Water and nutrient use efficiency of three tree species in monoculture and mixed stands and potential drivers in the Loess Hilly Region, China

Background Mixed plantations utilize resources through specific strategies. However, the impact mechanism of mixed plantations on the water and nutrient use efficiency of different tree species remains unclear. Methods Robinia pseudoacacia mixed ( R. pseudoacacia and Amygdalus davidiana , RPAD; R. pseudoacacia and Armeniaca sibirica , RPAS) and monoculture stands ( R. pseudoacacia , RP; A. davidiana , AD; and A. sibirica , AS) were used to determine mixing species relative to monoculture effects on leaf water (WUE), nitrogen (NUE), and phosphorus (PUE) use efficiencies and their potential drivers in the Loess Hilly Region. Results Mixing R. pseudoacacia and A. sibirica is a good strategy to maximize soil nutrient resources. Compared to monocultures of all species, R. pseudoacacia and A. davidiana in RPAD and A. sibirica in RPAS had higher WUE and lower NUE and PUE; P limitation of R. pseudoacacia in mixed stands was lower than that in RP. Furthermore, R. pseudoacacia WUE was positively correlated with stand density and crown area, A. davidiana and A. sibirica were the opposite. Regarding all tree species, WUE was influenced by leaf dry matter content, leaf tissue density, and soil available P, while NUE, PUE, and N:P ratio were by average tree height and specific leaf area. Notably, irrespective of tree species, WUE exhibited a negative correlation with NUE and PUE, but a positive correlation with N:P ratio. Conclusions Mixed plantations affect water and nutrient use efficiency by altering the above- and below-ground niche through tree structure, and there is a degree of trade-off between WUE, and NUE, PUE.