Variation of Sap Flow and Stem Diameter of Populus euphratica under Drought Stress——A Case Study along Yingsu Section Karyotype Analysis of Section Populus in Populus Soil Water and Fertilizer Factors on the Trade-off of Growth and Lignification of Populus euphratica Seedling Sensitivity Analysis of P. euphratica Radial Growth to Groundwater Changes in the Different Transects of the Lower Reaches of Tarim River