| Ragweed (Ambrosia artemisiifolia L.), native to the Southwest Americas, has become a major invasive plant in China. It can be found in more than 20 provinces and spread over 10 counties in Hunan Province, it is extremely difficult to control or eradicate. Knowledge of the invasive mechanism is very helpful to controlling the spread of it. In this paper, the ecological characteristic of ragweed population and community were studied using field survey and laboratory experiment method. The favorable factors of invasive ecological strategy and community characteristics were also analyzed. The purpose of the present study is to reveal the mechanisms underling its successful invasion, and try to find some effective ways for its control.1. This field survey on invasion of A. artemisiifolia was conducted in Hunan Province of China. The invasive pattern was studied by randomly sampling. Such 14 variables as "slope-orientation", "slope-deflection", "wateriness of topsoil", etc. were be quantified to decide the different invasion of this alien noxious weed. The principal component analysis demonstrated that all the ecological message included in those variables could be decided by three most principal factors, "Factor of Damp and Light"(First Principal Component),"Factor of Environment"(Second Principal Component), and "Factor of Botanic Biodiversity" (Third Principal Component).In principal, the occurring frequency of A. artemisiifolia will be related to 11 ecological factors, it has high positive relativity with several variables as "slope-orientation", "slope-deflection", "wateriness of topsoil" etc., and negative relativity with "number of other species", "coverage of weed", etc.2. A survey was conducted with the square intercept method on species and quantity of weeds in 6 kinds of habitats where A. artemisiifolia grew in Hunan Province between April and October. Results showed that a total of 83 indigenous species were collected and among them,41 species were common indigenous plants, belonging to19 families. Compare with native communities, invasion of ragweed caused significant (P<0.05orP<0.01) decline of species richness index,Simpson index and Shannon-Wiener index and little change of Pielou evenness indexes in all habitats involved. The variation in species richness index of ragweed invasion communities in the different season was small, diversity indexes and evenness indexes were increased, and the invasion communities were concentrated on ragweed in spring and summer.3. The growth indexes of A. artemisiifolia between April and September were measured, the dynamic changes of stem length, stem diameter and biomass could be developed by Logistic Curve, and the decisive coefficients over 0.94. At the same time, the growth indexes also used to model CAR equation between weather conditions and growth, CAR model for stem length, stem diameter and biomass was established, it can be used for prediction of growth. Germination rate, seedlings survival and flowering characteristics of ragweed seeds were examined through pot experiment and field survey in different habitats. The results showed that the germination rate was 74.4% in shady and wet habitats. In different habitats, seedling survival rate was 15.79-30.08%, probability of seedling developing into a mature plant was 1.20-5.68%. The probability of a seed developing into a mature plant was estimated (1.38ˉ5.87‰).4. The contents of physiological metabolites were measured, results showed metabolism of A. artemisiifolia in different stages were active, the contents of photosynthetic pigment and soluble sugars in the stages of inflorescence were highest, contents of flavonoid and total alkaloid in the stage of seed were highest. The accumulating of photosynthetic pigment and soluble sugars were benefit to photosynthesis, accumulating of flavonoid and total alkaloid could be helpful to resist the environmental stress. Study on photosynthetic characteristics in leaves of ragweed by Li-6400 portable photosynthesis system, results showed A. artemisiifolia is a kind of heliophile, whose light compensation point is about 18.27-37.65μmol·m-2s-1, light saturation point is around 1106.8-1451.8μmol·m-2s-1. Compared with associated species, the net photosynthetic rate (Pn) and water use efficiency (WUE) were higher.5. The adaptability of different environment was studied through pot experiment, result indicated that seedlings grew well in the abundance of light and water, soil with better nutrient. Seedlings could survive in soil with lower nutrient and gentle drought through expanding the roots. The content of MDA and changes of antioxidant system were also studied under various environmental stresses (including heat, cold, drought and flooding). The results showed that four stresses induced oxidative damage as evidences by an increase in lipid peroxidation. The contents of MAD and activities of superoxide dismutase, guaiacol peroxidase increased in response to all stresses. However, catalase activities increased in response to heat but decreased to cold, drought and flooding. The contents of GSH and activities of anti-superoxide anion radical decreased in response to all stresses.The plant has a higher capacity for scavenging oxygen radicals in heat, flooding stresses than in cold and drought stresses. Rapidly spread of ragweed owed to the diversified resist mechanism of various environmental stresses and it can acclimatize itself to different habitats.6. Genetic diversity and genetic differentiation of A. artemisiifolia were studied by using ISSR makers, ISSR amplification was conducted with 6 primers for 75 individuals from 5 populations, The main results were follows:(1) 29 loci amplified polymorphic were detected, of which 25 (PPL=86.21%) were polymorphic loci. Genetic diversity analysis showed that Nei's gene diversity(He)was 0.2545 and Shannon's genetic diversity index (I) was 0.3125 in species, it indicated high genetic variation at species level. The total genetic diversity (Gst) was 0.2961. Genetic differentiation mainly occurred within populations (70.39%), and without populations (29.61%).UPGMA cluster analysis indicated that two populations of Changsha and Linxiang clustered in a clade, Jiangyong and Lingling clustered in an other clade, the two clade clustered in aclade at last. Genetic distance was correlated with geographical distance.7. By using a multiple De wit replacement series design method, the morphological characteristics, competition effects and response of A. artemisiifolia and two southern turf grass (Festuca arundinacea, Cynodon dactylon) were investigated comparatively on potted plants during their seedling stages. To define the relative competition between A. artemisiifolia and two turf grass, experiment was conducted to explore the effect of planting time, sowing densities and proportional mixtures on growth of them. The main result showed that the relative competitive ability of A. artemisiifolia significantly increased with the increasing of proportional mixtures. The relative competitive ability of A. artemisiifolia is lower at the high densities and in the mixtures, together with the advance of planting time of turf grass.To sum up, the successful invasion of A. artemisiifolia depends on both of the high invasiveness of the invader (such as high probability from seedling to sexual maturity, adaptation to environmental stress and high tolerance to environmental heterogeneity, high genetic variation and competitive ability) and high invisibility of ecosystems (such as abundant light and moisture environment, high level of disturbance). The prediction indicated the rapid continuation spread in Hunan Province, South china and Southwest china were the next imaginable invasive areas of the plant according to the climate (high air temperature and abundant moisture). The government must pay more attention to pre-warning and control of A. artemisiifolia.
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