Investigation of bud burst, shoot growth and leaf expansion in Populus euphratica of different ages Clonal growth characteristics of the endangered species Populus euphratica oliv and Populus pruinosa schrenk Distribution Pattern of Seedlings of Populus euphratica after River Overflowing in the Lower Reaches of Tarim River Characteristics of the Populations Age Structureof Three Typical Plantin Kumtag Desert