Abstract: A field experiment was conducted to study the leaf N, P, and chlorophyll contents, photosynthetic gas exchange parameters, and chlorophyll fluorescence parameters in pure Ulmus pumila orest, pure Robinia pseudoacacia forest, and U. pumila-R. pseudoacacia mixed forests ［1：1 (1B1C), 1:2 (1B2C), and 2:1 (2B1C)］ in different growth periods. From May to September, the plant leaf N and P contents in pure and mixed forests all presented a decreasing trend. By the end of growth period, the leaf N content of U. pumila and the P content of R. pseudoacacia in 1B2C were obviously higher than those in pure forests. In the mixed forests, the chlorophyll content of U. pumila was obviously higher than that of  R. pseudoacacia, and the chlorophyll content of U. pumila in 1B2C reached the highest in July. The photosynthetic rate (P_n) of  U. pumila  and R. pseudoacacia  in mixed forests was higher than that in pure forests, and the P_n of R. pseudoacacia in 1B2C reached the highest (18.54 μmol·m^-2·s^-1) in July. The transpiration rate (T_r) and stomatal conductance (G_s) of R. pseudoacaciam  in mixed forests were higher than those in pure forests, and the T_r and G_s in mixed forests were in the order of 1B2C>1B1C>2B1C. In September, the quantum yield of PSII electron transport (Ф_PSⅡ) of U. pumila  in mixed forests was obviously higher than that in pure forest. The photochemical quenching coefficients (q_P) of U. pumila and  R. pseudoacacia  in pure and mixed forests had no significant difference, but the nonphotochemical quenching coefficient (NPQ) of the two tree species in 1B2C was significantly lower than that in corresponding pure forests. It was suggested that mixed planting  U. pumila  and R. pseudoacacia  could significantly improve the leaf nutrient contents and photosynthetic capacity of the two tree species, and the optimum mixed ratio of  U. pumila and  R. pseudoacacia was 1: 2.

Key words: Ulmus pumila, Robinia pseudoacacia, mixed forests, leaf nutrient content, photosynthetic physiological characteristics, Cuscuta australis, parasitism, defense, biological control, invasive plant.