| As an important life support system in the city,green spaces contribute to the improvement of human well-being.Affected by urbanization and human preference,the construction of urban green spaces usually leads to the replacement of native species by alien species,resulting in the homogenization of species composition across cities in different climatic zones.However,along the climate gradient,whether the tree species diversity（indicated by the Simpson,Shannon-Wiener,and Pielou indices）in green spaces is homogenized across cities is unclear.In addition,deciduous broad-leaved trees are commonly introduced into relatively warm cities,while evergreen broad-leaved trees are introduced into cities in relatively cold regions.This condition may lead to the homogenization of life from composition（evergreen vs.deciduous）of tree in different cities along a temperature gradient.Therefore,studying from the perspectives of plant species composition,species diversity indices and life form composition（evergreen vs.deciduous）will help to improve people’s understanding of the homogenization of community structure in urban green space across cities.Urban green spaces mainly provide cultural services and regulating services.Affecred by the distance decay effect,the mismatch between the spatial patterns of green spaces and population will lead to the inequalities of per capita green space area and ecosystem services in a city.The inequality of per capita green space area in different location in a city has been studied.However,there is a lack of quantitative research on the inequality of per capita ecosystem services provided by green spaces.This will limit people’s understanding of the relationship between the inequality of per capita green space area and the inequality of per capita ecosystem services of green spaces.Among cities,affected by economy and climate,the greening level of different cities is different,which also lead to the inequalities of per capita green space area and ecosystem services in different cities.Yet,the inequalities of per capita area and ecosystem services of green spaces in different cities have never been quantified.Taking 34 cities as cases,we compared the change trends of tree species composition,species diversity indices and life form composition of green spaces in cities and natural vegetation outside cities along the temperature or precipitation gradient,to explore whether the community structure of green spaces across cities is homogenized and the factors driving homogenization.Based on the green space coverage pattern,land rent pattern,population pattern and the structural attributes of the green spaces from field surveys along the center-edge gradients of the 20 cities,we further quantified the inequalities of per capita green space area and ecosystem services within cities,and explored the relationships between the spatial inequality of green spaces and socioeconomic factors.We also quantified the inequalities of per capita green space area and regulating services between the 20 cities.Finally,through scenario analysis,we explored the impact of the optimization of green space coverage pattern,population pattern and land rent pattern on the improvement of the spatial equalities of per capita green space area and ecosystem services within cities.The main results are as follows:（1）The similarity of tree species composition in green spaces across 34 cities（the average Jaccard value is 0.20）was higher than that in natural vegetation across 11natural regions（the average Jaccard value is 0.08）,showing the homogenization of tree species composition in green spaces across cities.There were 88 tree species widely used in more than half of the studied cities,and 60%of them were used in cities outside their natural distribution areas.The similarity of tree species composition in green spaces between cities decreased with an increase in the temperature difference or precipitation difference between cities.This shows that even if human activities exist,the temperature and precipitation are still important factors affecting the composition of tree species in cities.（2）In the humid area of eastern China,along the temperature gradient,the increasing slope of Simpson diversity index of trees in urban green spaces（0.012）was only～1/2 of that in natural forests（0.030）;the increasing slope of Shannon-Wiener diversity index of trees in urban green spaces（0.027）was only～1/5 of that in natural forests（0.138）.The increasing slope of the proportion of evergreen broad-leaved trees in the green spaces along the temperature gradient（3.857）was slower than that in natural forests（5.208）,and the decreasing slope of the proportion of deciduous broad-leaved trees in the green spaces along the temperature gradient（-3.271）was slower than that in natural forests（-4.310）.It confirmed that the species diversity and life form composition of trees in green spaces across cities was homogenized along the temperature gradient.（3）In temperate regions of China,along the precipitation gradient,the tree species diversity indices（including Simpson,Shannon-Wiener,and Pielou indices）in urban green spaces did not change significantly,while the tree species diversity in natural vegetation deceased significantly with the decrease of precipitation,and the regional vegetation types changed from forests to grassland and then to desert.It confirmed that the tree species diversity in green spaces across cities was homogenized along the precipitation gradient.（4）In the 20 cities,the equality of per capita green space area along the urban center-edge gradient was the highest（the median Gini coefficient of the 20 cities is0.357）,followed by per capita regulating services provided by green spaces（Gini=0.360）,per capita total ecosystem services（Gini=0.430）,and per capita cultural services（Gini=0.431）.Taking Changchun City as an example,the inequality of per capita total ecosystem services within the city（Gini=0.565）was more than 4 times that of per capita green space area（Gini=0.132）.The equality of per capita green space area was positively related to the subjective quality of life,while the equality of per capita ecosystem services was positively related to the economic competitiveness of cities.In addition,the equality of ecosystem services in large cities was higher than that in small cities.（5）The per capita green space area of the 20 case cities ranged from 5.3 to 54.6 m2,which was significantly positively correlated with the mean annual temperature,precipitation,humidity,urban population,and gross domestic product（GDP）.The per capita regulating ecosystem services of the 20 case cities ranged from 4.58 to 69.22USD yr-1,which was significantly positively correlated with the mean annual temperature,mean annual precipitation,humidity,but had no significant relationship with urban population,GDP and built-up area.The equality of per capita green space area between the 20 cities（Gini=0.278）was higher than that of per capita regulating services（Gini=0.319）.In addition,the equalities of per capita green space area and regulating ecosystem services between cities were higher than those within cities.In conclusion,we confirmed the homogenization of species composition,species diversity indices and life form composition（evergreen vs.deciduous）of trees in green spaces among cities in different climatic regions.It is generally believed that the homogenization of community structure in green spaces across cities will cause a series of adverse effects,such as the simplification of tree species and the decline of people’s desire for travel.In this study,we also recognized that the homogenization will meet people in different cities to pursue similar aesthetic preferences a certain extent and promote the equality between cities.Due to the problem of inequalities of per capita green space area and ecosystem services within cities were more serious than that between cities,we also used scenario analysis to find that the spatial optimization measure that green spaces and population are not concentrated in the central area of a city is conducive to improve the spatial equality of per capita green space area and ecosystem services within cities.Therefore,this study will help to promote a deeper understanding of the homogenization of community structure in green spaces across cities;and will provide a reference for decision-making on the selection and allocation of species in green spaces.This study will also help to promote the understanding of the multi-dimensional and multi-scale inequality of green spaces,and will help to evaluate the current policies and provide guidance for urban planning,and will help to address the challenges of exploring strategies for sustainable urban development and human well-being improvement in the context of environmental justice and public health. |
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