The differences of water use efficiency (WUE) among three Populus deltoids clones, and the cloning and characterization of related gene, PdERECTA Relationship on Leaf δ13C and Nutrients in Leaf and Soil of Different Nitraria tangutorum Populations in Tsaidam Basin Leaf δ13C of plants in different vegetation succession stages on karst hillslope of Northwest Guangxi, China