Shallow Groundwater Changed The Traits Of Phragmites Australis In The Yellow River Delta

Global warming induced sea level rise has a huge impact on the coastal wetlands.How the coastal wetland plants adapt to the changing natural environment is a hot topic in current ecological research.Plant functional traits can objectively express the response mechanism of plants to environmental factors.In order to explore the response mechanism of coastal wetland plants to environmental changes,11 sampling points were set up in the coastal wetland of the Yellow River Delta(YRD)in Xianhe Town,Dongying City.The shallow groundwater level and salinity,plant community characteristics,soil physicochemical properties,and plant functional traits were investigated and analyzed.According to shallow groundwater level,salinity and soil physicochemical properties,11 sampling points were divided into 4 groups(A,B,C,and D).To study the effects of shallow groundwater,soil physicochemical properties on the functional traits of phragmites australis,the population characteristics and leaf traits(including leaf dry matter content(LDMC),thickness(LT),area(LA),specific leaf area(SLA),stomatal area(SS),stomatal density(SD),C-N-P ecological stoichiometry,and carbon stable isotopes)were measured.The cluster analysis,principal component analysis,and redundancy analysis were employed to explore the key factors that affect plant functional traits.The study would provide theoretical basis for ecological restoration and wetland protection in the YRD.The main findings of this study are as follows.(1)The salinity and water level of shallow groundwater in the study area decreased gradually from the coastline to landward.The water level fluctuated between 0.20?1.00 m in different months.As both sides of the study area are adjacent to the sea,the salinity of shallow groundwater was increased gradually.The salinity of shallow groundwater changes gently in different months with the average salinity of sample groups A,B,C,and D were 57.08 g/L,40.93 g/L,29.82g/L and 27.31 g/L respectively.Shallow groundwater significantly changed the soil physiochemical properties.With the gradual decline of shallow groundwater salinity and water level,the soil conductivity gradually increases.The average soil conductivity of sample groups A,B,C,and D were 3.23 mS/cm,4.00 mS/cm,5.27 mS/cm,and 4.55 mS/cm respectively?The soil bulk density increased with the increase of soil depth.The species composition of plant community was relatively simple in the study area.17species plant species were find belonging to 8 families,mainly including Tamarix chinensis,Phragmites australis,Suaeda salsa,and other halophytes.Under brine groundwater condition(Group A),the plant density(89.85 herb/m~2)was higher than that under saline groundwater condition(Group B 15.92 herb/m~2,Group C 18.73 herb/m~2,and Group D 46.18 herb/m~2respectively).With the salinity decrease of saline groundwater,the aboveground biomass of herbs increased gradually(Group B 0.18 kg/m~2,Group C 0.24 kg/m~2,and Group D 0.38kg/m~2 respectively),The P.australis was the dominant species and constructive species in the community.(2)With shallow groundwater salinity and water level declined,the coverage,density,plantheight,and aboveground biomass of P.australis increased significantly(P<0.05).With higher salinity and water level of shallow groundwater(Group A),P.australis showed higher density(4.16 herb/m~2),coverage(43.89%),aboveground biomass(0.33 kg/m~2),important value(0.5±0.06),and richness(23.84%).With lower salinity and water level of shallow groundwater(Group B,C,and D),P.australis showed lower density,coverage,aboveground biomass,important value and richness.(3)The salinity and water level of shallow groundwater significantly changed the leaf morphology and stomatal characteristics of P.australis.With the decrease of salinity and water level of shallow groundwater,the leaf thickness,leaf area and stomatal density of P.australis decreased,and the dry matter content and stomatal area of abaxial surface increased.Under brine groundwater condition,the leaf area(16.81 cm~2),dry matter content(287.98g/kg),specific leaf area(22.61 g/cm~2),and total stomatal density(6.85 N/mm~2)of P.australis were significantly lower than those under saline groundwater condition.Under saline groundwater condition,with the decrease of groundwater salinity,because the soil conductivity exceeds the optimal growth‘critical value'(3.0 ms/cm)of P.australis,more resources were allocated to the construction of leaves.Therefore,the leaf dry matter content and leaf area of P.australis increased,the leaf thickness decreased,and the leaf stomata changed from small and dense to large and sparse.(4)The foliar C-N-P contents of P.australis increased with the decrease of groundwater salinity(P>0.05).Under brine groundwater condition,the foliar C-N-P contents were slightly lower than that under saline groundwater condition,and the foliar C:N and N:P ratios were higher than that under saline groundwater condition.The foliar C content was negatively correlated with foliar N and P contents,and positively correlated with the C:N and C:P ratios.There was a significant positive correlation between foliar N and P contents,a significant negative correlation between C:N ratio and N content.The foliar P content was negative correlated significantly with C:P,N:P ratio.The lower foliar C content indicated relatively weak C storage capacity and resistance to environmental disturbance of P.australis.The higher foliar P content is an adaptive strategy of P.australis to water and salt stress and barren soil habitat.The average foliar C:N:P ratio of P.australis was 216.9:9.1:1,which was approximate to that of other plants in the YRD and lower than that of forest leaves at a global level.The determination coefficient of the correlation between P and C:N:P ratio is generally higher than that of C and N,indicating that foliar P was key controlled C:N:P ratio.Combined with the availability of soil P in the study area,the growth of P.australis was limited by soil N and P,but the limitation of soil N is more significant with the foliar N:P<10.(5)In the study area,with the decrease of groundwater salinity,foliar?¹³C of P.australis increased gradually.The foliar?¹³C was generally between-23.65 and-21.39,with an average value of-22.63±0.44,which was significantly higher than that of P.australis in other areas of the YRD,indicating that P.australis had higher water use efficiency(WUE)in this study area.The foliar?¹³C was significantly affected by plant physiology,nutrient status and soil conditions.The foliar?¹³C was positively correlated with foliar C and P content of P.australis and soil conductivity,negatively correlated with the C:P ratio.The higher foliar P content indicated that P.australis increased carbohydrate synthesis and biomass by improving P use efficiency in water deficiency environment.The decline of groundwater level will lead to the decrease of soil salt content.Therefore,foliar?¹³C increased and WUE increased with the decrease of salinity of shallow groundwater.(6)PCA,RDA,and correlation analysis showed that the main influential traits of 17functional traits in the study area were SLA,leaf C:P ratio,leaf P content,stomatal density,and population density.SLA was mainly affected by groundwater level and salinity;leaf nutrient content was mainly controlled by groundwater salinity and soil conductivity;stomatal density was mainly regulated by groundwater level and soil bulk density;population density was mainly governed by soil bulk density and soil water content.