Responses And Thresholds Of Typical Salt Marsh Species To Flooding-salinity Stress In Yangtze Estuary

Coastal wetlands are the transition zones of land and ocean ecosystem which are one of the most productive in nature ecosystems.They play an important role in maintaining the ecological balance of habitats and regulating the carbon cycle.As an important part of coastal wetlands,salt marsh species have high primary productivity.It is beneficial to the maintenance of regulating climate,purifying water quality,and maintaining coastline and the structure and function of wetland ecosystems.Flooding and salinity are the main environmental factors of coastal wetlands,affecting the survival,growth and distribution of salt marsh species.However,sea level rise in the future may cause flooding stress and saltwater intrusion,which will cause salt marsh speices to face the high challenge of flooding and salinity interaction stress,which affect the structure and function of the coastal wetland ecosystem.The ecosystem can maintain a stable state by self-regulation when the flooding-salinity stress are small.Once the disturbance exceeds the survival threshold of salt marsh species,it will cause irreversible changes in ecosystem and seriously affect the structure,function,stability and sustainable development of the coastal wetlands.Therefore,the research of effect and threshold of salt marsh species to flooding-salinity stress will provide effective management suggestion before major changes in the system,which also makes sense for ecosystem protection and sustainable management.In this study,three typical salt marsh species(i.e.Scirpus mariqueter,Spartina alterniflora,and Phragmites australis)were investigated in the Yangtze River Estuary.The elevation simulation experiment carried out in Nanhui Dongtan of Shanghai,and the salinity stress and flooding-salinity interaction stress simulation experiment carried out in the greenhouse.Regularly monitor the survival rate,plant height,number of tillers,and ear setting percentage of three typical salt marsh species,and measure the aboveground and underground biomass at the end of the growing season.The model of logistics fitting were used to determine the elevation threshold,the salinity threshold and the interaction threshold of flooding-salinity stress of three typical salt marsh species,which to investigate the effects of elevation,flooding,salinity and flooding-salinity interaction on their survival,growth and reproduction.The results were as follows:1.Respones and thresholds of typical salt marsh species to habitat elevation in Yangtze Estuary.(1)The plant height,the number of tillers and the aboveground biomass of S.mariqueter,S.alterniflora and P.australis increased firstly and then decreased with increasing elevation.The largest number of tillers and the maximum aboveground biomass of three salt marsh species at elevation of 2.80 m,2.96 m and 3.30 m,respectively.(2)The elevation thresholds of three typical salt marsh species in the Yangtze River Estuary were as follows: S.mariqueter(2.36 m)< S.alterniflora(2.51 m)< P.australis(2.79 m).When the elevation were lower than the threshold respectively,the three salt marsh plants died;The daily duration flooding threshold was as follows: S.mariqueter(6.25 h/d)> S.alterniflora(6.00 h/d)> P.australis(5.33 h/d).When the average duration of flooding continued above this threshold,three salt marsh plants died.(3)The optimum habitat zone elevation were 2.58-3.02 m for S.mariqueter,2.73-3.19 m for S.alterniflora and 3.04-3.56 m for P.australis.And S.alterniflora had a stronger competitive in the overlap area.2.Respones and thresholds of typical salt marsh species to salinity stress in Yangtze Estuary.(1)The survival rate of S.mariqueter,S.alterniflora and P.australis appeared a decreasing trend with the increasing salinity,and the survival rate of S.alterniflora under the same salinity was significantly higher than S.mariqueter and P.australis(P<0.05).(2)The growth and breeding of S.mariqueter,S.alterniflora and P.australis were all significantly affected by salinity.The plants height,the biomass,the number of tillers and ear setting percentage of S.mariqueter and P.australis appeared a decreasing trend with the increasing salinity,but them of S.alterniflora showed the first increasing and then decreasing trend,which all were the highest when the salinity was 10 ppt.(3)The salinity threshold of three saltmarshes were as follows: S.alterniflora(43.58 ppt)> S.mariqueter(21.30 ppt)> P.australis(13.35 ppt).When the salt concentration was higher than the salt tolerance threshold,three salt marsh plants died.3.Responses and thresholds of typical salt marsh species to flooding-salinity interaction stress in Yangtze Estuary.(1)The survival rate,the plant height,the number of tillers and the ear setting percentage of S.mariqueter,S.alterniflora and P.australis were all decreased gradually with increasing flooding-salinity interaction stress.The three factors of “salinity”,“flooding depth” and “flooding-salinity interaction” on the biomass accumulation of three salt marsh species from high to low were: salinity > flooding-salinity interaction > flooding depth.(2)The threshold equation of flooding(x)and salinity(y)interaction stress of S.mariqueter,S.alterniflora and P.australiswe were as follows: y =-0.2886 x + 19.22,y =-0.2539 x + 43.76 and y =-0.1141 x + 15.925.(3)Compared with single factor stress,flooding-salinity interaction stress had more significant inhibition on the survival and growth of three salt marsh species.S.alterniflora had stronger tolerance to flooding and salinity interaction stress than S.mariqueter and P.australis.