Effects Of Seasonal Floods On Species Diversity Of Plant Communities In The Yellow River Wandering Reach

Floodplain wetland ecosystem is located at the junction of river and land,which is affected by both land and water environment,and has great uncertainty and ecological vulnerability.As a ecological barrier for both aquatic and terrestrial ecosystems,floodplain wetland vegetation plays an important role in ecosystem services,which can maintain the stability of the beach and the dynamic equilibrium of material flow between water and land,improving biodiversity of floodplain wetland.The hydrologic rhythm of the river can change the soil seed bank and the soil nutrient level of the wetland,thus has a significant impact on the plant community structure of the floodplain wetland.The wandering reaches of the Middle and Lower Yellow River are the most affected by the process of water and sediment regulation by the Xiaolangdi Dam,it is not clear how seasonal flooding from the Xiaolangdi Dam will affect the plant species diversity of floodplain wetlands along wandering stretches of the river.Therefore,by comparing the changes of soil seed bank,plant species composition and soil environmental factors under the pre-flood and post-flood in four typical plant communities(Tamarix chinenisi community,Phragmites australis community,young Tamarix chinenisi community,Erigeron canadensis community)of floodplain wetland in the Middle and Lower Yellow River,our study focus on the influence of sediment deposition in flood process.Based on the statistical analysis methods of correlation analysis,Non-metric multi-dimensional scale,ANOVA analysis and Boosted regression tree analysis,the mechanism of plant community assembly and species diversity maintenance in floodplain wetland was discussed,it reveals the effects of flood on soil seed bank and physical and chemical properties of typical plant communities in floodplain wetland of wandering reach of the Yellow River,and quantifies the driving effects of various factors on maintaining plant species diversity.The main findings are as follows:1)The density of the seed bank in the wandering reach of the Middle and Lower Yellow River is between 7175-33325 seeds/m~2,and the seed bank size decreases obviously after the flood,but the seed bank still retains activity and diversity,is critical to the restoration of vegetation after flooding.The dominant families in the soil seed bank were Poaceae and Asteraceae,and the dominant species were Calamagrostis epigeios,Cyperus rotundus,Symphyotrichum subulatum,Potentilla supina.The seed quantity of Cyperus rotundus and Symphyotrichum subulatum were very large,which accounted for more than half of the total seed density in the soil seed bank.The species composition of the soil seed bank changed obviously after the flood.The Shannon-Wiener Diversity Index,Simpson Diversity Index and Margelef Index of the soil seed bank basically showed a significant increase trend post-flood.The silting depth of Phragmites australis community(dominated by Phragmites australis and Typha angustifolia)was the deepest(8.05 cm),and the silting depth was positively correlated with the seed bank density(p<0.05).2)Except for the community of Phragmites australis community,both Total organic carbon(TOC)and Total nitrogen(TN)increased significantly after flood(p<0.05),and the stoichiometry variation of soil carbon,nitrogen,phosphorus in different plant communities had no significant regularity after flood.The mean particle size(MD)of sediments in different plant communities was significantly smaller than that in the situ soil.The decrease of the percentage of fine sand in the soil after flood was the main reason for the significant decrease of MD in some plant communities(p<0.05).Total phosphorus content in sediments was higher than in situ soil,but carbon and nitrogen content was significantly lower than in situ soil(p<0.05).The content of C,N and P in sediment and soil had a good correlation with p H and MD(p<0.05).3)The vegetation of the floodplain is mainly herbaceous,and Tamarix chinensis was the only shrub and one of the dominant species.The dominant families of vegetation were Poaceae and Asteraceae,which is similar of the soil seed bank.In Poaceae,the Phragmites australis,Calamagrostis epigeios and Cynodon dactylon were the main perennial species,but annual Poaceae had no competitive advantage.Other dominant species were Typha angustifolia,Symphyotrichum subulatum,Cyperus rotundus,Erigeron canadensis.Symphyotrichum subulatum is a successful invasive alien species,which was dominant in all four communities.After the flood,there is a significant changed of the species composition of the soil seed bank.The Shannon-Wiener Diversity Index,Simpson Diversity Index and Margelef Index of the vegetation basically showed a significant increase trend.The species composition of post-flood vegetation was more similar to that of post-flood soil and sediment seed banks.The species composition of post-flood vegetation was more similar to that of post-flood soil and sediment seed banks.There was a significant negative correlation between soil C,N,P and community diversity index(p<0.05).The results of boosted regression tree analysis showed that the Shannon-Wiener diversity index of soil seed bank was the most important factor to determine the Shannon-Wiener diversity index of vegetation community,and the interpretation degree was 30.10%.The total interpretation of soil seed bank and soil physical and chemical properties were 44.05%and 40.82%,respectively.The flood changed the species composition of soil seed bank and the distribution pattern of soil nutrient elements through water flow and sediment,which affected the species composition and diversity of aboveground plant community.In conclusion,our study focused on the changes of soil seed bank,soil physical and chemical properties,aboveground vegetation community before and after water and sediment regulation of Xiaolangdi Dam.We quantified the role of soil seed bank and environmental factors in maintaining the species diversity of aboveground plant communities,and revealing the key factors of maintaining the species diversity of river floodplain wetland communities,it provides a theoretical basis for the restoration and maintenance of wetland communities in the Middle and Lower Yellow River.