**7. Species and environmental factors**

The species distribution of different green space types is greatly affected by human factors, so this study only ranks the species distribution in different loops. Because there are many species, only the species with relatively high frequency are shown in the ranking chart. The canonical correspondence analysis (CCA) results of species-environment clearly show the distribution of species in different loops and the average value of relative occurrence frequency. In the tree species environment CCA ranking diagram, the characteristic value of axis1 is 0.411 and the characteristic value of axis2 is 0.255. The two axes show a high correlation between species and environmental factors, which are 0.795 and 0.777, respectively. The interpretation rates of the two axes can reach 42.0% and 68.1%, respectively. The distribution and occurrence frequencies of different tree species in the 2nd ring and the 5th to 6th

ring are not different, but the occurrence frequencies in other rings are significantly different. For example, the average value of occurrence frequency of ash tree in each ring is in the order of: 3rd to 4th ring >2nd to 3rd ring > inside the 2nd ring >5th to 6th ring >4th to 5th ring, while the average value of occurrence frequency of juniper tree in each ring is in the order of: inside the 2nd ring >5th to 6th ring >4th to 5th ring >3rd to 4th ring >2nd to 3rd ring. The characteristic values of axis1 and axis2 in the shrub species-environment CCA ordination diagram are 0.328 and 0.255, respectively. The correlation between species and environmental factors shown in the two axes is relatively high, 0.769 and 0.670, respectively. The interpretation rates of the two axes are 33.6% and 59.7%, respectively. The frequency of occurrence of most shrub species in the 4th to 5th ring is relatively low. Among them, *Buxus megistophylla*, *Buxus sinica*, *Prunus cerasifera*, and *Lonicera maackii* have the lowest distribution frequency in the 4th to 5th ring, while the highest frequency is in the 5th to 6th ring. The average frequency of occurrence in the 2nd ring, the 2nd to 3rd ring, and the 3rd to 4th ring is not much different. The characteristic values of axis1 and axis2 in the herbaceous species environment CCA ordination diagram are 0.206 and 0.156, respectively. The correlation between the species and environmental factors shown in the two axes is also relatively high, which are 0.750 and 0.753, respectively. The interpretation rates of the two axes are 36.3% and 63.8%, respectively. The species of herbaceous plants are significantly more than trees and shrubs, and they are highly concentrated in each ring. Most herbaceous plants have the highest occurrence frequency inside the 2nd ring and in the 5th to 6th ring. Among them, plants such as *Eleusine indica*, *Digitaria sanguinalis*, *Ophiopogon japonicus,* and *Viola philippica* have a higher frequency inside the 2nd ring and in the 5th to 6th ring, and they are distributed in all other rings with little difference in their frequency.

Relevant studies [17–19] show that the high-intensity human interference, invasion of alien species, and reduction of native species caused by urbanization have led to the homogenization of species distribution. This study found that the similarity index of urban green space plants in Beijing is high, indicating that the cultivation and allocation of urban green space plants in Beijing are greatly affected by human activities, especially the species of trees and shrubs on the streets are relatively single, which makes the urban plants tend to be homogeneous. On the issue of protecting urban plant diversity, the first principle to be followed is to respect nature and maintain and protect urban plant diversity guided by respect for nature. According to international excellent experience, the goal of urban biodiversity protection is to introduce nature and forests into cities. The area of green space in the city is small, and the core of protecting urban plant diversity is to use limited green space, scientifically and reasonably carry out ecological allocation, and maintain and restore urban biodiversity. In the early stage of urban development planning, precious plants and areas with high biodiversity value should be protected in a key way, and in garden design, both ornamental value and ecological benefits should be used, native species should be encouraged, exotic species should be introduced reasonably, corresponding plants should be selected and configured under different environmental conditions according to local conditions, species with different ornamental values (fruits, leaves, branches, flowers, etc.) should be configured reasonably, and shade-tolerant ground cover plants should be used scientifically to increase urban plant diversity. Grass germplasm resources are an important part of biodiversity [42] and have the characteristics of strong adaptability, low maintenance and management costs, and unique ornamental value [43, 44], especially ornamental grasses, which play an extremely important role in urban landscape design. In the urban area of Beijing, most native

#### *Assessment of Urban Biodiversity: A Case Study of Beijing City, China DOI: http://dx.doi.org/10.5772/intechopen.106264*

species have extremely high ornamental value and are natural ground cover plants. The application and rational allocation of native species in urban green space plants can reflect the regional characteristics and urban personality of the city and can also reflect the local characteristics of urban landscaping in the region. It will also promote the harmonious coexistence between man and nature and achieve a symbiotic and co-prosperous relationship.

The richness and diversity index of urban plants also change with the degree of urbanization. At present, due to different research sites or methods, there are two main trends of the impact of urbanization on urban plant diversity: one is that plant diversity increases with the strengthening of urbanization [45] and the other is that the plant richness increases along the change from urban area to urban-rural fringe to suburb [26, 46]. McKinney [47] found in the summary of the research on the impact of urbanization on species richness that 65% of the plant research results show that with the slow improvement of urbanization level, the plant species richness continues to increase. However, in this study, within a certain range, the plant richness and diversity index decreased with the increase of urbanization gradient and then showed a gradually increasing trend with the development of the city. Different from previous studies [36], in this study, the richness index of shrubs is lower than that of trees, which may be due to the fact that trees are the main body of artificial green space, shrubs are used in combination, and less species are selected. It is found that the plant diversity index inside the 2nd ring is slightly higher than that in the 2nd to 3rd ring, which may because the landscaping construction inside the 2nd ring pays attention to the introduction and allocation of shrubs and herbs, which makes the diversity of trees, shrubs, and grasses inside the 2nd ring relatively high. Previous studies have shown that woody plant richness in residential areas and parks is higher than that of other land use types [48]. This survey found that the species diversity index of shrubs is more significantly affected by green space types.

The difference of quadrat species diversity index with the change of environmental factors is not significant, but the distribution of species is greatly affected by environmental factors. CCA ranking analysis shows that the average value of distribution and occurrence frequency of plant species in each loop is different, which may be because the microclimate formed in different environments is more suitable for the survival of some species, or because different soil environments affect the distribution of species [49]. It may also because different urban construction areas have different urban landscape design schemes.

Research has shown that urbanization leads to the increase of alien species, while the diversity of native species decreases [29, 45, 50, 51]. The number of introduced species of urban plants in Beijing shows a gradually increasing trend outside the 2nd ring. The main reason may be that in the later urban construction, the introduction and cultivation of exotic garden plants have been gradually increased. At the same time, it shows that more and more attention is paid to the introduction of alien species in urban construction to achieve better plant configuration and ornamental effect, but the introduction of alien species will ignore the phenomenon of native species.

At the beginning of urban development and after it develops to a certain extent, the richness and diversity index of urban plants increase. In urban construction, not only must foreign species be introduced for the design and configuration of garden plants, but more attention must be paid to the selection of local species, that appropriately increase the types and quantity of shrubs and the types of turfgrass. If not necessary, avoid using a large area of single turfgrass, which can not only make the tree, shrub, and grass form a close-to-natural multilayer green space community

structure but also give full play to its ecological benefits to the greatest extent and make the urban biodiversity play its value [52].

Beijing has a wide geographical scope and a huge number of green areas. Because the survey objects are some sample points in the urban artificial green space, the number and area of the selected sample points are limited, so the results of plant species, family and genus distribution, biological diversity characteristics, and other aspects of research are not comprehensive enough. If you want to have a more accurate and in-depth study on the plant diversity of Beijing's urban green space, it is necessary to comprehensively investigate all plant communities in the city and establish a complete Beijing urban green space biodiversity monitoring system to monitor the dynamic changes of urban biodiversity for a long time, so as to provide services for the protection of biodiversity in Beijing.
