The expansion pattern of saltmarshes at Chongming Dongtan and its underlying mechanism

The range expansion of saltmarshes in the Yangtze Estuary depends on accretion of dynamic mudflats resulting from the large amount of silt brought down the Yangtze River. The formation and succession of saltmarsh vegetation is a result of the interaction between saltmarsh plants and their living environment. Over the last 15 years, rapid invasion of large areas of the saltmarshes of Chongming Dongtan by an exotic plant Spartina alterniflora has occurred. Two types of advancing fronts are found: the S. alterniflora-mudflat (SM) front and the S. alterniflora-Scirpus mariqueter -mudflat (SSM) front. The dynamic and natural characteristics of the coastal wetlands at Chongming Dongtan, both in terms of sediment deposition, salt marsh succession and biological invasion, make it an ideal place for studying the interaction between the range expansion of saltmarsh vegetation and the hydrodynamic regimes. In this study, we invested the seedling dispersal, establishment and formation of expansion pattern at the advancing fronts during the years of 2011 and 2012. At the same time, the accretion/erosion regime and hydrodynamic conditions at the saltmarsh expansion fronts were measured. The research aims were to: 1) test whether and how the hydrodynamic regimes govern the spatial structure of range expansion of saltmarshes at these two advancing fronts; 2) determine the interactions between the range expansion of saltmarsh and hydrodynamic regimes, and 3) understand the mechanisms underlying the range expansion of saltmarshes and their implications for the spatial structure of tidal saltmarshes. The results showed that the accretion/erosion regime and hydrodynamic condition, especially in the critical stage of seedling establishment during April-June, were the most important factors responsible for the formation of these two typical expansion patterns. The rate of range expansion at the SM front was much faster than the SSM front. The different colonization behaviors at these two types of advancing fronts could be related to the differences in hydrodynamic regimes. At the site with a regime of autumn/winter erosion and spring/summer accretion, the sediment accretion and seedling recruitment of S. alterniflora were much higher, while the wave energy and current velocity were relatively gentle during the critical period of seedling establishment. At such site, the original pioneer species of S. mariqueter was replaced by S. alterniflora and a SM pattern of range expansion developed. In contrast, at the site with a relatively stable autumn/winter accretion regime and relatively strong wave energy and current velocity during the critical period of seedling establishment, the original pioneer species of S. mariqueter could remain within the advancing front and a SSM pattern of range expansion developed. Based on these results, the impact of abiotic and biotic factors governing the range expansion of and its implications on the spatial structure of tidal saltmarshes are discussed. The results from this study can provide a valuable insight into the bio-physical interactions on the formation of expansion patterns and the mechanisms underlying the range expansion of saltmarshes in the Yangtze Estuary, which could be important for the maintenance of coastal saltmarsh resource and biodiversity as well as the dynamics prediction of coastal saltmarshes under the impact of climate change.