Effects Of Environmental Factors And Competition On Regeneration Of Dominant Species Community In Chongming Dongtan

Chongming Dongtan wetland is located at the Yangtze River Estuary,which is one of the most important wetlands in the world.The plant communities composed of Phragmites australis,Spartina alterniflora,Scirpus mariqueter,Scirpus triqueter and other local dominant species are the basis of the wetland ecosystem,providing space for living and foraging of all kinds of benthic animals and waterfowl.Therefore,it's necessary to study the spatial pattern and regeneration of the plant communities for predicting the future development of the wetland.The seed stage is the beginning of the regeneration.Previous studies mostly focused on the distribution and growth of adult plants,but overlooked the role of seed germination and seedling growth in the early life history stage in community assembly.In this study,we investigated the growth and soil seed bank of several dominant plant species in Dongtan to understand the impact of tide on plant growth and seed dispersal.Then we conducted the controlled experiment to explore the germination pattern of dominant species seeds under different salinity and competition,and extrapolated to pot experiment to explore the response of invasive Spartina alterniflora and native Scirpus mariqueter to salinity and competition,which can further improve our understanding of regeneration mechanism of wetland plant community.The results include:(1)The composition of different plant communities varies along the elevation gradient.In the middle to low marsh close to the sea,the community is dominated by Scirpus mariqueter and Scirpus triqueter;in the middle marsh,the community composition is similar to that in the middle to low marsh,with occasional Carex scabrifolia patches;in the middle to high marsh,Phragmites australis and Spartina alterniflora are dominant species.There were significant differences in soil water content,salinity and average daily immersion time among different communities and seasons.In March,soil water content was the highest while salinity was the lowest;the immersion time in March and May was significantly higher than that in November.And the immersion time of Phragmites-Spartina community was the shortest,while that of bare flat community was the longest,and that of sedge community was between them.The salinity of Phragmites-Spartina community was significantly higher than that in other two communities,and up to 1.7% in May while each community was below 1% in other months.In Phragmites-Spartina dominated community,Phragmites australis population accomplished growth rapidly through vegetative reproduction in the early spring,focusing on the resource allocation to individual growth.However,the germination and growth of Spartina alterniflora was longer,and the reproductive allocation was increased.In bare flat community with little aboveground vegetation,there were also differences in growing season between Scirpus triqueter and Scirpus mariqueter with similar life forms.Scirpus triqueter begun to germinate and grow in the early spring while Scirpus mariqueter lagged behind,but their niche overlapped more since summer.Scirpus triqueter,Scirpus mariqueter and Carex scabrifolia were dominated in sedge community.The adaptability to water stress might be the main reason for species distribution in this area.(2)In Phragmites-Spartina and bare flat community,the seed stock in soil were not affected significantly by season and sampling depth,while in sedge community,the seed stock was significantly affected by season.The number of seeds collected in March was significantly higher than that in May.There were significant differences on the flux of seeds carried by tidal water among different communities and seasons.The number of seeds in Phragmites-Spartina and sedge community was significantly higher than in bare flat community,and that in November and March was also significantly higher than in May.Interactions in sedge community showed significant effect for the seeds were less than bare flat community in November.(3)1% salinity significantly inhibited the germination speed of three dominate species Phragmites australis,Spartina alterniflora and Scirpus mariqueter,and there were also significant differences among species.The germination rates of Phragmites australis and Spartina alterniflora in monoculture were not significantly affected by salinity,while the germination rate of Scirpus mariqueter was reduced from 90% to less than 10%.Besides,Spartina alterniflora in mixture significantly promoted the germination of Scirpus mariqueter,whereas it was significantly inhibited by Phragmites australis.(4)The aboveground biomass and seed production of Scirpus mariqueter were still significantly inhibited in 1% salinity.The facilitation between Spartina alterniflora and Scirpus mariqueter during germination period disappeared in the adult stage,which was different from the result of petri dish experiment.On the contrary,the survival rate and seed production of Scirpus mariqueter in mixture were significantly lower than those in the high-density monoculture.At this time,the high-density monoculture showed the highest survival rate and seed production.In different stages of community succession,there are different factors affecting the growth and competition of plants.Regeneration stage is the key period of community construction.Seed germination and seedling growth are very important to the development of community.In the early stage,the formation of community is mainly dominated by environmental factors,and the basic community structure is formed through environmental screening.As the community continues developing,the interspecific relationship especially competition gradually becomes a new key factor.In the restoration of wetland communities,the effects of environmental factors and competition on community regeneration should be considered to improve the efficiency of vegetation restoration.