Seasonal Dynamics Of Weed Seed Bank In Wasteland Of Shanghai And Its Difference Along Urban-rural Gradient

The habitat degradation,alien species invasion and other ecological and environmental problems caused by rapid urbanization,have exerted lots of impact on urban plants diversity.As one of the three components of urban vegetation,weeds have the characteristics of high sensitivity,strong plasticity and adaptability to heterogeneous habitat,thus weeds become a model object to study the effect of urbanization on vegetation and its response.Research on the difference of soil seed bank in urban and rural gradients and their relation with aboveground weed community can help to reveal the effect of urbanization on weed species diversity.In this study,the soil seed bank of wasteland in Shanghai was chosen as the research object,and 24 plots were selected in urban-suburban-rural gradient for community survey in spring and autumn,in which soil sample was collected for seed bank germination experiment.The difference between urban and rural areas in species diversity and density of soil weed bank were analyzed,with the comparison of the similarity in species composition between soil seed bank and aboveground weed community.Below are the key findings of this research:1)The weed seeds of wasteland in Shanghai are mainly annuals,especially summer annuals,small growth form species and native species.The seed density in autumn(2675ind./m2)was much higher than that in spring(838ind./m2).In urban-rural gradient,the proportion of annual and small growth form species showed an increasing trend,while the difference of seed density and the proportion of exotic species(about 40%)was not obvious.2)There were no significant differences in richness and Shannon-Wiener index of soil seed bank,likewise the above-ground plant community in the wasteland on urban-rural gradient of Shanghai.The richness and Shannon-Wiener index in spring survey were 32 and 3.0 respectively,while in autumn survey the numbers were 22 and 2.6 respectively.The richness and Shannon-Wiener index in spring soil sample were 11 and 2.0 respectively,while in autumn soil sample the numbers were 8 and 1.7 respectively.3)There were significant differences in species composition between soil seed bank and aboveground plant community in Shanghai urban wasteland.Characterize species similarity by Jaccard similarity coefficient.Between the soil seed bank and the above-ground weed communities in spring,the similarity of urban areas(0.14±0.06)was significantly lower than that of suburban areas(0.24±0.08).Between the soil seed bank in spring and the above-ground weed communities in autumn,the similarity of urban areas(0.09 ± 0.06)was significantly lower than that of rural areas(0.17±0.06).4)The weeds species composition of wasteland in Shanghai,both in the soil seed bank and aboveground community,showed obvious seasonal differences between spring and autumn.The average value of similarity(Jaccard similarity coefficient)between spring and autumn both below 2.5.And in urban areas,the similarity of the soil seed bank between spring and autumn(0.11±0.05)was significantly lower than that of aboveground community(0.2010.03).This suggested that highly degree of urbanization leads to a disordered phase change between soil and ground.To sum up,the weed seed bank of urban wasteland in Shanghai is dominated by low growth form and summer annual species.The relatively high proportion of exotic species by human disturbance and the low similarity between soil seed bank and above-ground weed communities,make urbanization as a comprehensive environmental factor,has no significant effect on the weed soil seed bank.