**5. Introduced species**

Among the 536 plant species surveyed in the urban area of Beijing, 175 species are introduced species, accounting for 32.6% of the total, including 95 domestically introduced species and 80 foreign introduced species. Trees and shrubs account for a large proportion of the introduced species. Among them, 41 species of arbors are introduced domestically, accounting for 37.6% of the total arbor species, 16 species of arbors are introduced abroad, accounting for 14.7% of the total arbor species. The proportion of shrubs introduced from China is 33.6%, and that from abroad is 14.1%. In the survey, there are 253 native herbaceous species, accounting for 79.1% of the total herbaceous species, and the total number of introduced herbaceous plants is only 20.9%.

The number of introduced species in different rings in Beijing urban area is unevenly distributed, of which 99 species are introduced in the 2nd ring, accounting for 45.4% of the total number of species in the 2nd ring. Outside the 2nd ring, with the increase of urbanization gradient, the number of introduced species showed a gradual increase trend. There are 90 introduced species from the 2nd ring to the 3rd ring, accounting for 37% of the total species. There are 100 introduced species from the 3rd ring to the 4th ring, accounting for 38.6% of the total species. There are 108 introduced species from the 4th ring to the 5th ring, accounting for 41.4% of the total species. There are 129 species introduced from the 5th ring to the 6th ring, accounting for 34.8% of the total species.
