Comparative Studies on Photosynthetic Characteristics off Taxodium distichum and Taxodium ascendens Seedlings under Simulated Soil Water Change in the Hydro-Fluctuation Belt of Three Gorges Reservoir Area Nutrient Dynamics in Soils of Taxodium distichum Seedlings under Simulated Soil Water Changes in the Hydro Fluctuation Belt of the Three Gorges Reservoir Region Effect of soil water change on photosynthetic characteristics of Taxodium distichum seedlings in the hydro-fluctuation belt of the Three Gorges Reservoir Area Influences of mimic soil water change on the contents of malic acid and shikimic acid and root-biomasses of Taxodium distichum seedlings in the hydro-fluctuation belt of the Three Gorges reservoir region