Water Use Efficiency and Carbon Isotopic Compositions in Tetraploid Clones of Black Locust under Drought Resistance Ecosystems carbon storage and carbon sequestration potential of two main tree species for the Grain for Green Project on China‘s hilly Loess Plateau Effects of Water Stress on Biomass and Photosynthetic Characteristics of Tetraploid Black Locust (Robinia pseudoacacia L.) Clones THE SOIL AMELIORATION EFFECT OF POPLAR-BLACK LOCUST MIXED PLANTATION ON SAND SOIL AND THE INTERACTION OF MUTUAL SUPPLEMENT OF NUTRIENT BETWEEN TREE SPECIES