| Vegetation restoration is most effective approach on soil and water conservation To combine the benefits of artificial vegetation and natural vegetation and promote the succession process for artificial vegetation naturalization, is an important strategy ecosystem restoration. This paper, aimed at artificial Robinia Pseudoacacia forests, studied on growth characteristics, changes of soil moisture and nutrition succession process of undergrowth, relationship between undergrowth and environmental factors. discussed the methods evaluating vegetation naturalization degree, and a technology to promote the process of vegetation naturalization is also presented. The main results are as follows:1. The distribution and growth of Robinia Pseudoacacia in zonal environment was mainly influenced by precipitation on Loess Plateau. Low soil water content was a main limit factor to the distribution and growth of Robinia Pseudoacacia, but not to the restoration of zonal vegetation. The Robinia Pseudoacacia has very strong ability of water-utilization. Its adaptation strategy for water deficit seems to decrease its growth speed and height. Where it is seriously dry condition, the plant may sacrifice some components (withered tip).2. The rate of soil fertility restoration on Robinia Pseudoacacia woodland was faster than that on abandoned land. After plantation, soil organic matter, total nitrogen, available nitrogen, total phosphorus, available phosphorus and available potassium, were increasing with the growth process . On the contrary, the pH value tended to decrease. Compared with the soil of 5a Robinia Pseudoacacia woodland, six indexes were used to described above of 50a has increased by 218%, 206%, 434%, 22%, 85% and 52% respectively. From southern to northern part on the Loess Plateau, soil organic matter and total N content of tree sites tend to increase with growth age, however, the soil pH tend to decreased.3. Species composition and biomass of Robinia Pseudoacacia woodland undergrowth in different succession stage was similar to that of vegetation in abandoned land. From the results in forestry-grassland region of Ansai site that soil moisture did not hinder the growth and succession of undergrowth, although trees consumed much water in soil, and make soil moisture dryer and dryer.4. 9 kinds of functional traits of Robinia Pseudoacacia and Artemisia gmelinii. were measured, The results showed that almost all functional traits of Robinia Pseudoacacia in different region had not significant difference at same growth stage, which means that Robinia Pseudoacacia did not form new ecotype to adapting the different environment condition, especially to water resources change, and its adaptive strategy to environmental change was to adjust its growth speed, even to lose some of its components. However the Artemisia gmelinii was different from Robinia Pseudoacacia. which had fine correlation with environment factors. It means that the adaptability to environment in the Loess Plateau of Artemisia gmelinii was much better than that of Robinia Pseudoacacia.5 The evaluation model of vegetative naturalization index (VNI) was presented with variables including vegetation coverage, community synusium structure, species composition and aboveground biomass. The result found that VNI may describe the situation of vegetation naturalization in the research. By using of VNI, the vegetation naturalness index of Robinia Pseudoacacia forests undergrowth and vegetation on abandoned land in forest-steppe zone on Loess Plateau. The result showed that the VNI of two types of vegetation tendency of was basically coincident with succession stage. But VNI of undergrowth at later young forest stage and early middle age stage tend to decrease for the density canopy, and VNI of latter basically tended to increase in whole succession process. Once the canopy opened, and the undergrowth could get full sunlight, the succession speed of undergrowth was accelerated, and VNI value was enhanced rapidly. Correlation analysis indicated that VNI value of undergrowth had fine correlation with environmental factors. Favorate soil fertility could improve the process of vegetation naturalization. Soil water content was negatively correlated with VNI, which means that low soil water content did not effect the positive succession of undergrowth.
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