The Dynamics Of Plant Community Distribution Of The Salt Marshes In The Yangtze River Estuary As Influenced By Spartina Alterniflora Invasions

Coastal salt marshes,which are fragile and sensitive to environmental changes, provide great valuable ecosystem services for human society.Plant community ecology of salt marshes is an important and basic issue of salt marsh ecology.The salt marshes in the Yangtze River estuary play very important roles in maintaining biodiversity and estuarine ecosystem stability.However,over the last decades they have severely suffered from human activities such as land reclamation and Spartina alterniflora invasions,and consequently the structures and functions of the ecosystems are greatly threatened.The aims of this thesis were to:1)examine the plant community dynamics,succession of the salt marshes and the main factors which affect the plant community in the salt marshes in the Yangtze River estuary;2) evaluate the impacts of S.alterniflora invasion on plant community dynamics,and better understand the ecological mechanisms for the successful invasion of S. alterniflora in the Yangtze River estuary.Remote sensing technique,field surveys, transplanting experiments and controlled experiments were employed to achieve these goals.The major findings are summarized as follow:1)Plant zonation was found along the elevation gradient,from lower marshes to higher marshes.This spatial pattern of plant distribution shown in sequence of plant community succession was:bare flat→Scirpus mariqueter community→Phragmites australis community.However,S.alterniflora invasion has disrupted this natural succession pattern.In the marshes with higher salinity,S.mariqueter might be entirely eliminated.The new sequence of plant community succession would then be: bare flat→S.alterniflora community→P.australis community.On the other hand, in the marshes with lower salinity,the sequence of plant community succession would be:bare flat→S.alterniflora community→P.australis community.5.alterniflora invasion resulted in both generation of a new plant zonation pattern and reduction of range of S.mariqueter and P.australis in the Yangtze River estuary.2)The plant community dynamics in the Yangtze River estuary was affected by multiple factors such as the development of wetlands(mudflats),human activities. Diking at Chongming Dongtan reduced vegetated area,while the areal changes in plant community at Jiuduansha was due to wetland(mudflat)development.In the last 10 years,S.alterniflora invasion played the most important role in plant community dynamics.The area of plant community increased with the constant and rapid development of mudflat,but the area of S.mariqueter was reduced due to the competitive exclusion by S.alterniflora.(3)The competitive balance between S.alterniflora and P.australis could be altered by growing conditions,and their distributions in salt marshes could be changed by environmental factors.RNE(Relative Neighbor Effect)analysis indicated that competitive dominance of S.alterniflora occurred under the conditions of high salinity,sandy sediment and high immersion.Interactions between environmental factors were detected in this study.At high salinity level,increased N level favoured the competitive superiority of S.alterniflora over P.australis.However,under the condition of low salinity,P.australis showed competitive superiority when N level increased.The environmental factors that increase the competitive dominance of S. alterniflora over P.australis could partially explain the successful invasion of S. alterniflora into P.australis communities.The results obtained here also imply that eutrophication,sea level rise and saltwater intrusion may further favor S.alterniflora invasions in coastal ecosystems.(4)S.alterniflora,P.australis and S.mariqueter showed different adaptive capacities to the environments of the salt marshes in the Yangtze River estuary.In general,the performances of the three plants were correlated with the elevational variation of the tidal marshes,which results in the regular changes of environmental factors along the elevation gradient.S.alterniflora was found to survive within 0.4 m below mean high water level(MHWL),P.australis could survive within 0.6 m above MHWL,and the lower limit of S.mariqueter varied about MHWL,depending on the salinity.Under certain conditions,an elevational increase could promote the growth of the three plants,the percentage of sexual reproduction,and the intensity of interspecific competition.In low marshes,the distribution of the plants was determined by the ecophysiological tolerance to physical stresses.On the other hand, the physical stress was weakened in high marshes,and thus interspecific interactions became more intensive.As a consequence,the distribution of plants was determined by the outcome of interspecific competition.S.alterniflora was found to have high tolerance to the physical stresses in low marshes and high competitive capacity in middle marshes.Therefore,S.alterniflora had great impacts on the plant distribution and plant community dynamics in the Yangtze River estuary.Furthermore,this study indicated that the plant zonation in salt marshes in the Yangtze River estuary was a consequence of interactions between biotic and abiotic factors.(5)According to the characteristics of plant community and environmental heterogeneity,S.alterniflora has adapted itself to the biotic and abiotic environments in the Yangtze River estuary,and intentional introductions have obviously facilitated its successful invasions in the Yangtze River estuary.The ecological mechanisms for successful invasions of S.alterniflora into the salt marshes in the Yangtze River estuary were the interactions between the invasiveness of S.alterniflora and the invasibility of the salt marsh ecosystems under the disturbance of human activities in the Yangtze River estuary.(6)S.alterniflora invasions in the Yangtze River estuary have strong negative effects on the native plants via competitive exclusion,which may in turn threaten the structure and functions of the native ecosystems.Thus,a plan for controlling further spread of existing S.alterniflora populations in the Yangtze River estuary is urgently needed.