| The distribution patterns and invasive characteristics of20different noxious invasive plants (as notified by the Ministry of Agriculture in Hainan) were studied in different ecosystems studied. Our results verified the distribution of11kinds of noxious species(i.e., Chromolaena odorata, Praxelis clematidea, Mimosa pudica, Parthenium hysterophorus, Mikania micrantha, Wedelia trilobata. Sorghum halepense, Conyza sumatrensis, Alternanthera philoxeroides, Lantana camara and Eichhornia crassipes) in the study area, the invasion and their potential invasibility of tropical rain forest by invasive plants were also studied.1. The results indicated that invasive species were mostly concentrated in the northeastern and southwestern regions of Hainan in comparison with the southeastern region, and they were least distributed in the central region. Plantations and abandoned land and farmland ecosystem were the main habitats heavily invaded by these species. These invasive species showed a distinct successional transition among their respective plant community. Invasive plants rather affected certain functional groups than the whole communities in grassland ecosystems, when the coverage of invasive plants was within a certain extent(3.9%-49.1%), it was positively correlated with native plant richness. However, it was negatively correlated with native plant richness in abandoned agricultural farmland and forest edges, indicating different ecological effect in different habitats.2. Two years’rigorous observation of invaded sample plots in northwest of Hainan indicated that herbaceous invasive plants had higher mortality and recruitment rate in the community, but the invasive plant populations showed a decreasing trend overall. Both the population density and the population coverage of Chromolaena odorata, Lantana camara, Mimosa invisa, Rhynchelytrum repens and Parthenium hysterophorus, the population density of Praxelis clematidea, Mimosa pudica, as well as the population height of Praxelis clematidea, Parthenium hysterophorus, Bidens pilosa, Conyza sumatrensis, Rhynchelytrum repens showed a decreaseing trend. Drastic change is land use pattern resulted in75.9%of the reduction of sample plots.3. In less disturbed ecosystems such as natural secondary forest and forest edge showed resistance to the invasion. Six species of noxious invasive plants and13species of general invasive plants were found in Bwangling National Nature Reserve, a typical tropical montane forest. The frequency of occurrence, horizontally and vertically along the slope were0.85and0.77, respectively. Most invasive plants only inhabited highly disturbed areas with high light intensity. The invasive plant species accounted for13.5%of the total species richness. Only2.7%of the total seed reservoir in the soil seed bank comprised invasive plants and their seed were concentrated more towards the roadside and the concentration decreased as we go away from the road.The frequency of above ground occurrence of invasive plants and their presence in the soil seed bank were both negatively correlated with the overall canopy density of the community.4. Four species were choosen, namely, Chromolaena odorata, Praxelis clematidea, Lantana camara and Mimosa sepiaria, to study their germination and seeding growth characteristics along different positions of the canopy gap. The results indicated that L. camara and C. odorata could only germinate in the central portion along the expanse of canopy gap, while M. sepiaria and E. catarium could germinate in all the three positions of the canopy gap. The seed germination rate was delayed and the seedings mortality was increased if seeds were treated with forest litter cover. Seedling mortality was persistent after germination. All the seedlings of M. sepiaria and L. camara died in three months after sowing. Only few seeding of E. catarium and C. odorata existed in the core area of the canopy gap and their number accounted for8.1%and3.6%of the germinants and1.1%and0.2%for the total sown seeds, respectively. |
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