| Ecological restoration has been evaluated as an important strategy to recover the degradation of coastal wetlands,to protect the biodiversity and to enhance its ecological services.Yangtze Estuary endemic salt marshes characterized by Scirpus mariqueter is identified as the most favorable habitat for local biodiversity,the extent of which has been declined due to the rapid expansion of exotic Spartina alterniflora.Based on an ecological engineering aimed at controlling this invasive species,the newly formed tidal flats have made it possible to recover of native S.mariqueter marsh.At present,the restoration of S.mariqueter in the Yangtze estuary is mainly carried out by the transplanting technique with soil microcosms consisting of plants and corms.However,the problem of high input and low output has been increasingly noticed.This study aims to propose a cost-effectiveness technique for restoration of S.mariqueter based on ecological theory.It looks forward to providing the scientific basis and technical supports for large-scale restoration projects and enhanced the restoration efficiency of coastal salt marshes.To explore the process and mechanism of colonization and expansion of salt marshes on tidal flats,and develop the scientific and effective methods for the restoration of S.mariqueter,we carried out the restoration experiment in three typical experimental sites of dynamically changing tidal flat outside the project of ecological control of Spartina alterniflora and improvement of birds habitats at Chongming Dongtan.The main research results were as follows:1.The applicability of facilitation theory to the ecological restoration of S.mariqueter in Yangtze estuary was verified.We first carried out a transplantation experiment with different size of initial S.mariqueter patches along an elevation gradient on the tidal flats.We aimed to test the value of facilitation theory(positive intraspecific interactions to facilitate organism success)and stress-gradient hypothesis on saltmarsh restoration for the newly formed tidal flats in Chongming Dongtan.The results showed that:(1)under a certain range of intertidal stress-gradient,increasing size of the transplanted patches could significantly enhance the survival rate and shoot density of S.mariqueter(P<0.05);(2)the hydrodynamic conditions on the intertidal flats were closely related to the elevation of tidal flat(P<0.05),while the stronger hydrodynamic stress at the low-elevation zone could minimize the positive bio-physical interactions.Our results indicated that improvement of transplantation design of target species in coastal saltmarsh restoration may motivate the positive intraspecific interactions,resulting in significantly higher restoration success.2.The study on manual support to improve the colonization of S.mariqueter seedlings was carried out.To increase seedling emergence rate in areas with frequent hydrological disturbance,we then examined the restoration strategy which add manual support during critical periods.The results indicated that the setting of manual support will not inhibit the germination and initial growth of seeds,and the seed density has no significant effect on the emergence rate in the laboratory without significant interference environment(P<0.05).In the field,under the condition of dynamic tidal flat,the seed emergence rate with manual support was significantly higher than that of n the direct sowing with a depth of 5 cm.Adding manual support could create the Windows of Opportunity(WoO)for seed emergence and could significantly improve the emergence rate of S.mariqueter(P<0.05).3.The WoOs for the colonization and establishment of S.mariqueter salt marsh in the tidal flats was identified.To improve the understanding of the processes that control salt marshes establishment,we investigated the WoOs for establishment and colonization of S.mariqueter salt marsh on a large-scale restoration site in the Yangtze Estuary.A set of field survey and measurements were conducted to identify the potential physical and biological thresholds that could provide a mechanistic insight into the establishment and colonization of this pioneer marsh.The successful colonization and expansion of S.mariqueter marsh on the tidal flat require for passing both physical and biological thresholds to open up the WoOs for establishment and colonization.That is,the concurrence of propagules availability in the early growing season,the appropriate elevation of tidal flats,sound sedimentary regime and the competition to exclude invasive species,are presumably to be essential for S.mariqueter marsh establishment and colonization success.The seedling establishment needs to pass a tipping point of tidal flat elevation which is higher than 2.2 m,and the sedimentation dynamics should be benign.At the same time,the cover of S.alterniflora should be controlled down to less than 10%of salt marsh total cover and surpass the threshold of propagule availability.The present study is the first to underline the importance of WoO related thresholds in the establishment and colonization of the pioneer salt marsh in the Yangtze Estuary.4.A new cost-effective restoration strategy of seed-sowing by applying the concurrence of WoO concept was presented.To test the effects of the concurrence of WoOs on the establishment and colonization process of S.mariqueter and in the design of a cost-effective technique for restoration practice,a seed-sowing experiment on the bare tidal flats was then conducted.The thresholds have to be passed until the pioneer S.mariqueter marsh could be successfully established and expanded.This study is the first to underline the importance of WoO related thresholds in the establishment and colonization of the pioneer salt marsh in the Yangtze Estuary.In applying such ecological insights,our seed-sowing field experiment proved that that by surpassing these thresholds,S.mariqueter salt marsh can be successfully restored in a cost-effective way.The cost of seed-sowing practice in this study could be reduced to 1-1.5 yuan/m~2,as the seed-sowing on the bare tidal flat was much easier and needed much less labor and transportation.A new cost-effective restoration strategy of seed-sowing by applying the concurrence of WoO concept can open up a new way for the large-scale restoration of salt marshes.Our results reveals that restoration techniques based on seed sowing and plant transplanting developed by ecological theory might be feasible for the restoration of intertidal salt marsh.The successful establishment and colonization of salt marshes on tidal flats is the result of bio-physics interaction.The WoO theory,to some extent,explains the mechanism of the establishment of pioneer salt marshes.Before carrying out the ecological restoration of salt marshes,understanding relevant ecological theories and applying them to ecological restoration practice can provide the scientific basis and technical supports to large-scale restoration projects and enhanced the restoration efficiency of coastal saltmarshes in the Yangtze Estuary. |
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