Phylogenetic Structure And Diversity Of The Understorv Herb-layer In The Urban Forests Along Urbanization Gradient In Shanghai

Rapid urbanization,as an inevitable trend of the development of human society,has caused many ecological and environmental problems such as habitat degradation,biological invasion and human disturbance,and has had significant influences on urban flora.Weeds,as a special part of urban flora,are characterized by high sensitivity,short life history and strong plasticity,and understory herb-layer is also important for the regeneration of tree seedlings,which make them an excellent object to study the impact of urbanization on urban ecosystem.Research on the phylogenetic diversity and phylogenetic structure of the community can help reveal how historical events and modern factors affect community construction.In this study,92 urban forest plots in Shanghai were chosen as the research object for community survey in spring and autumn.Phylogenetic trees which constructed using the DNA data of the species pool of the common understory herb-layer species in Shanghai and the degree of urbanization was quantified based on the land use types.Using these approaches the impact of urbanization on the understory herb-layer in the urban forests were analyzed at three levels:species diversity,phylogenetic diversity and phylogenetic structure.The main results are as follows:1)221 weeds belonging to 77 families and 182 genera were observed in the understory herb-layer in the urban forests in Shanghai.In life forms and growth forms,the weeds in the urban forests in Shanghai are mainly smallg growth form species,accounting for 62.3%.The proportion of annual species and perennial species are similar,accounting for 53.2%and 46.8%respectively.There was a significant negative correlation between the species richness of the understory herb-layer in the urban forests in Shanghai and the degree of urbanization.There were no significant differences in Shannon-Wiener index on the urban-rural gradient in spring,and there was a significant negative relationship between Shannon-Wiener index and urbanization degree in autumn.2)The phylogenetic tree of the species pool of the common understory herb-layer species in Shanghai,which was constructed based on the DNA data and the APG III classification system are not significantly inconsistent on large scales,such as orders and families.However,in some families that have many species,the relationship of some species is different.There was a significant positive correlation between the diversity of the understory herb-layer in the urban forests in Shanghai and the species richness.The NTI and NRI indexes were negatively correlated with the species richness in autumn,which means he phylogenetic structure of autumn tends to be overdispersed with the increase of species richness,but there is no significant relationship in spring.3)There was a negative relationship between the phylogenetic diversity of the understory herb-layer in the urban forests in Shanghai and the degree of urbanization,but no significant relationship was found between the phylogenetic structure and the degree of urbanization of the urban forests in Shanghai.Only the NTI index in autumn and the degree of urbanization are negatively correlated.In addition,the phylogenetic structure of the understory herb-layer in the urban forests in Shanghai tends to be clustered in the autumn with the increase of canopy density.There was a significant negative correlation between phylogenetic diversity and canopy density in evergreen and deciduous mixed forest.To sum up,urbanization has an influence on the species composition of the understory herb-layer communities of the urban forests in Shanghai;it significantly reduces the species diversity and phylogenetic diversity of the communities,but its effect on the phylogenetic structure is not significant.This urbanization pattern is a result if both the competitive exclusion and the urban area's special comprehensive environmental factors.This study provides major advances in the better understanding of the impact urbanization has on urban vegetation and constitutes the theoretical basis for the design of the urban green infrastructure.