Water and nutrient use efficiency of three tree species in monoculture and mixed stands and potential drivers in the Loess Hilly Region, China - Lu, Senbao (Chinese Academy of Sciences. The Research Center of Soil and Water Conservation and Ecological Environment) ; Chen, Yunming (University of Chinese Academy of Sciences) ; Sardans i Galobart, Jordi (Centre de Recerca Ecològica i d'Aplicacions Forestals) ; Peñuelas, Josep (Centre de Recerca Ecològica i d'Aplicacions Forestals)