Soil salinity which is closely related to sodium (Na) content greatly affects vegetation pattern in beach wetlands. Tidal flat reclamation is a popular way to increase land area in coastal regions of East China, leading to landscape change and successional development towards mesophytic communities in beach wetlands. The Hangzhou Bay is a world-famous estuary gulf with extremely strong tide, i.e. the Qianjiang Tide, and located in the vicinity of northeastern Zhejiang Province in East China. The Andong Wetland located in Cixi City of Zhejiang Province along the Hangzhou Bay is a key spots for northeast Asian birds migration in China. However, it has been strongly affected by human activities, especially reclamation; effects of human activities on vegetation and soil traits have not been studied yet.
To investigate the effect of reclamation on soil Na content and on the relationship between Na content and vegetation pattern, we surveyed plant communities inside and outside the seawall (affected and not affected by reclamation) by transect sampling method in the Andong Wetland from 2003 to 2005. We also collected soil samples and determined soil Na content by X-ray fluorescence analysis. We calculated Margalef index and Simpson index as measures of species diversity and importance value as a measure of plant dominance.
Species diversity was higher inside than outside the seawall. Vegetation inside the seawall consisted of 11 plant species, which was relatively more dominated by mesophytes than that outside. Inside the seawall the most dominant species was Spartina alterniflora and the second most dominant was Phragmites australis. However, only three species (Sp. alterniflora, Scirpus xmariqueter and P. australis) occurred in the natural wetland outside the seawall, and Sp. alterniflora and Sc. xmariqueter were the two dominants. Both the Margalef index (R²=0.732, P<0.01) and the Simpson index (R²=0.924, P<0.01) were negatively correlated with soil Na content. Inside the seawall, soil Na content was positively correlated with the important value of Sp. alterniflora (R²=0.802，P<0.01), but negatively correlated with that of P. australis (R²=0.607, P<0.01). However, outside the seawall, soil Na content was significantly correlated with the importance value of neither Sp. alterniflora nor Sc. xmariqueter. With increasing duration since reclamation, number of mesophytes increased, whereas that of halophytes decreased. We conclude that long-term land reclamation significantly decreases soil salinity which greatly influences plant species diversity and dominance in wetlands.