Environmental Adaptability And Biogeomorphological Process Modeling Of Typical Salt Marsh Vegetation In Yangtze River Estuary

Coastal wetlands play vital ecological service functions such as carbon sequestration,pollution reduction,shoreline stabilization and biodiversity maintenance.However,as a typical biogeomorphic ecosystem,coastal wetlands are highly sensitive and fragile,making them highly vulnerable to threats from global change and human activities.Under the dual influence factors of sea level rise and invasion of alien species,China's salt marsh ecosystem is undergoing severe degradation.Due to the dynamic characteristics and complex feedback mechanisms among biology,ecology and geomorphology,the scientific protection and management of salt marsh ecosystems remains an urgent and arduous task.Taking the Chongming Dongtan Wetland at the mouth of the Yangtze River as the research area,this dissertation comprehensively applied various research methods such as environmental control experiments in ventilated greenhouse,field observation,remote sensing image interpretation,threshold analysis and model simulation to explore the environmental adaptability and bio-geomorphological interaction mechanism of typical salt marsh plants in the Yangtze River estuary.The main research contents include the physiological and ecological responses of two typical Poaceae salt marsh plants to waterlogging-salinity interaction environmental stress,the ecological adaptability and ecological threshold effect of pioneer salt marsh vegetation at the coastal frontier,the characteristics and mechanisms of coastal salt marsh bio-geomorphological interaction,and the application of eco-geomorphological coupling model in salt marsh vegetation restoration strategy.The results of this dissertation can provide guidance for biodiversity conservation,ecological restoration and management of coastal wetlands.The main results and conclusions of this study are as follows:(1)Responses of different salt marsh plants to simulated waterlogging-salinity stress.Through rigorous environmental gradient control experiments in a ventilated greenhouse,the growth,propagule(spike),chlorophyll content and salt secretion characteristics of native Phragmites australis and exotic Spartina alterniflora under waterlogging-salinity interaction stress were investigated.The results show that the combined treatments of waterlogging and salinity lead to significant decrease of ecophysiological parameters and the absence of sexual reproduction organs in P.australis.In contrast,S.alterniflora performed well under both stresses and showed significant salt secretion adaptability.Low salinity(5 ppt)and shallow waterlogging(50 mm)are suitable growing conditions for S.alterniflora.At moderate salinity(15ppt),there were no significant differences in biomass,leaf area,sexual reproduction organs and chlorophyll content.With the increase of salinity,the contents of Na⁺and Cl^-in the leaves of both species increased,whereas the content of K⁺decreased.At medium(15 ppt)and high(30 ppt)salinity,the concentration of ions in S.alterniflora remained relatively consistent,and the amount of salt secretion significantly increased.The physiological activities of osmotic regulation and salt secretion in S.alterniflora mitigated the nagative effects of salinity stress.However,for P.australis,the combined effect of hypoxia and salinity may damage its photosynthetic organs,disrupting its ionic homeostasis.In the coastal wetland of the Yangtze estuary,exotic S.alterniflora is more adaptable to the complex environment of flooding stress and salinity change than native P.australis.The relevant results provide a basis for the optimization of salt marsh model parameters.(2)Ecological adaptability of salt marsh pioneer vegetation at the tidal flat front.By selecting a fixed monitoring sample zone in the Chongming Dongtan Wetland,the growth,reproduction characteristics(seed bank and corm)of the pioneer plant Scirpus mariqueter and deposition/erosion dynamics of coastal wetland in the selected sample zone were regularly observed to explore the ecological adaptability of the pioneer vegetation of the salt marsh to the tidal marsh frontier habitat.The results showed that the colonization,growth and reproduction of S.mariqueter in the coastal frontier are very sensitive to elevation changes,reflecting the ecological adaptability of plants at landscape and individual scale.S.mariqueter that span flooding intensities usually exhibited morphological and reproductive plasticity.With the decrease of elevation,S.mariqueter tendency to allocate more biomass to the below-ground tissue.The elevation threshold interval of reproductive organ yield(2.38-2.50 m)was higher than that of morphology(2.05-2.14 m).The elevation threshold of asexual reproductive organ(corm)production shifted to the lower elevation(by about 0.15 m)relative to that of sexual reproductive organ(spike)production.With the prolongation of the flooding time,the ratio of corm:spike increased,indicating the reproductive plasticity.This study revealed that the combination of morphology and reproductive response of pioneer sedges contributes to its survival and expansion in heterogeneous habitats at the foremost coastal flat.(3)Bio-geomorphological characteristics and mechanisms of coastal salt marsh.Based on environmental control experiment and field observation results of S.alterniflora,P.australis and S.mariqueter,the structure optimization and parameter improvement of the Salt Marsh Model for Yangtze Estuary(SMM-YE)were carried out.The interactions between biotic processes(plant growth,colonization,and hydrodynamic tolerance)and abiotic processes(tidal transport of propagules,sediment input,and sediment erosion/deposition dynamics)are coupled into the model.The SMM-YE can simulate the processes of salt marsh vegetation dynamics,tidal flooding,sediment sedimentation and tidal flat elevation changes.Comparisons with field measurements showed that the model provided a satisfactorily description of interannual variations in vegetation growth and lateral expansion,sedimentary dynamics and elevation changes,and the formation of edge cliff of salt marshes frontier.In particular,observations and simulations revealed that sediment supply regulates the vegetation behavior and the rate of geomorphological change.The sites with high sediment supply have higher plant formation and growth rates,higher rates of vegetation expansion and sedimentation rates,resulting in a faster seaward progradation of marsh-edge cliff,compared with the sites with low sediment supply.The results showed that the interaction between biological and abiotic processes in the estuarine coastal tidal flats,together with the supply,jointly determine the heterogeneity of the biomorphology of salt marsh wetlands in high sedimentary systems(such as the Yangtze River estuary).The results also highlighted the great potential of process-based biogeomorphological coupling models in the formulation of coastal wetland management and assessment of future system resilience.(4)Application of ecological and geomorphological coupling model in salt marsh vegetation restoration strategy.Based on the ecological restoration project of S.mariqueter,the re-vegetation process of native wetland plants in the Yangtze Estuary at the front of the tidal flat was simulated by using the optimized SMM-YE model,and the risk of the second invasion of S.alterniflora to the restoration of S.mariqueter was predicted by scenario analysis.During the period of 2014-2019,the area planted S.mariqueter increased by 26.7 times,and plant density and biomass growth reached the level of mature sedge community.The establishment of S.mariqueter promoted sedimentary accretion at an annual sedimentation rate of 9.0 cm yr^-1,which was significantly higher than that in mudflat.The 6-year monitoring revealed that vegetation reconstruction using belowground propagules(corms)was an effective method under the strong hydrodynamic distrubances in the tidal mudflat,because the morphology and germination rate of the corms promoted the anchoring function of the seedlings.Due to the positive feedback between vegetation expansion and sedimentary accretion,both the restoration community and the original community of S.mariqueter are expanding.Subsequently,the second invasion scenario of S.alterniflora was simulated.The prediction suggested that high-speed invasion of S.alterniflora would colonize and replace the whole S.mariqueter community in the study area during 2014-2019.Therefore,the results of this study highlight the importance of combining the reintroduction of S.mariqueter with the removal of S.alterniflora in ensuring the restoration of native coastal sedge vegetation.As a long-term management strategy,it is recommended that continuous field monitoring of the risk of secondary invasion of S.alterniflora,with the assistance of decision-making tools(such as predictive models),to ensure the successful restoration of native salt marsh vegetation.In summary,the typical salt marsh plants in the coastal wetland of the Yangtze River Estuary have multiple ecological adaptations to environmental changes by phenotypic plasticity.However,different species and different eco-physiological indicators respond to environmental changes to dissimilar degrees,and there are explicit threshold effects.The process-based bio-geomorphologic interaction model can well simulate and predict the vegetation dynamics,elevation dynamics and formation and migration of marsh-edge cliffs of coastal wetlands.In addition,the model has vast application potential in vegetation restoration at salt marsh fontier and risk management of alien species invasion in coastal wetlands.In the future work,it is necessary to strengthen the response and adaptation of typical salt marsh plants in the Yangtze River Estuary to hydrodynamic effects and sedimentary dynamics,and further combine the process-based SMM-YE with hydrological processes and sediment transport to achieve accurate assessment of the vulnerability or resilience of salt marsh communities and biogeographic landscapes under future environmental changes.