[Composition and niche of the existing herbaceous plants in the water-level-fluctuating zone of the Three Gorges Reservoir Area, China].

To understand the usages of available resource by dominant plants, their niches and the mechanisms of inter-specific competition and co-existence in the water level fluctuation zone, we studied the spatial distributions and niche characteristics of existing dominant herbaceous species at a typical water-level-fluctuation site of the Three Gorges Reservoir Area, Zigui. The results showed that there were 39 herb species in total, which belonged to 18 families and 32 genera. Gramineae, Compositae, Polygonaceae and Euphorbiaceae were the dominant families. Cynodon dactylon, Setaria viridis, Bidentis tripartitae and Digitaria chrysoblephara were the dominant species, with high importance value and niche breadth. In addition, at 145-155 m, 155-165 m and 165-175 m altitude section, the three major species, expressing the highest ecological niche breadth, which were in order of C. dactylon ＞ Polygonum lapathifolium ＞ S. viridis, S. viridis ＞ D. chrysoblephara ＞ C. dactylon, and S. viridis ＞ B. tripartitae ＞ P. orientale, respectively. The niche overlap of the species between the different altitudes zone was relatively high. The species which had a broad niche could co-exist with those occupying narrow niche. The niche overlap could not be determined by niche breadth lonely. Furthermore, after seven times of water level fluctuations, most of the species were annual herbs, and the degree of niche differentiation was low in the area. Due to scarce resources and unstable habitats, the inter-specific competition was strong, and the vegetation was at the primary successional stage.