| Pure Ulmus pumila, pure Robinia pseudoacacia and three kinds of proportion mixed forests (Ulmus pumila: Robinia pseudoacacia =2:1; Ulmus pumila: Robinia pseudoacacia =1:1 and Ulmus pumila: Robinia pseudoacacia =1:2) were studied in the Loess Plateau. Through two years field experimental observation in the Yangling District of Shaanxi Province, the soil and plant nutrient content, leaf structure characteristics, the root characteristics and soil respiration, photosynthetic physiological characteristics and plant growth characteristics of the seasonal dynamic changes were analyzed. The relationships of the plant physiology characteristic and ecology function of Ulmus pumila and Robinia pseudoacacia mixed forest were discussed in the Loess Plateau. The findings were helpful to reveal the ecological environment change compatible strategy of the mixed forest plant growth in the Loess Plateau, and provide a theoretical basis for mixed forest model selection and afforestation recovery and reconstruction of vegetation. The main conclusions are as follows:(1) The changes of soil and plant nutrient content of pure Ulmus pumila, pure Robinia pseudoacacia and three mixed forests in the Loess Plateau were clarified. The soil N, soil P and soil organic matter content in mixed forest were all increased than in the two pure forests, especially the mixed forest of Ulmus pumila: Robinia pseudoacacia =1: 2 (1B2C) to improve soil nutrient content greater than other treatments. Plant N and plant P content of different organs were higher in the leaf and lower in the stem. In the three mixed forests, the plant N and plant P content were: 1B2C > 2B1C (Ulmus pumila: Robinia pseudoacacia =2:1) > 1B1C (Ulmus pumila: Robinia pseudoacacia =1:1). These indicated that because of the nitrogen-fixing tree species- Robinia pseudoacacia, the soil nutrient level in mixed forest had been improved, and it also raised the plant N and plant P absorption and utilization of nutrients, and particularly increased the plant N and plant P nutrition absorption of Ulmus pumila in the mixed forest. Among the five treatments, the mixed forest of 1B2C can increased the total N and total P content more significantly.(2) The seasonal dynamic changes of leaf structural characteristics of pure Ulmus pumila, pure Robinia pseudoacacia and three mixed forests were revealed. The leaf dry weight and leaf thickness of pure Ulmus pumila were bigger and there were smaller changes of leaf dry matter content (LDMC) in the different treatments, and it shows that LDMC was possibly insensitive to the different forest type's changes. Specific leaf area of Ulmus pumila and Robinia pseudoacacia in the mixed forest 1B2C was larger than other treatments. Specific leaf area and leaf thickness, LDMC and leaf dry weight with different degrees relevance, leaf thickness of Ulmus pumila and Robinia pseudoacacia compared the effects of specific leaf area was greater than LDMC and leaf dry weight.(3) The seasonal dynamic changes of root characteristics and soil respiration of pure Ulmus pumila, pure Robinia pseudoacacia and three mixed forests were analyzed. The root surface area, root diameter and specific root length were the highest in July and the lowest in May in different seasons, but the change of root biomass was different. The root biomass and root surface area of Ulmus pumila in mixed forest 1B2C which were highest, and root biomass, root surface area and specific root length of pure Ulmus pumila were the lowest. Parameters of fine roots with different degrees relevance, it shows that different site conditions display the different fine roots parameters. Daily variation of soil respiration rate was single-peak curve, the peak value appears in the afternoon 12:00-15:00 pm. The daily variation tendency of soil respiration rate was mainly related with the soil surface temperature. The soil respiration rate also has the obvious seasonal changes, the mixed forest 1B2C was highest soil respiration rate in different treatments. Soil temperature and soil respiration has a strong positive correlation, especially the depth of 5cm soil temperature. The relationship between soil respiration and soil moisture often display discrete, and do not have the obvious regularity, explaining that in addition to temperature and soil moisture effects, other factors on soil respiration can not be ignored.(4) The relationship among leaf photosynthetic physiological characteristics and water use efficiency (WUE) of pure Ulmus pumila, pure Robinia pseudoacacia and three mixed forests were discussed. Photosynthetic rate of Robinia pseudoacacia in the mixed forest 1B2C was the highest among the different treatments, but the transpiration rate was determined by the temperature. WUE of Ulmus pumila in the mixed forest 1B2C and pure Robinia pseudoacacia were higher than other treatments, and WUE of Ulmus pumila was bigger than Robinia pseudoacacia. The photosynthetic nitrogen use efficiency (PNUE) has markedly improved of three kinds of proportion Ulmus pumila in the mixed forest than pure Ulmus pumila. PNUE of the three mixed forest: 1B2C> 1B1C> 2B1C. The results showed that there were higher nitrogen use efficiency of Ulmus pumila and Robinia pseudoacacia in the mixed forest 1B2C, particularly Ulmus pumila in the mixed forest. From the correlation of leaf N content, photosynthetic rate and PNUE, it indicated that with the leaf nitrogen content increasing, the photosynthetic capacity enhancing, but leaf nitrogen content increases cannot represent the plant have high nitrogen utilization efficiency.(5) Plant growth dynamics and the biomass allocation patterns of pure Ulmus pumila, pure Robinia pseudoacacia and three mixed forests were studied. We found that during the prime season of plant growth, the more biomass was allocated in leaf growth in order to obtain more photosynthate to promote plant growth. At the end of period of growing, plant assimilation substance will accumulate to the root system so as to preparing for the disadvantageous environment or passed the winter. This is a plant strategy to adapt to the environment. Using the model W=aDb (W for biomass, D for tree diameter, a, b as the coefficient) we set up the regression equation about plant biomass and the tree diameter, and found that a higher correlation coefficient is existed, so it can be used as plant biomass estimation model of Ulmus pumila and Robinia pseudoacacia.
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