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.

人们早已确定了城市地区在引进外来植物物种以及随后在农村地区传播中的作用。但是，由于缺乏有关外来物种种植频率及其定殖早期阶段的数据，因此主要被间接考虑。在这项研究中，我们利用远处可见的高大观赏植物——amp草（Cortaderia selloana）的奇花异草，研究了超过600公里的人工林和归化林（即种子散布的林）的空间动态。²并超过11年。使用距离采样方法，我们在2002年和2013年分别检测到216个和241个种植林分。在2002年，我们还发现了853个归化林分，这些林分与市区附近的人为栖息地密切相关。这种空间联系本来可以代表从城市地区扩散到自然栖息地的初期阶段。但是，在11年之后，归化林分与市区之间的这种紧密联系并没有减弱，这表明它是一种持续存在的格局。在2002年和2013年期间，只有不到15％的归化个体在自然栖息地中成长，几乎都局限于不威胁当地生物多样性的芦苇床边缘。我们的研究表明，将外来植物大规模引入城市地区，以及随后的大规模归化，并不一定意味着它会传播到自然栖息地。相反，它可以与人为栖息地保持紧密联系。