Effects Of Environmental Factors On Seed Production And Dispersal Of Dominant Species In Chongming Dongtan

Seed production and dispersal are important stages of plant community regeneration,representing the future development potential of populations and playing an important role in vegetation restoration.Chongming Dongtan is a wetland of international importance with abundant resources that provide suitable foraging habitats for a variety of bird species and zoobenthos.The environment of this area varies significantly along the elevation and has a simple species composition,making it an ideal ecosystem for studying community assembly mechanisms.In this thesis,I revealed the effects of salinity and tidal action on the seed production and dispersal of dominant plants in Chongming Dongtan through controlled environment experiments,field investigations and manipulation experiments,and preliminarily elucidates the influence mechanisms on the spatial distribution patterns of salt marsh plant communities and soil seed bank dynamics.The main findings are as follows:1.Salinity significantly inhibited the vegetative growth and seed production of Scirpus mariqueter,while propagule type had a significant effect on vegetative growth of Scirpus mariqueter,Spartina alterniflora and Phragmites australis and seed production of Spartina alterniflora.Vegetative growth of Scirpus mariqueter seedlings were higher than that of ramets,but seed production of seedlings was more inhibited by 1%salinity treatment.Vegetative growth and seed production of Spartina alterniflora ramets were significantly higher than those of seedlings,and seed production of ramets was promoted by 1%salinity treatment.Vegetative growth parameters of Phragmites australis ramets were significantly higher than those of seedlings,and no seed was produced under all treatments.2.Salinity had significant effects on sedimentation of Spartina alterniflora seeds,with a significantly higher sedimentation rate at 1%salinity than in freshwater.Temperature and hydrological condition had significant effects on sedimentation of Scirpus mariqueter,Spartina alterniflora and Phragmites australis seeds.The sedimentation rate of Scirpus mariqueter seeds in room temperature under different salinities was significantly higher than that in the low temperature treatment,while the opposite trend was observed for Spartina alterniflora and Phragmites australis seeds.The sedimentation rate of all three plant seeds were significantly higher under shaking treatment than under standing treatment.Under saline,shaking treatment close to the field environment,Spartina alterniflora seeds settled the fastest,Scirpus mariqueter the second,and Phragmites australis the slowest(half sedimentation time is 3.0 days,8.3 days and 12.7 days,respectively).3.Along Tuanjiesha transect,middle marsh(seed production 2256 m^-2·a^-1)was the main source of seed dispersal.Seed flow was dominated by Carex scabrifolia,and dispersal occurred mainly from May to August(when Carex scabrifolia seeds matured).Seeds were transported from middle marsh to low marsh.In this area,seaward seed flow(236 m^-2·a^-1)was higher than net interception(202 m^-2·a^-1)and landward seed flow(193 m^-2·a^-1).Along Dashitou transect,low marsh(seed production 5080 m^-2·a^-1)was the main source of seed dispersal.Seed flow in this area was dominated by Scirpus mariqueter.From September to November(the season of Scirpus mariqueter seed maturation),seed flow size was significantly higher than in other seasons,with a large amount of seeds being lost by tidal transport to higher tidal zones or the sea.In this area,seaward seed flow(2162 m^-2·a^-1)was higher than net interception(1938 m^-2·a^-1)and landward seed flow(1102 m^-2·a^-1).4.Along Tuanjiesha transect,plant communities from high to low elevation were dominated by Imperata cylindrica,Phragmites australis,Carex scabrifolia,Scirpus mariqueter and Scirpus triqueter,respectively.Ramet density,canopy height,and aboveground biomass of plant communities were significantly higher in November than in other months.Soil seed bank consisted of Scirpus triqueter,Carex scabrifolia and Scirpus mariqueter seeds.Seed number in soil seed bank did not differ significantly among seasons and elevations,but was significantly higher in deep soil(10-20cm)than that in surface soil(0-10cm).Dominant species along Dashitou transect was Scirpus mariqueter.Ramet density,canopy height and aboveground biomass of plant communities were significantly higher in November than in other months,and aboveground biomass was significantly higher in low marsh than in middle marsh.Soil seed bank was dominated by Scirpus mariqueter,with a small amount of Carex scabrifolia.In November when Scirpus mariqueter seeds matured,seed number in soil seed bank was significantly higher than in other months.Seed number of soil seed bank was higher in low marsh than in middle marsh,and was higher in surface soil(0-10cm)than in deep soil(10-20cm)during this season.Seed number of seed bank did not differ significantly among different elevations and between different soil depth in other months.Under the influence of tidal movement,soil seed bank composition differed from aboveground plant community composition,and seeds that were longer-lived and easily dispersed with the tide had a greater chance of occurring in the soil.Salinity and propagule type are important factors influencing plant seed production,while tidal movement is the main factor affecting seed dispersal.Determinants of seed dispersal in different areas in Chongming Dongtan differ.Tuanjiesha transect was less disturbed and close to the natural state,and seeds of each species spread more freely in areas of different elevation through tidal movement.Dashitou transect was more seriously affected by anthropogenic disturbances,and the dike restricted the distribution and seed dispersal of high marsh plants,which led to seed accumulation there.Therefore,in the restoration practice of in Chongming Dongtan salt marsh,Scirpus mariqueter populations should be restored by either sowing seeds or ramet transplanting according to environmental stress.In addition,the remnant Spartina alterniflora should be found and removed as early as possible in the restored area,and the recolonization of Spartina alterniflora seeds after long-distance transport by tidal water should be closely monitored and prevented.