Ecological water requirement is a hotspot in recent years. A lot of researches have been done to calculate the quantity of ecological water needed and to elucidate the mechanism why and how ecological water is required. Little has been done on the thresholds of ecological water requirement. Ecological thresholds of wetland plant species to the environmental gradients is one of the key issues to ascertain the thresholds of wetland ecological water requirement.
Yellow River Delta, located in the middle of East-Asian Continent, is one of the three main estuarine wetlands in China. Suaeda salsa salt marsh, covered most forelands in the Yellow River Delta, is an important habitat for the birds from East-Asia and from East-Asia to Australia. It is also important to keep the natural succession of the wetland eco-system. In recent decades, with the exploitation of wetland resources and the rush of environmental problems, this habitat suitable to Suaeda salsa is degrading. The present paper was composed to answer the question what a water table depth degree and what a soil salinity degree is suitable for Suaeda salsa.
Based on a extensive investigation on the habitation of Suaeda salsa in the Yellow River Delta, we contrived three districts (A、B、C) to do our research. District A was located in the very estuary of the Yellow River, and Suaeda salsa was the only dominant species and had a great coverage in the district. District B was set in the region where the wetland restoration project has been carried out, and since this region was irrigated with fresh water from the Yellow River every year, it formed freshwater vegetation with reed and other hydrophytes, and Suaeda salsa can seldom be found. District C was set in a saline badlands with rare plant species and a low coverage of vegetation. Suaeda salsa in District C distributed only in a few small patches. We investigated 69 sampling sites in the three districts all together and obtained detailed data on the vegetation and the growth of Suaeda salsa in every sampling site.
Based on the correlation analysis of biomass, density, height, coverage, abundance of Suaeda salsa, we chose biomass as the biological index for further analysis. And the ecological thresholds of Suaeda salsa to the environmental gradients of water table depth and soil salinity in the Yellow River Delta were analyzed using the Gaussian Model. The results indicated, the optimum water table depth to the growth of Suaeda salsa was about -0.42m, and the ecological thresholds zone was [-0.92，0.08]（m）, and the optimum ecological thresholds zone was [-0.67，-0.17]（m）. To the soil salinity gradient, the optimum was about 12.71 g/kg, and the ecological thresholds zone was [5.17，20.25]（g/kg）, and the optimum ecological thresholds zone was [8.94，16.48]（g/kg）. Furthermore, the effect of water-salinity interaction to the growth of Suaeda salsa was discussed through analyzing the relationship between water table depth and soil salinity and their interactions. By Ward cluster analysis using Gamma distances, 7 different kinds of clustered sampling sites were found. With the changing of the water table depth and soil salinity, obvious succession of the plant communities was discovered.