Composition And Distribution Of Invasive Plants And Their Effects On Herbaceous Community In Shenzhen

Since cities are the main distribution areas of most exotic plants and the critical starting point of the invasion process of many invasive plants,the species composition of urban invasive plants and their effects on diversity are important issues of urban ecosystems,attracting more and more attention.Taking Shenzhen as an example,a field survey was conducted on 600 urban plant plots(100m×100m)in built-up areas.The composition and distribution of invasive plants and their effects on herb community diversity were analyzed.Based on the field vegetation survey data in the built-up area of Shenzhen,the effects of the invasive plants Bidens pilosa and Sphagneticola trilobata on the species diversity and functional diversity of the herb community were investigated based on different green space types.To provide some scientific basis for revealing the relevant influencing mechanism and preventing and controlling the damage of urban invasive plants.The main conclusions of this paper are as follows:(1)Composition of invasive plants in built-up areas of ShenzhenA total of 214 species,152 genera,and 55 families of invasive plants were recorded in the built-up areas of Shenzhen,and 18 species of grade 1 malignant invasive plants were recorded.Twenty-three species were seriously invaded at level 2.Level 3 local invasive included 17 species.Level 4 general invasive included 19species.Thirty-three species remain to be observed,and 29 species were suggested to be excluded.There are 75 domestic species in China.Among them are 86 species of biennial herbs,70 species of perennial herbs,32 species of shrubs,and 22 species of trees.The top three families were Gramineae,Asteraceae,and Fabaceae,accounting for 32.77%.The exotic invasive plants were mainly concentrated in the monospecies family/monospecies genus(1 species)and the few species family/few species genera(2-4 species).Although the family’s distribution is very obvious tropical floristic attributes,the origin is mainly America(North and South America).Bidens pilosa,Sphagneticola trilobata,and Mikania micrantha have become the dominant exotic invasive plants in the built-up areas of Shenzhen.(2)The distribution of invasive plants in built-up areas of ShenzhenDapeng New District had the most significant proportion of invasive plant species,while Futian District had the smallest.The proportion of invasive herbs and lianas in different green space types was higher than that of trees and shrubs.Generally,the more intense the artificial disturbance,the smaller the proportion of invasive plants,especially herbaceous plants.The invasive plants of grades 1-4 were Leucaena leucocephala,Lantana camara,Solanum torvum,Ricinus communis,Mimosa bimucronata,and Solanum erianthum.Among them,the unit green space,road green space,park green space,and residential green space are more distributed.Mikania micrantha was the most invasive vine plant in the 1-4 grade.Both Bidens pilosa and Sphagneticola trilobata appeared in the top five invasive herb values in different administrative regions and greenbelt types,suggesting that they had a wide invasion range and were dominant in the herb community in the built-up area of Shenzhen.(3)Invasive plant species diversity and community stability in built-up areas of ShenzhenIn the quadrat with different invasion degrees,the diversity indices of each invasive grade of herbs showed mild invasion>Moderate invasion>Heavy intrusion.The Simpson dominance index,Shannon-Winner diversity index,Margalef richness index,and Pielou evenness index of the quadrats coexisting with other plants were significantly higher than those of the quadrats composed entirely of invasive plants and those without invasive plants(P<0.05).TWINSPAN classification divided 5995herbal prescriptions into 62 groups,among which 12 groups had more than 2%plots(4780 plots),accounting for 79.7%of the total plots.Axonopus compressus,Bidens pilosa,and Sphagneticola trilobata were the dominant groups in the quadrat.According to the stability value of M.Gordon 1/D,only the quadrat of native plants>Only invasive plant quadrats>A quadrat where invasive plants coexist with native plants.This indicates that the invasive plants will have fierce competition with the species in the community after the invasion,leading to the unstable state of the community.(4)Effects of Bidens pilosa and Sphagneticola trilobata on species diversity and functional diversityMargalef species richness index,Shannon-Wiener diversity index,Simpson dominance index,and Pielou evenness index exhibited a significantly negative correlation with the invasive plants’coverage(P<0.05,0.5865<~2<0.9356).Functional richness index(FRic),Functional evenness index(FEve),and Rao’s quadratic entropy index(FD_Q)had specific correlations with the invasive plants’coverage(0.0000<~2<0.2211).Community-Weighted Mean plant height(CWM_H)had a specific positive correlation with invasive plants’coverage(0.0716<~2<0.2262).Compared with the non-invasive samples,the light invasion of Bidens pilosa significantly increased the species diversity(P<0.05).Still,the severe independent invasion of Bidens pilosa and Sphagneticola trilobata significantly reduced species diversity(P<0.05),and all levels’independent and joint invasions of Bidens pilosa and Sphagneticola trilobata also significantly increased CWM_H(P<0.05).As found by analyzing different green space types separately,the independent and the joint invasion of Bidens pilosa significantly increased the species diversity of all green space types(P<0.05).At the same time,Sphagneticola trilobata had no significant effect on the community species diversity.The ecological effect between Bidens pilosa and Sphagneticola trilobata maybe was antagonistic.The results provide a reference for further studies on plant invasion’s effects on urban herbaceous plant communities and supply a basis for effective control of urban alien plant invasions.