Do Woody Plants Prevent the Establishment of Common Reed along Highways? Insights from Southern Quebec

The common reed (Phragmites australis) is one of the most invasive vascular plants in northeastern North America. A competitive genotype from Eurasia has recently invaded road and agricultural ditches, which facilitate the dispersal of the plant over long distances. However, large tracts of roadsides-apparently propitious for the establishment of the plant-are not invaded by the grass. We hypothesized that the absence of the invader is associated with physical and biological characteristics of roadsides. To test this hypothesis, we collected field data and developed two statistical models to explain the presence or absence of the common reed along a highway of southern Quebec highly invaded by the plant but with contrasting patterns of common reed distribution. The models explained 23 to 30% of the total variance and correctly predicted the presence or absence of common reed 71% of the time. The models suggest that a dense woody plant cover over a drainage ditch limits the establishment and/or expansion of the common reed, probably by competition for light and space. Also, shaded ditches are not subjected to a frequent maintenance, and are therefore less disturbed, probably further reducing common reed invasion because the germination of their seeds is less likely without soil disturbance. This study yields insights on the potential of woody plants for controlling the expansion of invasive grasses, and could help to justify the preservation of dense shrubs and tree hedges along right-of-ways.