| Soil fauna is an important component of the ecosystem and plays an important role in substance cycling and energy flowing. It can sensitively responsible to the changes of environment. Ecotone system often exhibits relative more complicated characters and can sensitive response to changes of environment, provides relative more diverse habitats for creatures, displays relative higher biodiversity compared with the near ecosystems, and all these make ecotone system be more concerned by ecologists. At present, the ecological research of soil fauna community has focused on a single type of vegetation, but little research in vegetation types of these complex of ecotone system. The community composition, taxon distribution patterns and diversity of soil fauna were investigated on the timberline and forest-pasture ecotone. Using cluster analysis, this paper explored similarities of the soil fauna communities of two ecotones. By Canon correspondence analysis, the relationships between the major environmental factors and the soil fauna community, and the relationships between different environmental factors were explored. The purpose of this article was to understand the soil fauna community structure, ecological processes, the similarities and differences of the soil fauna communities under the different driving forces of alpine forst-grassland ecotone ecosystem, so as to provide a scientific basis for biodiversity conservation, ecological restoration and reconstruction of western mountain area of upper Yangtze River.We study on the soil fauna diversity and community structure of the typical timberline and forest-pasture ecotone in mountain area of western Sichuan from August 2008 to August 2009. The main conclusions are follows:1. A total of 111078 individuals were surveyed in the eleven plots on the timberline and forest-pasture ecotone habitat gradient, which belonged to 7 phyla,17 classes,35 orders and 157 groups. A total of 61241 individuals were surveyed on the timberline ecotone, which belonged to 131 groups. A total of 49837 individuals were surveyed on the forest-pasture ecotone, which belonged to 126 groups. The timberline and forest-pasture ecotone shared 103 groups.Nematodes was dominant in common groups which were very similar between two ecotones. There were large numbers of rare groups. At two types of ecotone, Acarina and Collembola were different in different seasons and different plots. The A/C value was 0.708 in timberline ecotone and was 1.024 in forest-pasture ecotone. In general, the A/C value was consistent with the characteristics of frigid zone.2. Both the density and richness of soil macro-, meso-, and micro-fauna were obviously declined with increasing soil depth under the timberline and forest-pasture ecotone. The group richness and density of soil fauna were significantly higher in the moss layer than that in other layer in coniferous forest of two ecotone which indicated that the moss layer of coniferous forest was more suitable for survival of soil fauna.3. There were larger differences of soil fauna diversity calculated by the different diversity indices. The Shannon-Wiener index showed gradual decreasing with altitude and the density-group index showed a unimodal shape aross the timberline. In the forest-pasture ecotone, the Shannon-Wiener index decreased slightly with disturbance gradient while the density-group index changed as "S"-type. In the timberline and forest-pasture ecotone, there is a clear displacement of groups of soil fauna.βdiversity values are quite small between the adjacent gradients, and increased gradually with the distance of the gradient.4. There were large differences of biomass, abundance, diversity of soil fauna between different seasons and between plots. Lumbricida occupy a large proportion of biomass of soil macro-fauna, followed by Diplopoda. The plot' means of soil macrofauna biomass in the timberline ecotone was larger than in the forest-pasture ecotone. In the timberline ecotone, the biomass of soil macrofauna was negatively correlated with altitude. In the forest-pasture ecotone, biomass of soil macro-fauna showed an "M"-type pattern with disturbance intensity. In most plots of the timberline and forest-pasture ecotone, the soil fauna density was the highest in autumn and diversity (H' and DG index) was the highest in summer.5. Using similarity and cluster analysis, the results showed the similarity of soil fauna community was mostly higher in the different habitats. The group compositions of soil fauna communities were quite different between different vegetation types, but were very similar between the same or similar types of vegetation. Serious disturbance of grazing resulted in larger differences in soil fauna community structure between plots under identical vegetation at high altitude.6. From the meadow to the coniferous forest in the two ecotones, soil environmental factors changed greatly. Soil organic matter increased and pH value showed a unimodal pattern with altitude. From the meadow to the coniferous forest in the transition, soil organic matter increased gradual and pH value decreased gradually7. The correlation of the soil animals and soil environmental factors showed a different pattern between the timberline and forest-pasture ecotone. In the timberline ecotone, the individual density of soil macrofauna had less affected by environmental factors, whereas soil microfauna had great affected by environmental factors. In the forest-pasture ecotone, the relationship of different size of soil fauna and environmental factors is highly similar, and soil fauna obvious had been affected by soil pH value, total N and litter thickness. The soil fauna of forest-pasture ecotone was more intensively influenced by environmental factors than of timberline ecotone.8. Different soil environmental factors had different degrees of influence to the different groups of soil meso-and micro-fauna.In the timberline ecotone, the most groups of soil meso-and micro-fauna were significantly affected by pH value. In the forest-pasture ecotone, the most groups of soil meso-and micro-fauna significantly had been affected by soil organic matter and litter thickness. The correlations of environmental factors are similar between in the timberline and forest-pasture ecotone. The groups of soil meso-and micro-fauna responded differently to soil environmental factors and the relationships were largely discrepant between two types of ecotone. Some groups were less affected by soil environmental factors.
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