Alien plant species along watercourses in the Natura 2000 network

Natura 2000 is the centerpiece of EU nature and biodiversity policy. It is a European wide network of nature protection areas aiming at assuring the long-term survival of Europe's most valuable and threatened species and habitats. However, alien plants do not stop their spread at the border of protection areas and invasive plants are reported to threaten aquatic and riparian ecosystems.
Our study aimed at assessing the plant invasion threat along watercourses within the Natura 2000 in Southern Belgium. A stratified random sampling, based on the natural region and the size of the watershed, was conducted. 187 sections of watercourses bank were visited between May and September 2013. For all recorded alien species, data were collected about local abundance, competitive impacts, regeneration potential and ecological conditions. 51 alien species were observed. Some were widespread (e.g. Impatiens glandulifera, with 17 % of linear banks invaded) whereas others were either rare or considered emergent alien species. Emergent species, showing low occurrence but high impact index, are appropriate target species for early detection programs and preventive management actions. Analyses showed that typical riparian species’ occurrence increased with the size of the watershed, indicating propagule pressure within protected areas through hydrochory. No link could be established between the presence of disturbance(s) and alien species in the considered N2000 sites.