A Research On Spatial-temporal Dynamics Of Salt Marsh Vegetation At The Intertidal Zone In Shanghai

Shanghai,with the huge amount of silt brought by the Yangtze River,is blessedwith plentiful coastal intertidal wetlands,which are also the potential land resourcesfor economical development.Intertidal wetlands could also provide ecological serviceand environmental value as well.The salt marsh communities colonized on thewetlands also have various functions such as improving water quality,mitigatingclimate change and conserving biodiversity.The results of this research on theintertidal vegetation populations dynamics in Shanghai will provide a sound basis forresource management and planning,preservation of biodiversity,sustainabledevelopment and utility of wetlands in the region.This study investigated spatial and temporal dynamics of salt marsh populationsof Yangtze River Estuary wetlands during the 1990 and 2008.A set of multi-temporalLandsat thematic mapper (TM) images were used,which basically covered the stateof the low tide at the time the images were taken and the areas of different stagesunder the impacts of human activities.These satellite images were geometricallycorrected by a series of nautical charts,using ERDAS software.Two spectralenhancement methods,Tasseled Cap (K-T) Transform and Normal DifferenceVegetation Index (NDVI),were used to interpret satellite images more efficiently.Moreover,several in situ field surveys were carried out to revise the imageclassifications and for the classification accuracy assessment.The main results of thisstudy were summarized as follows:1.The spatial distribution of salt marsh in Shanghai regionBefore the introduction of exotic plant Spartina alterniflora,the salt marshcommunities in intertidal zone of Shanghai region were mainly Phragmites australisand Scirpus mariqueter communities.S.alterniflora,as an ecological engineeringvegetation,was introduced to Shanghai intertidal zone during 1990s,and over the past10 years this species has gradually invaded large areas which could have been coveredby P.australis and S.mariqueter.P.australis was the dominant species by 1990 on the Yangtze River Estuarywetlands,it covered about 14100 hm~2,accounting for 80% of the total salt marsh area. However,half of the P.australis had been lost during 1990 and 2000.The mainreason was the1990s' reclamations,which happened in the high tidal mudflat,wheremainly colonized by P.australis communities.At the same time,S.alterniflora beganto establish on the Yangtze River Estuary wetlands.During 2000 and 2003,the areasof all the salt marsh vegetations kept increasing.To the August of 2003,the total areaof all the salt marsh vegetation had reached 21953 hm~2,including 7602 hm~2 of S.mariqueter,10075 hm~2 of P.australis and 4276 hm~2 ofS.alterniflora.After 2003,thetotal salt marsh began to decline mainly due to intense reclamation.Both S.mariqueter and P.australis communities shrunk by half.On the other hand,the exoticspecies S.alterniflora was still keeping increasing.To 2008,S.alterniflora haddominated 5698 hm~2 of the intertidal wetlands in Shanghai,accounting for the largestarea among all the salt marsh species.2.The spatial-temporal dynamics of salt marsh vegetation for ChongmingDongdan National ReserveIn Chongming Dontan,P.australis communities experienced serious degradationsince 1990 mainly because of the high altitude reclamation happen in 1992,1998 and2001.Afterwards,the P australis community began to increase slowly.There waslittle impact on S.mariqueter community from high altitude reclamation.From 1990the area of P.australis community had kept increasing for more than 15 years.Withthe rapid range population expansion of S.alterniflora,the area of P.australis beganto decrease since 2005,but the area of this species always maintained more than 2000hm~2.For exotic vegetation S.alterniflora,it had kept expanding from its initialintroduction,and had accounted for almost one third of the total intertidal salt marshat Chongming Dongtan Natural Reserve by 2008.3.Salt marsh population dynamics at Jiuduansha ShoalsJiuduansha Shoals are new formed neonatal islands,and they grow very fast dueto their unique location at Yangtze Estuary.The Jiuduansha shoals have never beencolonized by humans and have been in a natural condition since they were formed.From 1997 to 2008,the total salt marsh area had increased from 1094.6 hm~2 to 3600.6hm~2,with a mean increasing rate of 230 hm~2/a.S.mariqueter increased from 966.56hm~2 in 1997 to 1850.22 hm~2 in 2003,whereas the area of S.mariqueter began to decrease afterwards with the rapid range expansion of S.alterniflora population inMiddle Shoal and Lower Shoal.The mean increasing speed for P.australiscommunity was 70 hm~2/a,and the area increased from 168 hm~2 in 1997 to 924 hm~2 in2008.S.alterniflora had showed strong competitive capacity at Jiuduansha Shoals.Itsexpanding rate exceeded any of the indigenous species.The area of S.alterniflora hadincreased to 1708.57 hm~2 by 2008,accounting for 47.5% of the total salt marshvegetation,and had dominated the largest area on Jiuduansha Shoals.4.A CA model for population expansion of Spartina alternifloraAfter the initial introduction to Yangtze Estuarine wetlands,S.alterniflora hadshowed well adaptability in this region.It had become the dominant species in theintertidal zone of this area.The population expansion pattern of S.alterniflora wascompatible with the common feature of invasion,i.e.the initial colonization,a lagtime and the rapid range expansion.The intertidal environment was very suitable for S.alterniflora to grow and reproduct.In addition,there were little natural predators,sojust after 1-2 years of colonization and less than 3 years of lag time,S.alterniflorabegan its rapid population growth and range expansion.S.alterniflora had strongercapacity of competition and broader niche,which made it possible to invadesuccessfully in the intertidal zone.CA model of S.alterniflora population also indicated that in the region withlimited niche,such as Middle Shoal of Jiuduansha Shoals,the expansion of S.alterniflora was conformed with Logistic Growth model,whereas in the area withunlimited niche,such as Lower Shoal of Jiuduansha Shoals,the expansion of S.alterniflora was consistent with Exponential Growth model.So with the developmentof intertidal flat,more and more niches that suit for S.alterniflora would be formed,the range expansion ofS.alterniflora will be expected to continue well into the future,which means large area of intertidal wetlands will be dominated by S.alterniflora,putting seriously threaten to local ecosystem.