Replacement of estuarine communities by an exotic shrub: distribution and invasion history of Baccharis halimifolia in Europe

Baccharis halimifolia L. (Asteraceae) is a shrub native to North America which is invading estuarine communities in Europe. We report the invasion history and the distribution limits of B. halimifolia in Europe, with particular emphasis on the frequency of its presence in estuarine communities in Spain. B. halimifolia has been cultivated in Europe since the 17th century to present. It was first recorded as naturalized in the Bay of Biscay in 1906, where it forms currently stable and locally abundant populations in almost all the estuaries of Northern Spain and Western France. The ongoing invasion to the west could reach well conserved estuarine communities in Galicia (Spain). B. halimifolia also forms scattered populations in Northern and Southern France, Belgium, Netherlands, United Kingdom and Italy. In these countries it has experienced a rapid expansion during the last years. In Northern Spain, subhalophilous communities dominated by rush (Juncus maritimus) and/or sea couch (Elytrigia atherica), common reed stands (Phragmites australis) and ungrazed wet meadows are the most vulnerable to invasion. The subhalophilous communities are part of natural habitats of community interest according to the habitats directive 92/43/EEC. In some areas of Northern Spain these communities have been totally replaced by monospecific stands of B. halimifolia. In contrast, halophilous communities of the low marsh are resistant to invasion, suggesting that the survival of B. halimifolia may be limited by threshold values of salinity and waterlogging. With this study we want to raise awareness about the risk of replacement of estuarine subhalophilous communities by the ongoing invasion of B. halimifolia in Europe.