Studies on Vegetation Succession of Abandoned Arable Land in Loess Hilly Regions of Northern of Shaanxi Province-Analyses of Community Composition and Structure

Studies have been made on three main traits of communities' composition and structure in 18 abandoned lands of abandondment to analyze the species number-area curves, species diversity indices and species abundance pattern of communities. The results show that both power and exponent function could be used to fit the curves of species number-area. As the land abandoned time advances, the land community evenness decrease and the dominance index increase, the species diversity indices Simpson、Shannon、Brilliouin and Dahl decrease, but the Whittaker index steadily increase. The species patterns of initial succession stage effect logarithmic series distribution, which implies that niche pre-emption is the main factor in controlling the structure of the initial succession community. The species patterns of the middle and the later stage are arranged in lognormal distribution, their community structures could be affected by niche pre-emption, seed invasion stress of the nearby community, interspecies competition and other factors concurrently. Besides the above situations, some community patterns are of neither distributions . More statistical and theory models are needed to explain all this.