Ecosystems water use patterns of Quercus variabilis and Grewia biloba based on stable hydrogen and oxygen isotopes in the south aspect of Taihang Mountains Physiological Characteristics of Elaeagnus angustifolia and Grewia biloba G. Don var. parviflora Seedlings under Drought Stress Chlorophyll Fluorescence Characteristics of Elaeagnus angustifolia L. and Grewia biloba G.Don var.parviflora(Bge.)Hand.-Mazz. Seedlings under Drought Stress