| The Yangtze River Estuary Coastal Wetland has important ecological service functions in coastal protection,ecological services,biodiversity maintenance,carbon sequestration and pollution reduction.However,in the past 50 years,with the continuous decrease of the amount of sand coming from the upper reaches of the Yangtze River and the continuous rise of global sea level,it may have a potential impact on the coastal wetlands of the Yangtze Estuary.Compared with traditional ecology and geomorphology,Bio-geomophology is a new idea to systematically study the dynamics of ecosystem community structure by identifying the interaction between organisms and geomorphology.In order to clarify the dynamics of sediment erosion and sedimentation in the large estuary and the changes of incoming water and sediment on the estuary coastal wetland landforms and tidal flat organisms,it has certain value,and has special significance for threat factor analysis,ecological conservation and restoration of coastal wetlands.In this study,the Chongming Dongtan wetland in the Yangtze Estuary was selected as the research object.The E-site with partial muddy soil in the east and S-site with muddy sandy soil in the south were selected.The role of shaping landforms and the growth characteristics of tidal flat vegetation(Scirpus species)on different elevation gradients of tidal flats have analyzed the interrelationships of wetland biological topography.Based on remote sensing information,it interprets the changes in vegetation pattern on a multi-year scale,and combines the water and sediment flux data from Datong Hydrological Station to evaluate the response of the tidal flat vegetation and microtopography to the short-term changes in incoming water and sediment from the upstream.The main conclusions are as follows:1)According to the observation results of the vertical sediment erosion and deposition on the tidal flat from 2018 to 2019 in the study area,the annual average sediment deposition heights of the E site and the S site are 27 cm and 10 cm,respectively.At the beginning of the plant growing season,the average deposition amount of vegetation samples in E site was 2.8 cm,and the erosion trend of non-vegetation samples was-2.6 cm.The average deposition height of vegetation samples in S site was 0.9 cm,and there was no vegetation sample.The average amount of erosion is-6.3 cm.In the middle of the plant growing season,the average sediment deposition in the E site was 7.5 cm,and the average deposition amount in the vegetation plots was 1.45 times that of the non-vegetation plots 7.8 times,the vegetation has a significant effect on silting.There was a linear positive correlation between sediment erosion and vegetation density in the two plots,and the correlation between S site was extremely significant(p<0.01).At the beginning and middle of the plant growth season,there was a linear positive correlation between the sediment erosion and the above-ground biomass of the two sites,and the correlation between the middle of the plant growing season in the E site was extremely significant(p<0.01).The surface sediments of the tidal flat in the E site are mainly clay silt and silt,while the surface sediments of the tidal flat in the S site are mainly sandy silt and silt.During the vegetation growing season,the average particle size of the surface sediments in the two plots showed a trend of thinning to thickening from land to sea.2)The gradient of the elevation along the tidal flat decreases,and the plant height of the Scirpus spp.in the E site at the middle of the growing season gradually decreases(p<0.01).The above-ground biomass of the plant in the middle and high tidal flat area is the highest in the beginning of the growing season,and by the middle of the growing season,vegetation growth at each elevation of the beach gradually became consistent.In the S site,the above-ground plant biomass was highest in the high-tidal flat area at the beginning of the growing season.By the middle of the growing season,the middle and low tidal flat vegetation grew rapidly,and the above-ground biomass gradually became consistent with the high tidal flat vegetation growth trend.Above-ground biomass of the S site vegetation gradually decreased along the tidal flat elevation during each period of the growing season(p<0.01).At the beginning of the growing season,the underground biomass of the E site decreased along the elevation gradient(p<0.01),and the underground biomass of the S site decreased along the tidal flat elevation at each growth stage(p<0.01).Along the elevation gradient,there was no significant difference in biomass root-shoot ratio between two sites.The root biomass ratio of plant biomass in E site was significantly lower than that in S site(p<0.01),and plant height,density and biomass were significantly higher than those in S site.3)Based on the dynamic observations of the elevation and frontier vegetation of the Chongming Dongtan wetland tidal flat from 2013 to 2017,the results show that even at the historically low point of the Yangtze River,the tidal flat siltation and frontier vegetation expansion of the coastal wetland in the Yangtze River still uninterrupted.In two monitoring sites,the net increase of the tidal flat elevation was 24.3-57.5 cm,and the area of the frontier vegetation increased by 2.18-4.74 times.In addition,due to the surge in the Yangtze River runoff during 2016-2017,the sediment deposition rate and vegetation planting rate of tidal flats were higher than in other years.It can be seen that the short-term substantial increase in the amount of incoming water in the upstream can lead to a large increase in the amount of sediment transported,which meets the needs for the settlement and diffusion of tidal flat vegetation.However,due to the uncertainty of river sediment transport,the annual sediment transport volume recorded by Datong Hydrological Station in the upper reaches of the Yangtze Estuary may not be a suitable indicator to directly explain the tidal flats and vegetation dynamics of the Yangtze River Estuary.Based on the current river sediment transport volume and the positive feedback interaction of vegetation clusters and sediment deposition processes,the coastal wetlands of the Yangtze Estuary can resist the effects of sea level rise.However,if the amount of incoming sand continues to decrease,the risk of estuarine coastal wetland state transition will increase.In summary,when the tidal flat vegetation reaches a certain biomass(E site:100g/m~2,S site:54g/m~2)and plant density(E site:1650 stems/m~2,S site:1500 stems/m~2)in mid-growing season,it has a significant effect on sediment promotion.At the same time,in areas with high sediment deposition,it is helpful for plant growth.The elevation of tidal flats and vegetation area increased significantly from 2016 to 2017,which may be related to the short-term substantial impulse in river and sediment discharge from the upstream.At present,the amount of sediment transported upstream can still maintain the tidal flat siltation and vegetation settlement in the coastal wetlands of the Yangtze Estuary. |
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