In recent years, the relationship of biodiversity to ecosystem stability, productivity and other ecosystem functions have been researched by using theoretical approaches, experimental investigations and observations in natural ecosystems,however, results have been controversial. For example, simple systems were more stable than complex systems in theoretical studies, higher productivity was observed in man-made ecosystems with poorer species composition than in natural ecosystems with more diverse assemblages, etc. The role of biodiversity in ecosystem functioning, such as its influence on sustainability, stability, and productivity, still is not understood. Because accelerated soil erosion in various ecosystems has caused a decrease in ecosystem primary productivity, a logical way to study the relationship between biodiversity and ecosystem function will be to study the relationship between plant species diversity and soil conservation. In addition, biodiversity is a product of evolutionary history and soil erosion is a key factor controlling the evolution of the modern environment on the surface of the Earth. A study on the relationships between biodiversity and soil erosion processes could help to understand the environmental evolution of Earth and predict the future changes.

To test this, fifteen 10 m×40 m standard runoff plots were established to measure surface runoff, soil erosion and total P leaching in different secondary communities of semi-humid evergreen broad-leaved forests that varied in composition, diversity and level of disturbance and soil erosion. The following five communities were studied: AEI (Ass.Elsholtzia fruticosa+Imperata cylindrical), APMO (Ass. Pinus yunnanensis+Myrsine africana+Oplismenus compsitus), APLO (Ass.Pinus yunnanensis+Lithocarpus dealbatus+Oplismenus compsitus), AEME (Ass. Eucalyptus smith+Myrsine africana+Eupatorium enophorum) and ACKV (Ass. Cyclobalanopsis glaucoides+Keteleeria evelyniana+Viola duelouxii) in central Yunnan Province of China (101°28'18″ E, 25°24'09″ N, 1 950-2 015 m). Tree density, the diameter of trees at breast height and the hygroscopic volume of plant leaves were determined in each plot in order to analyze the relationship between plant species diversity and ecosystem function related to soil conservation and community structure. The degraded AEI, APMO, APLO and ACKV communities were restored naturally and the AEME was restored using plantation trees ofE. smith and E. maidenii at AEI. The sequence of the successional phases were: 1) AEI AEME ACKV; 2) AEI APMO APLO ACKV. The slope degree, slope position and soil types of all plots were similar so that the results of these plots could be compared based on differences in diversity and composition only.

The results indicated that surface runoff, soil erosion and total P leaching decreased according to a power function as plant species diversity increased and the three year average was 960.20 m³·hm^-2·a^-1, 11.4 t·hm^-2·a^-1, 127.69 kg·hm^-2·a^-1 in the plot with the lowest species diversity and 75.55 m³·hm^-2·a^-1, 0.28 t·hm^-2·a^-1, 4.71 kg·hm^-2·a^-1 in the plot with the highest species diversity. The low species plot was 12, 50 and 25 times, respectively, higher than in the high species plot. Soil conservation was enhanced with increasing plant species diversity. The coefficient of variation of surface runoff, soil erosion and total P leaching also followed a power function decreasing with increasing plant species diversity from 2001 to 2003. The coefficient of variation was 287.6, 534.21, 315.47, respectively, in the lowest species diversity plot and 57.93, 187.94 and 59.2 in the highest species diversity plot, which was 4, 3 and 5 times greater in the lowest species plot. Enhanced soil conservation maintained greater stability with increased plant species diversity. Plant individual density increased linearly and the degree of closeness and basic coverage increased logarithmically with increasing plant species diversity. The hydrological function of the leaves of the plant communities was strongest in ACKV, poorest in APMO, and intermediate in the AEME, APLO and AEI communities. The hydrological function of the leaves was enhanced as the plant species diversity increased. There were obviously relationships between plant species diversity with rainfall interception, coverage, plant individual density and they were related to soil conservation once more in the five successional forest communities.

The complex relationships of plant species diversity with above-mentioned ecological processes indicated that plant species diversity was an important factor influencing the interception of rainfall reducing soil erosion and enhancing the stability of soil conservation, but the causal mechanism is not known. This experiment showed that plant species diversity promoted soil and nutrient conservation and was able to predict primary productivity of the ecosystem, and was thus a good way to study the relationship between biodiversity and ecosystem stability. Rainfall interception could be assessed easily using the hygroscopic volume of plant leaves of the plant community. Because there were strong correlations between plant species diversity and ecosystem functioning as related to soil conservation, the patterns of plant species diversity will show a certain level of predictability on the interactions of life systems with surface processes of the Earth.