| The process of rapid urbanization has an influence on the flora composition,community structure and diversity distribution pattern of urban vegetation.As one of the important parts of urban vegetation,ruderals are characterized by strong tolerance and high plasticity.Ruderals can be widely distributed in cities without the control of human consciousness,and can respond quickly to the heterogeneity of urban habitats.In this dissertation,the process of urbanization in the area of"one major city with three vice cities and six major groups"of Hangzhou from 2000 was analyzed.The species composition and structure of ruderal communities were investigated in various habitat types on different urbanization gradients.Habitat factors were measured such as light intensity,soil pH,soil electrical conductivity,soil compaction,soil total nitrogen,total soil phosphorus,soil organic matter,and interference types.Based on the analysis of urban ruderal flora in Hangzhou,community types and ecological species groups were further divided,and the relationship between species composition and habitat factors of ruderal communities was analyzed.The distribution pattern of ruderal community diversity in urban heterogeneity habitats and its mechanism were explained.The adaptation strategies of ruderals to urban heterogeneity habitats were explored.The main conclusions of the paper are as follows:(1)During the ten years from 2000 to 2010,dramatic changes have occurred for land use types in Hangzhou.The area of changed land reached 5569.09 km~2.Among them,it was 92.71%of the total change in construction land,cultivated land and grassland.The net increase area of urban construction land reached 2418.5 km~2 and the annual change rate was as high as 9.46%,which has become the main type of land use in Hangzhou.The area of arable land decreased 2357.40 km~2 in the 10 years.It shows that the process of rapid urbanization in Hangzhou is accompanied by a serious loss of arable land.The process of urbanization in Hangzhou presents obvious regional differences.According to the strength of urbanization:Vice City(V),Yuhang Group,Liangzhu Group,and Tangxi Group(G1),Main City(M),Linpu Group,Guali Group and Yipeng Group(G2).(2)213 ruderal species belonging to 52 families and 157 genera were recorded in1665 plots.The species of Gramineae and compositae accounted for 29.30%of total genera and 26.30%of total species.The number of single species/genus and oligospecies/genus was accounted for 67.31%of the total number of families and 97.45%of the total number of genera.It shows that the urban ruderals in Hangzhou have“deterioration”phenomenon,which is similar to that in Shanghai and Harbin.The life form is dominated by the annual ruderal,which has highly tolerant and high plasticity.Referring to the classification principle“high level taxonomic units emphasize ecological appearance while low and medium level units focus on species composition and community structure”,the three-level classification method based on“life form,growth type,and dominant species”was established.The 1665 ruderal communities were divided into 3 life forms,20 growth types,and 125 dominant species community type.Combined with its distribution on the urbanization gradient(M-V-G1-G2),125dominant species communities are divided into 3 groups.The first group is widely distributed ruderal communities,most of which are species with strong adaptability,high plasticity,and wide distribution range,such as Stellaria media community,Rostellularia procumbens community,and Rorippa indica community.The second group is the suitable ruderal communities mainly distributed in the V and G1 regions,most of which are dwarf-growth annual ruderals,such as Paederia scandens community,Euphorbia humifusa community and Glechoma longituba community.The third group is dislike urban ruderal communities,which are distributed in the G2 region of relatively little human disturbance,such as Cerastium arvense community and Carpesium abrotanoides community.(3)The relationship between the number of ruderal community types and urbanization agrees with the moderate disturbance hypothesis.The community types are more in the areas of obvious urbanization(V,94)and relatively obvious urbanization(G1,75).The community types are less in the unobvious urbanization(G2,62)and the highest-degree urbanization(M,51).In addition,the duration of urbanization significantly impacts the species diversity of the community.The species richness and diversity index of the regional community are higher with less than 10-year urbanization.The species richness and diversity index of the regional community in the areas are smaller with urbanization time of more than 10 years and zero.(4)Ruderal community habitats in Hangzhou can be divided into eight types,including forest gap(FG),shrub-grassland gap(SG),gravel abandoned land(GA),soil abandoned land(SA),farmland(FL),lawn(LA),roadside gap(RG)and tree pond(TP).Among them,FG and LA are the most common habitat types.The numbers of plots/species in the plots are 430/147 and 352/130,respectively,and the numbers of community types covered are 20.10%and 16.33%of the total.Along the urbanization gradient,the proportion of FG gradually decreases from 40.00%to 13.86%;but the proportion of FL is gradually increasing from 1.46%to 30.74%.The ratio between LA and GA increases and then decreases along the urbanization gradient.The proportion of SA and SG in the four regions is about 10.00%.In all 7 habitats except TP,the dominant species of the community are mainly dwarf-growth annual ruderals.The environmental factors of different habitats are different.The light intensity of FG is the lowest,the soil conductivity value of SG is the highest,and the light intensity and soil compactness of SA are the highest.RDA analysis shows that soil pH and soil electrical conductivity are the most important habitat factors affecting species composition in FG and SG.Light intensity and soil pH have the greatest impact on species composition in FL.Light intensity is the most important environment factors affecting species composition in GA.Light intensity and soil compactness have the greatest impact on species composition in SA.In general,the most significant factors affecting the species composition are in descending order of their importance:light intensity>soil pH>soil compaction>soil conductivity.(5)Based on the distribution of ruderals in urbanization gradients and their relationship to habitats,213 ruderal species can be classified into four ecological groups.The first group is eurytopic species,which are widely distributed in seven or more habitat types.They are mostly low-growth and adaptable species including world-wide species,exotic species,and species with high resistance to injury,such as Echinochloa crusgalli,Pterocypsela indica,Stellaria media,etc.The second group is shady and wet-tolerant species,which are mainly distributed in forest gaps and shrub-grass gaps,such as Hydrocotyle sibthorpioides,Cardamine hirsuia,Commelina communis,etc.The third group is mesophytic species,which are mainly distributed in abandoned lands and farmland,such as Kalimeris indica,Humulus scandens,Leptochloa chinensis,etc.The fourth group is trample-and mowing-tolerant species,which are mainly distributed in the lawn,roadside gaps and tree pool habitat,such as Plantago asiatica,Dichondra repens,Euphorbia humifusa,etc.In summary,since 2000,with the advancement of urbanization in Hangzhou,land use types have changed significantly,resulting in significant differences in weed diversity and community composition on the gradient of urbanization.The differentiation of urban weed habitats on the gradient of urbanization and the formation of heterogeneous habitats are the major reasons.In addition,the length of time that the environment is facing urbanization pressure is also an important factor.Based on the distribution of weeds in urbanization gradients and their corresponding relationship with habitats,four types of ecological species groups can be divided and used to indicate the intensity of urbanization. |
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