The Mechanisms Of Spartina Alterniflora Invasion Into Mangroves In Zhangjiang Estuary

Spartina alterniflora is invading the entire Chinese coast, occupying mudflats through this range, forming mangrove-Spartina ecotone and displacing mangroves in the upper intertidal of south China. Multiple factors may interact to mediate the spread of S. alterniflora, however, the mechnaism was poorly understood.We carried out experiments in four habitats of Zhangjiang Estuary:mangrove, ecotone, Spartina and mudflat respectively. From the experiments of investigation on seed production, soil seed bank, seedling recruitment of S. alterniflora, we tested the effects of propagule pressure on S. alterniflora invasion; Besides, we did animal exclusion experiments on seeds and seedlings in the four habitats, combined with crab population dynamic along time and space, feeding assays on seeds and seedlings in laboratory and abiotic factors in the field. The results revealed the importance of plant competition and crab herbivory on the invasion of S.alterniflora; And, we also investigated the gaps invaded by S. alterniflora, to know the effects of disturbance on the invasion of this alien herbaceous which was significant for the prediction on the expansion of S. alterniflora. Base on the two years’experiments, the conclusions are as follows:1. S. alterniflora produced4325±518-5545±763seeds/m2in the mangrove zone of Zhangjiang Estuary.2. The seeds of S. alterniflora spread widely into the four different habitats and formed the soil seed bank.3. Seeds germination rate of S. alterniflora was suppressed by high salinity. The highest germination rate was36.4±3.1%in0PSU treatment and the lowest was21.4±1.9%in30PSU.4. The seeds in soil seed bank could germinate into seedlings in the following spring, but only successfully eatablished in unvegetated mudflat, all seedlings died in other three vegetated habitats within one growing season. 5. Crab herbivory and plant competition were the main factors affect the spread of S. alterniflora in the four different habitats. Sesarma plicata was the dominant species in mangrove, ecotone and Spartina, its predation on seeds and seedlings reached32.1±5.0%and70.9±4.1%within48h; Besides, the light intensity in the three vegetative habitats was62.5%-88.5%lower than mudflat, the competition for light was the key factor for mortality of seedlings in these habitats. Less consumer pressure and higher light intensity leads to the successful establishment in unvegetated mudflat.6. The potential of S. alterniflora invading into mangrove forest gaps relied on the size of the gap, large gaps were easier for the invasion of S. alternilflora.The results demonstrated that:Propagule pressure, competition and crab herbivory interacted on the spread of S. alterniflora, affecting the different patterns of S. alterniflora in mangrove zone of Zhangjiang Estuary. In the stage of dispersal, propagule pressure is the key factors limited the number of seeds in different habitats; In the stage of post-dispersal, crab herbivory pressure reduced the number of seeds in soil seed bank significantly; In the stage of seedling growth, both crab herbivory and competition for light with existing vegetation lead to the mortality of seedlings in the three vegetated habitats. According to these results, we’ve got a better understanding of the ecology of plant invasion. And, the results are useful for mangrove restoration and the control of S. alterniflora.