| The saltmarsh wetland,located at the land-sea transition zone,is one of the most valuable ecosystems in the tidal wetland,providing vital ecological services to human society and playing an irreplaceable role in maintaining coastal zone biodiversity.Scirpus mariqueter is a pioneer and key species of the Yangtze Estuary.The population of S.mariqueter is shrinking as a result of reclamation,exotic species(Spartina alterniflora)invasion,global climate change and sea level rise,posing a significant threat to the long-term stability of the saltmarsh wetland ecosystem and the sustainable development of coastal tidal wetland.Thus,it has become a focus of many scientists recently.Coastal wetland restoration techniques have been widely used in practice and have shown positive outcomes,but the ultimate results are in a quandary due to expensive investment,limited efficiency,and short-term benefits.Therefore,it is critical to expand our research into the processes of saltmarsh plant dispersal and colonization,as well as the long-term impacts of tidal flat sedimentation dynamics on saltmarsh,in order to restore and conserve the Yangtze Estuary's valuable coastal saltmarsh.In this study,the native pioneer species,S.mariqueter,of the Yangtze Estuary was selected and its seeds were used as the experimental material.The mechanism of its colonization,dispersal and growth process of S.mariqueter was studied with field monitoring,controlled experiments and remotely sensed image interpretation,and finally the response of saltmarsh plants to the sediment dynamics conditions of the tidal flats was studied based on the variability of waves,currents and geomorphological features.The main research results were as follows:(1)The dispersal rate of S.mariqueter in summer was much greater than that in spring,and seeds from last and current year promoted rapid dispersal and establishment of saltmarsh in summer.The timing of seed germination,dispersal and anchoring in summer coincided with the spring tidal cycle(15 days),providing a favorable window of opportunity for successful establishment of S.mariqueter.According to the results of remotely sensed image interpretation,the average interannual growth rate of the area of S.mariqueter in Chongming Dongtan was 69%during 2013?2020,with the total area increased from 3.6 ha to 98.2 ha.The total area of S.mariqueter increased twice as rapidly in summer as it did in spring,from 24.2 ha in May to 49.1 ha in August.The experiment revealed that the seeds of S.mariqueter had a high floating rate(>80%).The cumulative germination rate of S.mariqueter seeds from last year(2019)and current year(2020)was 73.3±6.1%and 28.9±1.3%,respectively.Although germination rate of the current year's seed was lower compared to that of the last year's seed,it played a critical role in the expansion of S.mariqueter in summer,accounting for about 27%of the total seed germination.On the one hand,the special germination mechanism that leaves sprout earlier than roots largely enhances the buoyancy ability of new seedlings,enlarging the floating potential of seeds/seedlings for long-distance and large-scale dispersal.On the other hand,the experimental results showed that the growth activities of soaked seeds were seed germination and leaf sprouting(7 days),root taking(2 days)and forking(6 days),which is highly consistent with the spring tidal cycle(15 days).The short process of seed germination,leaf growth,root taking and forking of between two spring tidal cycles helps understand the phenomenon that pioneer salt marsh species dispersed quickly on suitable tidal flat at large scale,and confirms the role of windows of opportunity(Wo O)in the germination,dispersal and seedling growth of salt marsh plants.(2)As a“seed trapper”and“seedling promoter”,mud cracks play an important role in the dispersal and establishment of pioneer saltmarsh species.According to the results of field observations and remotely sensed imageinterpretation,mud cracking,affected by alternating wetting-drying cycles of mud surface,tended to occur at the seaward edge of the low saltmarsh,and the extend of mud caracks was closely related to the water level lines,providing suitablegeomorphological conditions for establishment of saltmarsh plants.The field observations and experimental results showed that the seedling density in mud cracks were higher than those in patches,such as 562±80.67 ind/m~2and 417±57.17 ind/m~2 in the field,respectively.Moreover,the plant traits such as plant height and stem diameter in mud cracks were better than those in patches,9.34±0.80 cm and7.88±0.56 cm for plant heigh,respectively.The experimental results showed that the seed germination time of S.mariqueter in mud cracks was earlier than that in the patches.the average mud crack intensity factor(CIF)was 5.3%,and seedling number in mud cracks accounted for 48.3%of the total seedling number,indicating about 5%of the area of mud cracks contributed nearly 50%of seedlings in the process of plant establishment.In conclusion,the mud cracks at the seaward edge of the low saltmarsh could act as a promoter rather than a stressor for dispersal and establishment of pioneer saltmarsh plants.Mud cracks play two roles in the establishment and dispersal of saltmarsh:(a)as a“trapper”for seeds,which mainly refers to its fractured morphological characteristics acting as a trapper cage for saltmarsh seed when the hydrodynamics of tidal flat are relatively weak(e.g.,ebb tides).(b)as a“promoter”for seedlings,which mainly refers to its network structure of mud cracks on tidal flat changing the water cycle,moisture and oxygen content of sediment,increasing the aeration of internal sediment and the respiration of salt marsh plants;and increasing the effective nutrient content of saltmarsh plants.(3)Individuals colonized at seaward front of saltmarsh were subjected to stronger hydrodynamic and sediment dynamic disturbance than those at the inner saltmarsh.Thus,individuals at seaward front of saltmarsh were more resilient,and its phenotypic plasticity of plant traits was a response to the heterogeneity of sedimentation dynamics of tidal flats.Generally,the tidal flat of Chongming Dongtan has been continuously silting up with seasonal difference.From March 2018 to March 2019,the tidal flat elevation of Chongming Dongtan increased by 0.58 m(from 3.15 m to 3.73 m).Spatially,the tidal elevation generally decreased along the transect towards the sea.It varied significantly at the inner saltmarsh,while at the saltmarsh front and bare mud flat at seaward side varied slightly.Furthermore,the hydro/sediment dynamics at the low saltmarsh in Chongming Dongtan was much stronger than that at the inner saltmarsh.Specifically,the average water depth,effective wave height,current velocity,bed shear stress and suspended soil concentration at the low saltmarsh were 4.8,9,5.8,4,1.78 times than that at the inner saltmarsh,respectively.In order to adapt to the different hydro/sediment dynamics conditions of the tidal flats,S.mariqueter performed strong phenotypic plasticity in different seasons,and then showed different growth patterns.Taking the low salt marsh-bare mud flat junction as an example,the aboveground plant traits in the early growth period mainly showed a pattern of"(density)sparse-(biomass)low-(stem diameter)thick-(height)short",and roots traits showed a pattern of“(root length)short-(root diameter)thick”.Such as the average stem diameter of plants at the high,medium and low saltmarsh zones was 0.50±0.02 mm,0.55±0.02 mm and 0.60±0.02 mm,respectively.In the middle of the growing season,plants showed a pattern of thick and high stem,while the root traits showed a pattern of“(root diameter)thick–(root length)long”.For example,the total root length varied significantly between high saltmarsh and low saltmarsh(p<0.01).This plasticity in the traits of saltmarsh plants improved their tolerance to the changing environment.This study illustrates the colonization/establishment,dispersal and growth processes of saltmarsh plants based on seeds,tidal flat geomorphology(mud cracks)and sedimentation dynamics,and then proposes a new perspective on the ecological restoration of coastal saltmarsh wetlands,which provides a theoretical support for nature-based and economically efficient restoration techniques,as well as a reference for large scale ecological restoration of coastal wetlands. |
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