Tree Species Diversity And Homogenization In Urban Green Spaces Along Hydro-thermal Gradient Climate Transect In China

With the accelerating process of global urbanization,the role of human beings in the transformation of the ecosystem in the urbanization area is becoming stronger and stronger.Many studies have found that the species of urban green spaces plants between cities have shown a trend of homogenization,and they try to explore the driving factors.However,the current research on the homogenization of tree species in urban green spaces mainly focus on the urban agglomerations within the certain administrative division,or the relatively large cities,which are lack of social and economic constraints on the case cities.So it is impossible to exclude the interference of different urban economic development levels on the diversity of urban green spaces,and it is difficult to confirm whether there is a real homogenization phenomenon and explain its driving factors.In order to highlight the impact of climate on trees in urban green spaces and constrain the variable of urban economic level,this study set case cities on the hydro-thermal gradient transect in China,with their GDP per capita more than 100 000 yuan.This can highlight the change trend of tree diversity of green spaces between cities in similar economic development level and different climate conditions,and explore the mechanism of homogenization of green spaces between cities.Based on the investigation and observation of sample plots in case cities and literature review,the species data of urban green trees and forest trees outside the cities are obtained.The degree of homogenization of tree species in urban green spaces on the hydro-thermal gradient transect is quantitatively analyzed,and the relationship between the structural characteristics of urban green trees and the hydro-thermal gradient is explored.Based on the study of tree communities and field forest communities in five cities including Guangzhou,Hangzhou,Beijing,Xilinhot and Erlianhot,it is found that:(1)The similarity(Jaccard index)of the tree communities in the five cities on the hydro-thermal gradient was higher than that of the forest communities outside the cities.And there is significant difference between them.At the same time,the average of the common species proportion of the five urban green spaces tree communities is 4 times of the average of the common species proportion of the forest communities outside the city.It is found that the urban green spaces tree communities have the phenomenon of homogenization.With every 100 mm decrease of precipitation between cities,the Jaccard index of tree species in urban green spaces of the two cities increased by 0.02,and the Jaccard index of forest community outside the two cities increased by 0.01.With every 1 decrease of moisture index between cities,the Jaccard index of tree species in urban green spaces of the two cities increased by 0.16,and the Jaccard index of forest communitis outside the two cities increased by 0.06.The above results show that the tree diversity of case urban green spaces in China’s hydro-thermal gradient transect is more affected by human activities than that of natural ecosystem.(2)Due to the homogenization of urban green space trees,the growth rate of tree species diversity of urban green spaces with the change of hydro-thermal conditions is lower than the growth rate of tree diversity of forest community outside the city,and the hydro-thermal conditions are not the main factors affecting the α diversity of trees in urban green spaces,and the α diversity of urban green spaces on the hydro-thermal gradient transect also appears the homogenization phenomenon.The Shannon-Wiener index of the forest community outside the city is positively related to the precipitation,annual average temperature and moisture.The Shannon-Wiener index of the forest community outside the city increases by 0.07 with every 100 mm increase of the annual precipitation in the region.However,the Shannon-Wiener index of urban green spaces tree communities have no significant correlation with hydro-thermal conditions,but has a positive correlation with urban level(total GDP),built-up area and population.That is to say,the better the city’s economic situation,the more the urban built-up area and population,the higher the number of tree species in the urban green spaces,there is the "luxury effect".(3)The development trend of homogenization of trees in urban green spaces leads to the change of Pielou index of evenness of trees species in urban green spaces with annual precipitation,annual average temperature and moisture,which is totally different from that of trees species in forest community outside the city.For every 100 mm increase in annual precipitation,the Pielou index of urban green spaces tree community decreased by 0.03,while the forest community outside the city increased by 0.02.In addition,the Pielou index of urban green spaces tree communities also increased with the increase of built-up area and urban population,while the Pielou index of forest community outside the city did not respond to social and economic factors.In a word,human activities lead to changes in the distribution pattern of species in the natural state.Driven by the common factors of ecology,economy and society,the similar behavior of human beings to the transformation of urban living environment and the preference of some species make the phenomenon of the homogenization of urban green spaces trees appear between cities.The results of this study can be used as the basis of future research on urban ecology and ecosystem services.