Distribution pattern of exotic plants in the metropolitan area of Sapporo (Japan) in relation to life form and immigration date

To detect the invasiveness of exotic plants and their effects on native species, the distribution patterns of exotics and natives were examined in the metropolitan city of Sapporo (1,121 km2), northern Japan, by flora lists for 53 districts. The urbanization history has two phases, after the 1868 Meiji Restoration, and after the 1945 World War II End. We analyzed the plant distribution patterns digitized by flora list recorded 45 years after the World War II. The distribution was compared with life forms, immigration ages and environmental factors by numerical analyses. The findings are: (1) Exotics immigrated to the whole areas for short term, and the distribution patterns were determined more by social and economic factors, in particular population density, than by physical factors, such as elevation and geology. (2) Life form is related to the distribution of exotics; viz. annual exotics establish intensively in urbanized areas but do not exclude natives, while perennial exotics, in particular, immigrated before World War II are widespread. To restrict exotics, therefore, prompt treatment to new perennial exotics before becoming biologically invasive plants is important for conserving natural flora. (3) However, the number of exotics increased with the number of natives, showing that the exotics have not significantly deteriorated native flora in the present. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)