**6. Similarity index and diversity index**

The similarity indices of street green space and park green space between the 3rd to 4th ring and the 4th to 5th ring are the highest, which are 0.50 and 0.52, respectively, indicating that the species composition of the streets and parks in the two loops is the most similar (**Table 2**). The similarity index between the 2nd to 3rd ring and the 4th to 5th ring in the green space in the residential area is the highest, which is 0.48. The three types of green space with the lowest similarity index among the rings are different. On the whole, the similarity indexes between the street green space and the park green space from the 3rd to 4th ring and other rings are relatively high, reaching an average of 0.47 and 0.45, respectively, indicating that the species composition similarity between the street green space and the park green space from the 3rd to 4th ring and the corresponding green space types of other rings is high. The similarity index between park green space and residential green space from the 5th to 6th ring and other rings is relatively low, with an average of 0.35 and 0.39, respectively, indicating that some species of park green space and residential green space from the 5th to 6th ring are less distributed in the corresponding green space types in other rings.

The plant richness indexes of different life forms change differently with the outward expansion of the city. The richness indexes of trees and shrubs show a trend


#### **Table 2.**

*The similarity index between the various rings under different types of green space.*

of decreasing first and then increasing, while the richness indexes of herbs gradually increase with the increase of urbanization gradients. Among the plants of different life forms, herbs are the most abundant. The herb richness index of the 3rd to 4th ring is the highest, reaching 0.89, and that of the 2nd ring is the lowest, which is 0.83. The shrub richness was the lowest, and the shrub richness index was only 0.18 in the 2nd ring, which was relatively high. Trees are slightly higher than shrubs, and the tree richness index in the 3rd to 4th ring is the lowest, which is 0.40, and the highest in the 2nd ring is 0.71.

With the increase of urbanization gradient, the diversity index of trees and shrubs shows a trend of decreasing first and then increasing, while the diversity index of herbs has no obvious change law with the change of urbanization gradient. Among them, the species diversity of herb is the largest, the 5th to 6th ring with the highest herb index reach 0.98, and the lowest 4th to 5th ring also reach 0.86. The second is arbor, the highest diversity index is 0.60 in the 5th to 6th ring, and the lowest diversity index is 0.29 in the 3rd to 4th ring. The diversity of shrubs is the lowest. The lowest shrub index in the 4th to 5th ring is only 0.04, and the highest shrub index in the 2nd ring is only 0.16.

With the increase of urbanization gradient, the species evenness of trees, shrubs, and herbs has no obvious change. Among them, the species evenness of shrub is the lowest, with an average of 0.12, the highest within the 2nd ring is 0.19, and the lowest in the 4th to 5th ring is only 0.05. The evenness of arbor and herb is relatively high, with average values of 0.50 and 0.58, respectively. The highest evenness of arbor is 0.57 in the 2nd ring, and the highest evenness of herb is 0.64 in the 2nd to 3rd ring.

Due to the change of green space types, the species diversity index (richness, diversity, and evenness) of shrubs shows significant difference (*P < 0.05*), and the other species diversity indexes do not show significant difference. With the changes of

