Effects Of Environmental Variation On Growth, Distribution Of And Interspecific Interactions Among Dominant Marsh Plants At Chongming Dongtan

Salt marshes are wetlands in the transition zones between land and sea.They receive large amounts of water,organic matter and nutrients input,support diverse species of grazing food chain and detrital food chain,and play important roles in biogeochemical cycles.Salt marsh at Chongming Dongtan has sharp environmental gradients and simple species composition,and thus is an ideal system for studying organization of plant communities.Only when fully understanding of the structure, organization and dynamics of salt marsh plant communities has been achieved,can we predict the responses of salt marshes to anthropogenic activities and global change, and guide the conservation and restoration of declining salt marshes.In this study,we used field observation,manipulation and controlled environment experiments to discuss the relationships between performance of dominant plants and physical factors, to reveal the competition processes between invasive species and native species,and to illustrate the organization mechanism of plant communities at Chongming Dongtan. The major findings are summarized as follows:(1)Plant communities at Chongming Dongtan showed distinct zonation pattern at large scale:Scirpus species occupy the seaward area,Spartina alterniflora dominates the middle marsh and coexist with Phragmites australis in the high marsh. From 2005 to 2007,zonation pattern had changed a lot.Spartina alterniflora expanded seaward much more quickly than Scirpus mariqueter did,and greatly reduced its distribution area by competition.Phragmites australis zone had also expanded seaward,and formed mosaic patches with Spartina alterniflora in the high marsh.(2)Tidal effects and the associated edaphic factors(soil salinity,bulk density, water content,ORP and pH)play important roles in organization of salt marsh plant communities.Growth of dominant plants at Chongming Dongtan(Scirpus triqueter, Scirpus mariqueter,Phragmites australis and Spartina alterniflora)were influenced mainly by salinity and inundation.Scirpus triqueter was least salt tolerant,and its growth and reproduction were significantly inhibited at 16 ppt.It was well adapted to inundation and the belowground parts were less affected by submergence.Scirpus mariqueter was more salt tolerant and less tolerant to inundation than Sciprus triqueter.Phragmites australis was also salt sensitive,and performed better in low inundation conditions.Spartina alterniflora was the most salt tolerant species,and moderate inundation could promote its growth.In benign conditions(low salinity,low water level),the differences between invasive Spartina alterniflora and native species were minor,whereas in stressful environment(high salinity,high water level), Spartina alterniflora took more advantages over native species.Wide ecological amplitude was one of the most important reasons that enable the rapid invasion of Spartina alterniflora at Chongming Dongtan.Tidal transportation and scouring are determinants of events in early stage of life history such as seed dispersal and propagule establishment,which will in turn influence the distribution range of dominant plants.Results of trsplantation experiment indicated that both Spartina alterniflora and Phragmites australis could survive out of their current ranges.At the seaward sites,death of transplanted ramets was caused by tidal scouring rather than edaphic factors.Once established,Spartina alterniflora was able to evolve morphological adaption quickly,and formed low but dense ramets with greater allocation to belowground parts.On the contrary, Phragmites australis was not well adapted to the tidal effects and sharp change of water level,so its potential lower border was higher than that of Spartina alterniflora. Transplantation in low marsh had significantly promoted the expansion and invasion of Spartina alterniflora at Chongming Dongtan.(3)Interspecific interactions among three native sedge species with identical morphology were influenced by physical stress and disturbance regimes.Scirpus triqueter was significantly less salt tolerant than Scirpus mariqueter and Carex scabrifolia.It was dominant species in fresh water,but became competitive inferior at 16 ppt.The suppression of Scirpus triqueter by competing species increased significantly with increasing salinity.Scirpus mariqueter was competitive superior in most cases,and it was less affected by competitors at high salinity.Carex scabrifolia was inferior to Scirpus species,but it was most salt tolerant among these species. Disturbance did not reduce the competition intensity within the community,but changed the relative competitive abilities of 3 sedge species.Clipping(simulated grazing)significantly reduced the performances of all species,and favored Scirpus triqueter which had the highest root allocation.Accretion had less effect on the whole, and it enhanced the flowering ratio of Scirpus mariqueter and density of Scirpus triqueter.Phylogenetic distances had minor effects on competition intensity among these native species.It was tolerance to stress and different kind of disturbances that determine the competitive results,and hence the structure and composition of plant communities in different habitats.(4)Competitive effect of invasive Spartina alterniflora on native Phragmites australis was strongly affected by litter,and varied within a growing season. Competition in early growing season had the greatest influence on growth and reproduction of Spartina alterniflora and Phragmites australis,while Spartina alterniflora had facilitative effects on Phragmites australis in mid growing season. Standing litter and vegetation in mature communities of Spartina alterniflora and Phragmites australis significantly reduced the light intensity under the canopy,and promoted the growth of transplants of other species in early growing season,but inhibited them later.Litter also affected the growth of their own populations.Litter reduced the density of living ramets,and promoted the height growth of Spartina alterniflora in early growing season,but inhibited it later.Phragmites australis grew higher during the whole growing season with the presence of litter.Litter also reduced the biomass of Spartina alterniflora,but the negative effects on Phragmites australis only existed in mid growing season.(5)Competitive effects of Spartina alterniflora and physical conditions had significant effects on Scirpus mariqueter.Spartina alterniflora had strong competitive effects on Scirpus mariqueter,and it dominated the mixture in all treatments.As an aboveground competitor,Spartina alterniflora suppressed the growth and reproduction of Scirpus mariqueter mainly by shoot competiton.In the front of intertidal zone,elevated sediment reduced the tidal effects,which would facilitate the growth of Scirpus mariqueter.Nutrient addition had no significant effects on Scirpus mariqueter,but enhanced the biomass of Spartina alterniflora.Therefore, eutrophication might stimulate the invasion of Spartina alterniflora into Scirpus zone, which would cause severe ecological consequences.Only when we have considered all factors affecting growth and reproduction of plants,can we understand the structure and organization of plant communities at Chongming Dongtan and predict the future dynamics,which will provide guidance to management of the nature reserve.