional species pools are suggested.