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Exotic plant species of the St Lawrence River wetlands: a spatial and historical analysis

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Abstract Aim 

To evaluate the importance (number of species, plant cover) of the exotic flora in seven well-defined sectors of one of the most important transportation waterways in North America. To determine the impact of exotic species on wetland plant diversity and reconstruct the spread of some invasive species. Location 

St Lawrence River, southern Québec. Methods 

The exotic flora (vascular plants) of wetlands bordering the St Lawrence River was studied using 713 sampling stations (25 m2) along a 560-km long corridor. Results 

Exotic species represent 13.7% of the vascular flora of the St Lawrence wetlands. The relative plant cover occupied by exotic species is high in some of the fluvial sectors (42–44%), but low (6–10%) in the estuarine sectors. Wetlands (marshes) surrounding islands were particularly susceptible to invasion by exotic plants. Historical, abiotic and landscape factors may explain the differences observed between sites. Purple loosestrife (Lythrum salicaria L.) is the most common exotic species of the St Lawrence wetlands, but other species, namely flowering-rush (Butomus umbellatus L.) and reed canary grass (Phalaris arundinacea L.) are much more invasive. There is no linear relationship between the exotic species cover and the diversity of wetland plants; low diversity sites can be dominated by either exotic or native plant species. In the other sites, exotic species generally have little impact on plant communities and can contribute to increase diversity. Common reed (Phragmites australis (Cav.) Trin. ex Steudel) and reed canary grass, both considered as exotic species in this study, clearly have a stronger impact on plant diversity than flowering-rush and purple loosestrife. Main conclusions 

This study shows that the global impact of an invader cannot be adequately evaluated with only a few highly invaded sites. While nationwide strategies have been developed to control exotic species, large surveys are essential to adapt them to regional particularities.
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Keywords: Butomus umbellatus; Lythrum salicaria; Phalaris arundinacea; Phragmites australis; St Lawrence River; diversity; exotic species; herbarium specimens; invasive species; wetlands

Document Type: Research Article

Affiliations: 1: Centre Saint-Laurent, Environment Canada, 105 McGill Street, 7th Floor, Montréal, Québec H2Y 2E7, Canada 2: Centre de recherche en aménagement et développement, Université Laval, Sainte-Foy, Québec G1K 7P4, Canada, and

Publication date: 01 April 2003

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