Natural-based Restoration Techniques For Scirpus Mariqueter Marshes

Salt marsh vegetation are fundamental for coastal wetland ecosystem,in maintaining their structure and function.As a foundation pioneer salt marsh species in the Yangtze Estuary,Scirpus mariqueter plays an extremely important role in maintaining the stability of salt marshes ecosystem in the Yangtze Estuary.However,in the past 30 years,the area of S.mariqueter marshes have decreased rapidly in the Yangtze Estuary,as a consequence of the invasion of Spartina alterniflora,human reclamation and sea level rise.Although many efforts have been devoted to tackle the loss of S.mariqueter marshes,it remains challenging to reconstruct and rejuvenate S.mariqueter population efficiently,economically and conveniently under multiple stressors in the Yangtze Estuary.As S.mariqueter restoration schemes are largely impeded by the lacked of in-depth insights into the natural expansion processes of S.mariqueter propagules,this study manipulated a series of lab experiments and field experiments to mechanistically understand the dispersal and colonization of early-stage S.mariqueter propagules.Firstly,the natural dispersal processes of S.mariqueter marshes on a natural tidal flats in Chongming Dongtan were investigated.Secondly,the critical abiotic thresholds for S.mariqueter propagule dispersal and colonization were examined.Finally,based on these results,this study developed a natural-dispersal process based marsh restoration design,and experimentally showcase of such a design in newly formed tidal flats in the Yangtze Estuary.The main research contents and results are as follows:(1)The natural dispersal processes of S.mariqueter marshes in Chongming DongtanThe natural dispersal processes of S.mariqueter population in Chongming Dongtan was studied by remote sensing interpretation and field investigation.The results of spatiotemporal dynamic monitoring of salt marsh vegetation in the study area from 2014 to 2020 suggested that the population area of S.mariqueter only decreased from 3.1 hm~2 in 2014,it increased to 89.9 hm~2 in 2020,an increase of 29times.This suggested S.mariqueter had sufficient of propagules(seeds)sources to rapidly recover in suitable habitats in our study area via natural dispersal.S.mariqueter propagules(seeds and seedlings with buoyancy)are an important source of dispersal and colonization in the suitable habitats of the newly formed tidal flats.(2)Critical abiotic thresholds for S.mariqueter propagules dispersal and colonizationThe critical shear stress of S.mariqueter propagules(seeds and seedling)at different growth stages was measured on different sediment substrates by using U-GEMS erosion microcosm system.Present results suggested that the critical shear stress of S.mariqueter propogules were significantly affected by different growth stages and sediment substrates(P<0.01).On the surface of muddy and silty sediments,the critical shear stress for seed floating initiation was greater than 0.4 Pa.However,as the growth stage increased,the critical shear stress for seed floating initiation of S.mariqueter seedlings gradually decreased to 0 Pa,indicating that the chance of floating initiation and dispersal of S.mariqueter seedlings increased with current velocity.Compared to propagules on the muddy tidal flats,the seeds and seedlings of S.mariqueter have higher chances of floating initiation on silty tidal flats.By exposing S.mariqueter propagules(seeds and seedlings)of different growth stages to a gradient current velocity in a large Settling-Bucket.We found that the settling rate and velocity of S.mariqueter propogules were significantly affected by both propagule growth stage and current velocity(P<0.05).For seedlings at the same growth stage,their settling rate was significant declined,when the current velocity increased.For seedlings exposing to the same current velocity,their settling velocity increased significantly with a decreased settling rate as propagule growth stage increases(P<0.05).This suggested that seeds were easier to settle and seedlings were easier to dispersal under stronger current hydrology conditions.By means of a colonization experiment in that carried for S.mariqueter propagules(seed and seedling)under different disturbance-free periods,we found that longer disturbance-free period indeed assisted the colonization of S.mariqueter seedlings.The survival rate,bud length and root length of S.mariqueter first increased and then declined with the increased length of disturbance-free period under our experiment conditions.It was noted that the survival rate of seedlings in each growth stage approached to above 50%when the length of disturbance-free period was increased to more than 20 h/d(i.e.20 h/d and 22 h/d).(3)Application of natural restoration technology for S.mariqueter populationBased on the above results on the mechanism of S.mariqueter propagule dispersal and colonization,the natural restoration technology of S.mariqueter population based on intertidal natural dynamic process was developed.In verifying the cost-effectiveness of this design,we carried out a large scale field restoration experiment by carefully selecting suitable habitats for S.mariqueter colonization in newly formed tidal flats in Chongming Dongtan,and served the area with artificially added seed source.After natural dispersal and colonization of propagules,the S mariqueter marsh in the study area gradually expanded to a 17.7 hm~2 spatial coverage in one growing season.Present restoration experiments suggested that the natural restoration technology of S.mariqueter population based on intertidal natural dynamic process was indeed cost-effective as experimentally showcase in the Yangtze Estuary which can further provide scientific basis and technical support for natural-based salt marshes restoration schemes.