崇明东滩盐沼植被扩散格局及其形成机制

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

长江河口盐沼植被的形成和演化是生物与其生长环境相互作用的结果。以崇明东滩盐沼植被典型扩散前沿为研究对象,2011至2012年期间调查了盐沼植被扩散前沿实生苗扩散、定居以及形成的扩散格局,同时测定了盐沼植被扩散前沿的潮滩冲淤动态和水文动力条件。研究结果表明,崇明东滩盐沼植被在扩散前沿形成了互花米草-光滩(Spartina alterniflora-Mudflat,SM)和互花米草-海三棱藨草-光滩(Spartina alterniflora-Scirpus mariqueter-Mudflat,SSM)两种典型的扩散格局。冲淤动态和水文动力条件是影响盐沼植被扩散格局的重要因子,尤其是在4—6月盐沼植物实生苗传播和定居的关键阶段。在此基础上,分析了东滩盐沼植被扩散前沿的生物-物理相互作用以及盐沼植被扩散格局的形成机制。研究结果不仅有助于理解长江河口地区盐沼植被扩散的生物物理过程,并对全球气候变化和海平面上升条件下滨海生态系统动态预测与湿地保护与管理具有重要的意义。

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.