Quantitative Survey On Exotic Weeds In Nanjing And Comparison Of Invasive Characteristics Of Exotic Weeds Of Veronica

The invasion of exotic weeds by natural spread with aid of human, intentionalintroduction and unintentional introduction has caused a great losses in both economy andecology worldwide. In recent years, many researches on exotic weeds have been conductedincluding species surveys and damages of regional exotic weeds as well as biology, ecology,genetic diversity, control methods and utilization of important exotic weeds. At the sametime risk assessment and management measures of exotic weeds have tried to be proposed.However, the investigation about quantitative ecology of exotic weeds for a special regionhave been little documented. There was different viewpoints on standards and methods ofrisk analysis about the invasiveness of exotic weeds, which were caused mainly by theknowledge difference of risk analysis indexes.In this paper, the distribution, community characteristics and dynamics of exoticweeds in Nanjing, Jiangsu, China were studied to exhibit the occurrences and distributionsof genus Veronica and all other species in Nanjing region. The reproduction capacity,photosynthetic characteristics, allelopathy as well as genetic diversity of the selected fourexotic weed species of genus Veronica, V. persica, V. arvensis, V. didyma and V. hederifoliawere studied to elucidate the invasiveness of exotic weeds determined by the key factorsand to provide some theoretical basis for weed risk assessment.As the fundamental work, quantitative surveys on exotic weeds may be useful inassessment of infection and their integrated management. In different year and seasons, thespecies and distribution of exotic weeds were surveyed with square intercept method indifferent habitats in Nanjing. The results are showed as followed. (1) There weretwenty-one exotic weed species belonging to nineteen genera and eleven families recordedin autumn in the region. The human disturbance degree and soil humidity are the mainfactors affecting the occurrence and distribution of exotic weed species. According to the two factors, all samples were divided into four groups. GroupⅠincluded the samples fromdry land. The dominant exotic weed species were Amaranthus retroflexus L., Veronicapersica Poir. and Alternanthera philoxeroides (Mart.) Griseb. GroupⅡincluded thesamples collected in the wasteland and sideway where ever were non-managed ornon-irrigated farmland. The dominant exotic weed species mainly belonged to compositesuch as Erigeron annuus (L.) Pers.,Conyza bonariensis (L.) Cronq. and Conyza canadensis(L.) Cronq. GroupⅢincluded the samples collected in public parks where the number ofexotic weed species was the most. The dominant species were Ambrosia artemisiifolia L.,Plantago virginica L., Alternanthera philoxeroides (Mart.) Griseb., Aster subulatus Michx.,Erigeron annuus (L.) Pers., Conyza bonariensis (L.) Cronq. and Conyza canadensis (L.)Cronq. The last group mainly composed of the samples distributed in the waterside and thepaddy field. The dominant exotic weeds were Alternanthera philoxeroides (Mart.) Griseb.,Aster subulatus Michx. and Medicago hispida Gaertn. The analysis of the difference ofspecies richness among groups showed that invasion of exotic weeds had harmful influenceon biological diversity. (2) There were thirty-six exotic weed species recorded in spring,2002. The frequency of exotic weed emergence in 74 samples is 98.65％. The sequence ofthe average predominance of exotic weeds in different habitats is: Waterside＞Vegetablefield＞Wasteland＞Roadside＞Scenic spot＞Ridge＞Lawn＞Orchard＞Wall slot＞Fields＞Habitats around house＞Brae. Conyza bonariensis (L.) Cronq. Geranium carolinianum L.and Veronica persica Poir. were the most invasive weeds among the total 36 main weeds inNanjing region since they had the highest niche breadth.ased on literatures and field surveys, the distribution and damage of four exoticweeds of genus Veronica were studied in China. V. persica is regional malignant weedwhich caused severe damages in summer crop fields in Yangtze River drainage basin. V.didyma is important weed in Huang-Huai-Hai River basin. V. arvensis is common weedwith limited distribution area and slighter damages. V. hederifolia distributed only in scenicspots of Nanjing, Jiangsu and Dinghai, Zhejiang where it have not invade into crop fieldsyet. The damage rank of the four exotic weeds in China is V. persica＞V. didyma＞V.arvensis＞V. hederifolia. But in Jiangsu province, the rank is V. persica＞V. arvensis＞V.didyma＞V. hederifolia.Sexual and asexual reproductive of main exotic weeds in genus Veronica was studiedto analyze the characteristics and difference of reproduction capacity among them. The results are exhibited as followed. Despite the same or different habitat, the reproductivecapacity among individuals of those four exotic weeds varied significantly while theresource allocation proportion among different organ components was the same amongindividuals. Veronica persica had the higest seed setting rate, and V. didyma and V. arvensesecond. Although the seed setting rate per fruit was the lowest, thousand-grain weight of V.hederifolia was the highest. The seeds of all those four exotic weeds had dormancycharacteristic. Except V. hederifolia, the other three exotic weeds had asexual reproductioncapacity in which V. persica was the highest. The rank of reproduction capacity is V.persica＞V. arvensis＞V. didyma＞V. hederifolia.Alleopathic potency of those four species was biassayed through testing seedgermination rate, root length and dry weight of seedling and assessed comprehensively bysubordinate function value test. The results showed that: (1) those four exotic weedsexhibited the allelopathic effects; (2) higher concentration of three kinds of extracts fromthem could inhibit the seed germination, root length and dry weight of seedlingsignificantly, while the lower concentration of the extracts had positive effects on someindexes. The inhibitory effects increased with the increase of concentration of the extracts;(3) the allelochemicals of the four exotic weeds existed more in the aqueous extracts andethanol extracts and less in ethyl acetate extracts; (4) the rank of alleopathic effect amongthe four exotic weeds is V. hederifolia＞V. arvensis＞V. persica＞V. didyma.The research about photosynthetic characteristics showed that all those four exoticweeds had higher net photosynthetic rate in sunny habitat than that in shade habitat. V.persica has a higher fixation capacity of carbon dioxide and the compensation point of lightis the highest. V. hederifolia is adaptive to weak light habitat because the lamina leaf massper unit area (LMA), net photosynthesis rate (Pn) and light compensation point were thelowest among the tested four exotic weeds, which is accordant with its main habitat. Onlyfor V. didyma, the difference of Fv/Fm was significantly between sunny and shade habitat.The comparison of adaptive capacity to light intensity among those four exotic weeds byapplying a subordinate function value test showed that the rank is V. persica＞V. didyma＞V. arvensis＞V. hederifolia.Genetic diversity of those four exotic weeds was studied using AFLP marker method,and the result showed a relatively high level of genetic variation existed in inter and intra populations. But genetic variation at inter populations was richer than that of intrapopulation. V. persica had the highest genetic diversity. A UPGMA dendrogram based ongenetic distance could be used to distinguish five different weed species through analysis ofindividaual samples belonging to one population of those four exotic species and one nativeone. There was a greater proportion of diversity among species than that within species.Gene flow (Nm) among species was 0.607. The rank of genetic diversity throughcomparison of main parameters of genetic diversity of inter and intra populations byapplying asubordinate function value test is V. persica＞V. arvensis＞V. didyma＞V.hederifolia.Harmfulness reflects the real invasiveness while reproduction characteristics andgenetic diversity indicate potential invasiveness capacity. Based on above analysis, theranks of those four exotic weeds of parameters of both two aspects in real and potentialinvasiveness are consistent completely which indicated that the characteristics inhalmfulness, reproduction characteristics, genetic diversity and allelopathy are the keyfactors determining the invasive capacity of exotic weeds.