LAUSR

The role of urban areas in the introduction of alien plant species, and their subsequent spread in the countryside, has long been identified. However, it was mainly considered indirectly due to the lack of data on the frequency of planting of alien species and their early stages of colonization. In this study, we took advantage of the spectacular flowering of the pampa grass (Cortaderia selloana), a tall ornamental plant, visible from afar, to study the spatial dynamics of planted and naturalized stands (i.e. those resulting from seed dispersal) over 600 km2 and over an 11-year period. Using distance sampling methods, we detected 216 and 241 planted stands in 2002 and 2013, respectively. In 2002, we also detected 853 naturalized stands, closely associated with anthropogenic habitats around urban areas. Such spatial association could have represented the initial stages of a spread from urban areas to natural habitats. However, 11 years later, this very close association between naturalized stands and urban areas has not weakened, suggesting a persistent pattern. In both 2002 and 2013, less than 15% of naturalized individuals grew up in natural habitats, almost all limited to the edges of reed beds where they did not threaten local biodiversity. Our study shows that the massive introduction of an alien plant into urban areas, and its subsequent massive naturalization, does not necessarily means its spread to natural habitats. On the contrary, it can remain closely associated with anthropogenic habitats.