Studies On Ecological Characteristics Of Plant Communities On Salt Marsh In Jiuduansha Wetland, Yangtze River Estuary

Jiuduansha Wetland National Nature Reserve is newly formed islands of brackish silt shoals off coastline, which is influenced by the river and ocean tides cyclical or intermittent of vegetation cover. It can provide nutrient cycling, purification water, food production and many other ecosystem services, more ecological and economic value. Therefore more and more attention on its ecological and scientific values has been paid in recent years.Phytocoenology is one of the important branches of plant ecology, mainly studying species composition, community, structure, inerspecies interaction, distribution, succession with environments. Studying the distribution pattern, the factors influence the spatial and temporal distribution of salt marsh communities is essential and important for us to understand salt marsh ecosystem.The plant communities on typical tidal creeks of different gradient levels on Jiuduansha were taken as objects, the species composition of communities, population distribution pattern, population niche, and species diversity were studies. Meahwhile, the tidal flats of typical development at estuary were selected based on the hydrodynamic and hydrological characteristics. On these tidal flats, fixed observation sampled lines were established to investigate the successions of vegetations on the cross section (namely sampled lines on the tidal flats). The impacts of invasion by alien invasive Solidago candensis on the physical and chemical characteristics of soil, and on the composition of benthic animals were also investigated. The main results are as follows:(1) Most plant species of Jiuduansha are herbaceous angiosperms, some species with narrowly distribution. Though simple in species composition, Jiuduansha shows its rapidly increase in different taxa, from15species (belonging to15genera and7families) in2003to51species (belonging to46genera and17families). Among these51species, most belong to compositae and dicotyledon, some to tree or bryophytes. The plant communities are diverse and complex. The trend is clear that more terrestrial and mesic composition appear in the communities on higher elevation. The plant life types are characterized by annual and hidden bud plants. Some cultivated plants also grow well. The environments are possible suitable for some phanerophytes to grow.(2) Cosmopolitan and pan-Arctic distributions are the main elements in the flora on Jiuduansha. Scirpus mariqueter is the only species endemic to China. Among the51species, most belong to temperate distribution types, somen to cosmopolitan, and a few to tropical types. The composition geographic elements are related to the geographical location and sand environmental conditions of Jiuduansha.(3) There exist two vegetation subtypes, herbal marsh and salt meadow vegetation, of wich herbal marshes have Scirpus mariqueter and Phragmites australis two formation, and salt meadow has Phragmites australis and salt-tolerant species including Suaeda glauca and Tripolium vulgare, the invasion of Spartina alterniflora formed artificial vegetation, with the formation of sediment, Scirpus mariqueter and Scirpus triqueter appeared first, then Phragmites australis and other marsh plants, as hydraulic and vegetation interactive, tidal creeks are getting maturity, marsh vegetation gradually successes to salt meadow with the dominant species Phragmites australis, together with other salt-tolerant species such as Suaeda glauca, and Tripolium vulgare, some local sites invaded by Solidago canadensis, Conyza canadensis, Aster subulatus and other alien plants.(4) Spartina alterniflora has widely distribution across the herbaceous mash and halophytic meadow vegetation, showing its wide ecological adaptability on salt and ventilation.(5) Fifty-two sites were sampled on the typical tidal creeks on Upper and Lower shoals of Jiuduansha, the ecological investigation of plant communities were conducted. A total of22species were recorded in the52sites. These sites varied much among their species diversity. In the surveyed area, higher proportion of weeds and invasive plants reveal that the study area has been strongly influenced by human activities.(6) From the location of the22species on PAA three-dimensional ordination plot, we recognized six ecological groups, which are:Group1including Solidago Canadensis, Metaplexis japonica and Carex scabrifolia; Group2including Aster subulatus, Pterocypsela indica and Conyza canadensis; Group3including Chenopodium album, Polypogon fugax, Achyranthes bidentata and Pterocypsela indica; Group4including Scirpus marqueter, Scirpus triqueter, Tripolium vulgare, Suaeda glauca, Ohacelurus latifolius and Altermanthera philoxeroides; Group5and6including Phragmites australis and Spartina alterniflora, respectively; Phragmites australis and Spartina alterniflora were widely distributed in Jiuduansha, the former species is indigenous and the latter is alien invasive. Among the22species, Phragmites australis has widest niche, being3.1468, the species with niche width from2to3are Spartina alternifolia, Metaplexis japonica, Aster tataricus, Pterocysela indica, Carex scabrifolia, Scirpus mariqueter and Solidago canadensis, those with niche width from1to2are Tripolium vulgare, Scirpus triqueter, Suaeda glauca and Conyza bonariensis, while Ohacelurus latifolius, Altermanthera Philoxeroides, Achyranthes bidentata, Conyza canadensis, Miscanthus floridulus has narrower niche width from0to1, and the niche widths of Chenpodium album, Polypogon monspeliensis, Sonchus oleraceus, Eclipta prostrates are zero because of their distribution at one site.(7) In the similar habitats, the soil particles with dominant S. canadensis and other mesophytes are more coarse than those of P. australis and S. alterniflora, indicating that S. canadensis has promote the development of soil and the process of de-saline; Compared with other plant species, the content of N, P, K in soil invaded by S.canadensis are not significantly different with others, except in shallow soil in first grade of Lower shoal and forth grade of Upper shoal sites, promoting the accumulation the content of N, P, K in soil as invaded by S.canadensis; In different environments, from the Upper shoal (tidal creek of the first grade), as the micro-topography increasing gradually, the content of N, P of the soil invaded by S.canadensis increased; In Lower shoal (tidal creek with the third grade and the first grade), the invasion by S. canadensis increased the diversity of macrobenthic animal species.(8) On three surveyed section with different hydrodynamic and hydrological characteristics on South Jiangyang shoal, Upper shoal and Lower shoal, the trends of their community successions are as follows:On South Jiangya shoal:bare flatâ†'Scirpus communityâ†'Scirpus/Typha/Zizania communityâ†'ThphalZizanialPhragmite australia communityUpper shoal:bare flatâ†'Scirpus communityâ†'ScirpuslSpartina alterniflora communityâ†'Phragmite australia community;Lower shoal:bare flatâ†'Scirpus communityâ†'Scirpus/Spartina alterniflora communityâ†'Spartina alterniflora community.