Ecophysiological Responses Of Native And Exotic Coastal Wetland Plants To Waterlogging And Salinity Stress

The coastal wetlands are identified as the ecosystems with multiple important ecosystem services,including carbon sequestration,pollution reduction,habitat for protected fauna,and shoreline stabilization.However,coastal wetlands located in the transitional zone between terrestrial and marine interfaces,making them sensitive to impacts originated from interaction between upland and adjacent marine systems.Especially,the coastal wetland is vulnerable to sea-level rise caused by climate change due to their low elevation at the intertidal interface.As expected,the coastal wetlands would be at risk of prolonged waterlogging and saltwater intrusion depending on the various projections.Correspondingly,the pattern of saltmarsh vegetation and its ecological functions may change accordingly.Along China's coastline,the native Phragmites australis is the dominant primary producer and constitutes the community structure.However,Spartina alterniflora,which was introduced to China from North America in the late 1970s,has been expanding rapidly along China's coastline.The exotic S.alterniflora had a high degree of competitive suppression on many native species of China's coastal saltmarsh wetlands,and ecosystem properties were influenced due to the modification of the vegetation structure.In this study,the plant-soil mesocosms of P.australis and S.alterniflora collected from a coastal saltmarsh wetland of the Yangtze Estuary were grown under different waterlogging?non-waterlogging,long-term waterlogging?and salinity gradient?0,5,15 and 30 ppt?.By comparing the seasonal responses of plant growth and development indicators?shoot height,leaf area and biomass?,photosynthetic characteristics?net photosynthetic rate,apparent quantum yield and chlorophyll fluorescence parameters?and water use indicators?stomatal conductance,transpiration rate and water use efficiency?of the two saltmarsh plants,to explore their adaptability under changed hydrological conditions.It would be meaningful to provide a scientific basis to forecast the shifts of wetland vegetation patterns and system functions under sea-level rise at China's coastline.The main results and conclusions of this study are shown as below:1.Effects of waterlogging and salinity treatment on the growth parameters of P.australis and S.alterniflora.The growth parameters of P.australis slightly decreased under waterlogging treatments,and S.alterniflora slightly increased,but neither of the variables reached the significant level of difference?P>0.05?.The medium?15 ppt?and high salinity?30 ppt?significantly?P<0.05?reduced the shoot height,leaf area,aerial biomass and tassel biomass of P.australis,by 32.3±10.4%and 42.7±9.0%,25.4±9.5%and 46.1±8.2%,34.9±5.5%and 66.7±7.1%,and 38.6%and 100.0%,respectively?on average of the entire growing season?,especially in the middle?June-September?and later?November?growing season.On contrary,S.alterniflora revealed a significant decrease in growth parameters?P<0.05?only at high salinity,by17.7±7.8%,10.5±5.2%,26.5±9.0%and 40.7%,respectively.Under the combined treatments of high salinity and waterlogging,the growth parameters of P.australis were the lowest,and the shoot height,leaf area and aerial biomass were reduced by69.2±2.2%,72.2±8.8%and 83.1±3.8%,respectively,but no significant interactive effect was detected?P>0.05?.In addition,this study found that combined treatments of waterlogging,medium/high salinity led to the absence of sexual reproduction organs?tassel?in P.australis,whereas this phemonenon was not found in S.alterniflora.2.Effects of waterlogging and salinity treatment on the photosynthesis parameters of P.australis and S.alterniflora.The maximum rate of photosynthesis(A_max)and Apparent quantum yield?Q??of P.australasis slightly decreased under waterlogging treatments,while the S.alterniflora slightly increased.The photosynthetic parameters?except the maximum photochemical efficiency of photosystem II?of P.australis decreased significantly?P<0.05?under salinity treatments,but the photosynthetic parameters of S.alterniflora remained stable at high salinity?P<0.05?.Under the combined treatments of high salinity and waterlogging,the variables of A_max,Q?,minimum chlorophyll fluorescence?F₀?and maximum chlorophyll fluorescence?F_m?of P.australis were the lowest,by62.4±14.2%,40.0±4.1%,42.7±16.7%and 39.5±30.7%,respectively,but no significant interactive effect was detected?P>0.05?.The photosynthesis and chlorophyll fluorescence parameters of S.alterniflora under low salinity?5 ppt?were the largest,which indicated that low salinity may be the optimum for S.alterniflora.3.Effects of waterlogging and salinity treatment on the water use parameters of P.australis and S.alterniflora.The waterlogging treatments have a significant?P<0.05?impact on water use parameters of P.australis during the entire growing season,and a significant?P<0.05?effect on S.alterniflora was found during the late growing season.Under the waterlogging treatments,stomatal conductance?g_s?,transpiration rate?T_r?and water use efficiency?WUE?of P.australis and S.alterniflora increased by 20.0±12.4%and 14.2±8.1%,35.9±23.8%and 6.2±2.1%,and28.3±7.9%and 2.2±0.8%,respectively.Salinity treatments had significant?P<0.05?effects on water use parameters of both plants throughout the growing season?except S.alterniflora in April?.Medium and high salinity could significantly?P<0.05?inhibit g_s,T_r and WUE of P.australis,while water use parameters of S.alterniflora decreased significantly?P<0.05?only at high salinity.Under the combined treatments of high salinity and waterlogging,g_s,and WUE of P.australis were the lowest,by 54.0±29.5%and 55.6±35.3%respectively,and a significant interactive effect was detected?P<0.05?.In summary,S.alterniflora has better adaptability to waterlogging and salinity treatments than P.australis during a whole growing season.Under the condition of sea-level rise in the future,the combined risk of long-term waterlogging and saltwater intrusion may lead to the degradation of indigenous plant communities and the further invasion of S.alterniflora at China's coastal wetlands.