Effects Of Spartina Alterniflora Invasion On Regeneration In A Coastal Wetland And Regulation Mechanism By Nitrogen Coupling Factors

Coastal wetland is a complex ecosystem formed by the interaction between land and sea.Studying its community regeneration and the related mechanisms are helpful for maintaining the ecosystem stability and the sustainable development of the wetland.Soil seed bank as an important manifestation of the potential regeneration ability of the community can buffer the outside disturbance to the ecosystem through the dormancy form of‘storage effect’,showing important value for protecting species diversity and maintaining the community stability.Only investigating both field natural communities and soil seed bank structure under the influence of invasive species can we deeply clarify the mutual feedback relationship between the wetland plant communities and the environment in the context of plant invasion,and to predict the plant diversity pattern and community developmental trend of the reshaped coastal wetland.Chongming Dongtan wetland is a typical estuarine salt marsh wetland in the eastern China,and also a key area for wetland diversity protection.The large-scale spread and invasion of S.alterniflora population in the Chongming Dongtan wetland seriously threatened the structure and function of the ecosystem in the region.Although S.alterniflora invaded area in Chongming Dongtan wetland has been preliminarily controlled,such effects were difficult to maintain continuously in the later stages,which resulted in the occurrence of the tendency of‘secondary invasion’.In this study,the invasive species S.alterniflora communities,native species Phragmites australis communities,Scirpus mariqueter communities and ruderal communities and bare sites without vegetation coverage in the Chongming Dongtan wetland were taken as the research objects to analyze the differences and variation characteristics of the community regeneration in the invasive and native species distribution areas.Subsequently,based on the investigation of the community regeneration status and environmental factors,we further set up research plots and sampled the above-ground S.alterniflora communities and soil seed banks with different coverage to simulate the communities from the initial to the successful stages of the invasion.At the same time,we tried to find the key resource types and contents limiting the invasion process by analyzing the S.alterniflora adaptive adjustment strategy of functional traits and the coexistence mechanism of germinated seedlings in the soil seed bank.Finally,according to the key functional traits formed during S.alterniflora invasion,the overlapping resistance structure was constructed,and the relationship between the resource-mediated overlapping resistance structure and the dominance index of the target species in the soil seed banks were analyzed,taking P.australis community as a control.The main results were as follows:1.The species richness and density in the soil seed banks under the different plant community types were higher than those of the corresponding above-ground communities,and those at 0-5 cm seed banks were higher than those in 5-10 cm,except for the unvegetated sites.Among the soil seed banks under all vegetation community types,the species richness in the soil seed bank under the S.alterniflora community was the lowest,but the proportion of S.alterniflora seeds was the highest.Similarities of the species composition between the seed banks under S.alterniflora communities and the aboveground communities in the two soil depths were high,and the phylogenetic structures among the species in the seed banks in the two soil depths were in aggregation.The species composition variations in the seed banks at 0-5 cm and 5-10 cm depths were interpreted 3.03%and 2.25%by the aboveground communities,respectively,while those were interpreted 4.92%and 5.55%by the soil microbial biomass,respectively.The SEM model explained 98.1%and 91.8%of the species richness variations in the seed banks at the 0-5 cm and 5-10 cm depth,respectively,and explained 98.8%and 46.1%of the species density in the seed banks at the 0-5 cm and5-10 cm depth,respectively.The aboveground community biomass and species abundance directly affected the species richness in the 0-5 cm seed banks and species density,while its height and biomass only affected the species density in the 5-10 cm seed banks.The microbial biomass in 0-10 cm soil depth indirectly affected the species richness and species density in 0-5 cm seed banks,while affected the species richness in 5-10 cm seed banks.Soil physical and chemical properties only indirectly affected the species density in 0-5 cm seed banks.2.In the process of naturalization,the coverage of S.alterniflora communities showed strong plasticity in functional traits.From the early to the successful invasion stages,it realized coordination between the functional traits by reducing the height and chlorophyll content and increasing the aboveground biomass,to ensure the dominance in the competition.In the early invasion stage,S.alterniflora population showed the largest niche breadth in the communities,while in the successful invasion stage,its niche breadth decreased,but it still maintained the niche breadth advantage compared with the other species.At the initial stage of invasion,the niche overlap between S.alterniflora and the coexisting closely related seedling species from the soil seed banks was lower than that between S.alterniflora and the coexisting distantly related seedling species from the soil seed banks.However,in the middle and late invasion stages,the niche overlap between S.alterniflora and the coexisting closely related seedling species from the soil seed banks was higher than that between S.alterniflora and the coexisting distantly related seedling species from the soil seed banks.Although the seedling species richness from the soil seed banks under the different coverage of S.alterniflora communities were the same,the seeds of S.alterniflora maintained its highest proportion in the seed banks and its unique competitive advantage.3.During the natural regeneration process of S.alterniflora community,the regulation of soil nitrogen content on its functional traits was stronger than those of the other habitat factors.The experiments on the resource changes and soil seed bank germination and the seedlings cultivation showed that when the soil nitrogen concentration exceeded 1 g/kg,the plant density of S.alterniflora in the all germinated coexisting seedling species decreased.When the water depth above the soil surface exceeds 2 cm,the plant density of the native species in the all germinated coexisting seedling species decreased.Therefore,soil nitrogen had a limiting effect on the functional traits of S.alterniflora and the species diversity in the soil seed banks.4.The growth of S.alterniflora community tended to be stable in summer and autumn,and the trend of community height with different coverage tended to be consistent,and the peak height tended to be relatively concentrated,while,the seasonal variation trend of above-ground height of the native species P.australis community showed opposite to that of S.alterniflora community:the growth tended to disperse in summer and autumn,and the peak height increased with the increase of community coverage.The plant community structure reached the mature period in autumn,and after the germination of the soil seed bank from low to high coverage of S.alterniflora communities and P.australis communities,the dominance index of S.alterniflora and P.australis gradually decreased,and the dominance index of S.alterniflora was higher than that of P.australis.The overlapping resistance structure formed by S.alterniflora communities was synergistically affected by soil NH₄⁺-N,leaf total nitrogen and soil microbial biomass nitrogen,and the effect of this structure on the formation of soil seed bank in different soil layers showed an opposite trend（promoting the soil seed bank in0-5 cm soil layer and inhibiting the soil seed bank in 5-10 cm soil layer）.The overlapping resistance structure formed by the native P.australis community was only affected by soil NO₃^--N,and this structure only inhibited the formation of 5-10 cm soil seed bank under P.australis community.In summary,the changes of species diversity,functional traits and structure of the coastal wetland plant communities caused by S.alterniflora invasion,as well as the species coexistence and the formation mechanism of the germinated seedlings from the soil seed banks were systematically studied.During the invasion process,through preferentially allocating resources to the functional traits of the aboveground part,S.alterniflora population dominated the coastal salt marsh community.The overlapping spatial structure of plants formed by the synergistic changes of such functional traits screened seeds,helping to continuously keep the dominance of S.alterniflora seeds in the soil seed banks with multiple species coexistence.This process was regulated by soil nitrogen coupling factors.Appropriate soil nitrogen contents and types promoted the clonal growth and seed reproduction of S.alterniflora.