Study On Weed Ecology For Flora, Community, Risk Warning And Environmental Impact In Shanghai

High level of urbanization, developed transport, foreign exchange activity andlarge direct seeded area, wide planting of fruits and vegetables in agricultural andnumerous development zone and free county in Shanghai, makes this area stronglyunder the influence of external environment. Systematic survey is needed in order toclear weed flora and community features in Shanghai. At the same time, exoticspecies such as Geranium carolinianum and Alternanthera philoxeroides have alreadypassed in Shanghai and effected on agricultural production. However, we do not knowtheir specific impact. Furthermore, other parts of our country already have some otherexotic plant distribution that is a new dangerous on agricultural ecosystemconstruction in Shanghai what required an understanding of the invasion risk degree.For these problems in integrated management of weeds in Shanghai, weeds flora andcommunity ecology investigations were carried out in spring and autumn respectivelyand the flora composition, life type and occurrence situations of weeds were analysedin Shanghai suburban. Veronica hederaefolia, Euphorbia hirta, Ageratum conyzoides,Lantana camara and Eupatorium adenophorum caused serious damage outside ofShanghai. Based on19Environmental variables and distribution records of those fiveweeds in the world, their invasion risk to Shanghai were quantitatively assessed byusing MaxEnt ecological niche model and the software of ArcGis9.3. Simultaneously,Geranium carolinianum which was invading into shanghai recent years was studiedon allelopathic effects on wheat and impact on nutrients, microbial quantities andenzyme activities of peach orchard soil. Effect of Paspalum distichum and invasivespecies Alternanthera philoxcroides mixed-growing weeds on arthropods and riceyield was also studied and discussed the control economic thresholds of those two species. The major conclusions are summarized as follows:1. In Jinshan, Jiading, Baoshan, Nanhui, Songjiang, Fengxian, Qingpu, Minhangand so on area of Shanghai suburban，356and215random sites on differentecological conditions were surveyed for flora and synecology study of weeds inspring and autumn respectively.201weed species were detected in spring (141weedspecies were detected) and autumn (130weed species were detected) which belong to45families and138genera. Gramineae （including28genera and43species）andCompositae（including22genera and32species）are dominant families which accoutsfor21.39%and15.92%of weed species respectively.60.70%weed species are annualand biennial species that is consistent with the r-strategy taken by the short-livedplants in the high intensity and frequency interference habitats.2.38alien weed species were detected wich account for18.91%weed speciesin571survey sites of spring and autumn. Most of the alien weeds originate fromAmerica (57.89%) and Europe (28.9%).3. According to the clustering analysis,6major groups can be classified with thedominat species which are Galium aparine var. tenerum, Poa annua, Vicia sativa,Phragmites australis, Solidago canadensis and Veronica persica of spring weedspecies and7major groups can be classified with the dominat species which areAlternanthera philoxeroides, Leptochloa chinensis, Cayratia japonica, Digitariasanguinalis, Humulus scandens, Alternanthera philoxeroides and Solidago canadensisof autumn weed species in shanghai suburban.4. Among141spring weed species, Galium aparine var. tenerum (nichebreadth=0.3392), Poa annua (0.2680), Capsella bursa-pastoris (0.3016), Vicia sativa(0.1941) and Euphorbia helioscopia (0.2209) have a wider niche breadth. Among130autumn weed species, Setaria viridis (0.3157), Digitaria sanguinalis (0.2737),Leptochloa chinensis (0.2294), Alternanthera philoxeroides (0.2124) and Cynodondactylon (0.2354) have a wider niche breadth. Based on the ecological importancevalue of weeds,356and215survey sites of sping (classified as38site groups) andautumn (classified as40site groups) were classified respectively by using thesoftware of PC-ORD. Then ecological important value mean of site groups was computed. Based on the ecological important value mean, niche overlap wascomputed for construction of PAA two-dimensional ordination diagram and theminimal spanning tree of ecological similarity analysis. According to their nichecharacteristics, habitat types of42main weeds in Shanghai suburban could beclassified as dry habitat (including ridge, roadsides and abandoned land etc),mesophytic habitat (including fallow croplands, wheat field and corn field etc) andwet habitat (including tidal flat, ditches and marshland etc).5. According to the prediction results, the invasion risk order of five alien weedsis Euphorbia hirta, Ageratum conyzoides, Eupatorium adenophorum, Lantanacamara and Veronica hederaefolia. In our country, Euphorbia hirta andAgeratumconyzoides have a higher invasion probability, plant quarantine departments ofShanghai should strengthen the monitoring of these two weeds. However, the invasionrisk of Eupatorium adenophorum, Lantana camara and Veronica hederaefolia. inShanghai is very low.6. The allelopathic potential of aqueous extracts of aerial tissue of Geraniumcarolinum L.against wheat was investigated under laboratory conditions. The resultshowed that the synthesis effect of seed germination and growth that the aerial tissueof Geranium carolinum L. leaves to wheat seed were inhibition. The maximuminhibition of germination was the growth of seminal root, processing group was33.8%of control group when the concentration was0.100/g ml-1. The allelopathy ofG. carolinum L. leaves to wheat indicated that the aqueous extracts of the aerial tissueof G. carolinum L. had inhibitory effect on root, fresh weight, and photosyntheticefficiency of wheat．Its inhibitory effect enlarges along with the increase of theconcentration of the aqueous extracts. The most significant inhibition was the rootsystem whose project area, surface area, diam, tipes and crossings were5%less thencontrol group. Upon the whole allelopathy of G. carolinum L. is mainly inhibition ofwheat root growth.7. Four different treatments were processed on Geranium carolinum L. in thepeach orchard of Jiading Shanghai:①Not weeded and grown freely.②Artificial weeded and covered under the peach tree.③Artificial weeded and out of the peachorchard.④Chemical weeded as control. Temperature, water content, salinity andelectric conductivity of peach orchard soil were determined under four treatments.The results showed, artificial weeding and covering treatment reduced temperature,retained moisture, high salinity and conductivity of summer soil and also effectivelycontroled G. carolinum L. G. carolinum L. had an certain extent effect of nutrients,microbial quantities and enzyme activities of peach orchard soil: An obviousinhibition on soil urease, catalase and invertase was observed. The quantity of soilfungia and bacataria was decreased, while ammonia-oxidizing bacteria was increased.The contents of organic matter, total N and fast available K of G. carolinum L. natrualgrowth treatment were all lower than control, whereas available P was higher.8. The effects of weed community with dominant Paspalum distichum andAlternanthera philoxcroides on arthropods and rice yield were studied. The functionequations of weed biomass with insect individual number, rice yield and losspercentage, rice effective panicles, kernel numbers per panicle,1000-grain weight andwhite heads were established. The results showed that with the increase of weedbiomass, rice yield decreased distinctly, which mainly resulted from the decrease ofthe effective panicle number, kernel number per panicle. With the increase of weedbiomass, the individual number, the diversity, and the dominance index of insects alsoincreased, which was mainly related with the increase of dipteral insects andHomopteran insects. The individual number of spiders, however, not increased withthe increase of weed biomass. Multiple linear regression analysis revealed thatPaspalum distichum and Alternanthera philoxeroides was the main speciesaccounting for the decrease of rice yield, and the increase of the insect individualnumber was mainly caused by the biomass increase of Paspalum distichum. In thetransplanted rice filed where Paspalum distichum and Alternanthera philoxcroides aredominant weeds, the economic threshold for weed controlling is21g/m2, about14kg/mu.